JP2006011391A - Display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pixel structure capable of reducing the influence of an off current and attaining clear black display in a display device having a light emitting element. <P>SOLUTION: A pass element is provided at one electrode of the light emitting element so as not to flow the off current of a transistor for deiving the light emitting element through the light emitting element in a non-lighting period. The pass element allows the off current to flow to the outside. that is, the off current can be bypassed to the outside through the pass element and the influence of the off current is reduced. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

 The present invention relates to a display device that enables accurate display, particularly black display.

 In recent years, technical development relating to display devices having self-luminous elements has been actively conducted. In a pixel portion of a display device, a thin film transistor is often used as a semiconductor element, and lighting of the self light emitting element is controlled depending on whether the thin film transistor is on or off. When the thin film transistor is off, a minute current may flow. This current is called off-current. Even if the off-state current is very small, the self-luminous element is lit, which is a problem that is easily recognized by human eyes.

  As a conventional method for reducing off-state current, a lightly doped drain (LDD) structure is known. In this structure, an LDD region to which an impurity element is added at a low concentration is provided between a channel formation region and a source region or a drain region formed by adding an impurity element at a high concentration.

In addition, a so-called double gate structure having a conductive film provided to overlap with a channel formation region is proposed (see Patent Document 1). Patent Document 1 discloses that the off-current can be suppressed by always applying a constant voltage to the lower conductive film.
JP 2003-23161 A

 However, if the thin film transistor has an LDD structure or a double gate structure as in the above-mentioned patent document, the number of manufacturing steps increases, and the yield may be reduced.

  Thus, an object of the present invention is to reduce the influence of off-state current of a transistor connected to a light-emitting element in a display device by a method different from that of the above-described patent document.

 In view of the above problems, the present invention provides an element (hereinafter referred to as a “pass element”) that passes through one electrode of a light emitting element so that an off current from a transistor for driving the light emitting element does not flow to the light emitting element when the light emitting element is not lit. It is characterized by providing. With this pass element, the off-state current can flow outside the light emitting element, that is, outside. Thus, the present invention is characterized in that the off-current is bypassed to the outside through the pass element.

 One embodiment of the present invention will be described below.

 One embodiment of the present invention includes a light-emitting element, a first transistor for driving the light-emitting element, a light-emitting element, and a pass element connected to the first transistor, and the resistance value of the pass element Is a display device characterized in that it is smaller than the resistance value when the light emitting element is not lit and larger than the resistance value when the light emitting element is lit.

 In the present invention, the pass element can be any of a resistance element, a thin film transistor, a diode element, or a combination thereof.

 Another embodiment of the present invention is a light-emitting element, a first transistor for driving the light-emitting element, a light-emitting element, and a second transistor functioning as a pass element connected to the first transistor; And the second transistor is turned off when the first transistor for driving is turned off and turned off when the light emitting element is turned on.

 Another embodiment of the present invention is a light-emitting element, a first transistor for driving the light-emitting element, a light-emitting element, and a second transistor functioning as a pass element connected to the first transistor; And the second transistor is turned off when the first transistor for driving is turned off, turned off when the light emitting element is lit, and the resistance value of the second transistor is that of the non-light emitting element. The display device is characterized by being smaller than a resistance value when the light is turned on and larger than a resistance value when the light emitting element is turned on.

 Another embodiment of the present invention includes a light-emitting element, a transistor for driving the light-emitting element, a light-emitting element, and a p-type transistor connected to the transistor and functioning as a pass element. In the display device, the gate electrode of the transistor is connected to a power supply line, and one or the other electrode of the p-type transistor is connected to the counter electrode of the light-emitting element.

 Another embodiment of the present invention includes a light-emitting element, a transistor for driving the light-emitting element, a light-emitting element, and a p-type transistor connected to the transistor and functioning as a pass element. The gate electrode of the transistor is connected to the power supply line, one or the other electrode of the p-type transistor is connected to the counter electrode of the light-emitting element, and the resistance value of the p-type transistor is such that the light-emitting element is not lit. The display device is smaller than a resistance value when the light emitting element is turned on and larger than a resistance value when the light emitting element is turned on.

 In the present invention, the thin film transistor may have an LDD structure, a GOLD (gate overlap LDD) structure, or a double gate structure. By further combining the pass element of the present invention with a driving transistor having an LDD structure, a GOLD structure, or a double gate structure, a further reduction in off current can be expected.

 By providing the pass element in this way, when an off-current is generated in the driving transistor when the light-emitting element is not lit, the off-current can be flowed to the outside through the pass element. That is, the off-state current is not supplied to the light emitting element by the pass element. As a result, a clear black display can be performed.

 Embodiments of the present invention will be described below with reference to the drawings. However, the present invention can be implemented in many different modes, and those skilled in the art can easily understand that the modes and details can be variously changed without departing from the spirit and scope of the present invention. Is done. Therefore, the present invention is not construed as being limited to the description of this embodiment mode. Note that in all the drawings for describing the embodiments, the same portions or portions having similar functions are denoted by the same reference numerals, and repetitive description thereof is omitted.

 Note that although the transistor has three terminals of a gate, a source, and a drain, the source electrode terminal (source electrode) and the drain electrode terminal (drain electrode) cannot be clearly distinguished because of the structure of the transistor. Therefore, when describing a source electrode or a drain electrode of a transistor, it is described as one of the other electrode.

(Embodiment 1)
In this embodiment mode, a pixel structure is described with reference to FIG.

 The pixel shown in FIG. 1 includes a signal line 10, a scanning line 11, a switching element 13, a driving transistor 14, a light emitting element 15, and a pass element 16. In this embodiment, the case where a p-type transistor is used as the driving transistor is described. However, the present invention is not limited to the polarity of the transistor.

 The connection relationship of such pixels will be described. The switching element 13 is connected to the signal line 10 and the scanning line 11. The switching element 13 is connected to the driving transistor 14 and further connected to the light emitting element 15 via the driving transistor 14. The pass element is provided at a position where an off current from the driving transistor can flow to the outside, and is connected to the driving transistor 14 and the light emitting element 15 (P1). Such a pass element 16 can be expressed as an element connected to a driving transistor.

 The present invention is characterized in that the pass element is connected to the drive transistor 14 and the light emitting element 15 so that an off-current from the drive transistor can flow to the outside. It is not limited to.

 The operation of such a pixel will be described. When the switching element 13 is selected by the scanning line 11, a video signal is input from the signal line 10 to the driving transistor 14 via the switching element 13. Based on the driving transistor 14, the light emitting element 15 is turned on or off.

 When such a pixel configuration of the present invention is used, the off-state current of the driving transistor 14 flows to the pass element 16 when the light emitting element 15 is not lit. This is because the resistance of the light emitting element 15 is higher than the resistance of the pass element 16. As a result, there is no possibility that the light emitting element is turned on by the off-state current of the driving transistor 14.

 However, when the light emitting element 15 is turned on, it is necessary to prevent a current from flowing through the pass element 16. This is because if the current flows to the pass element 16, the light emitting element cannot be turned on with a predetermined luminance.

 Therefore, the resistance value Rp of the pass element 16 is R (on) when the light emitting element is turned on, that is, when the light emitting element is turned on, and when the light emitting element is not turned on, that is, the light emitting element is not turned on. When the resistance at this time is R (off), R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on) is satisfied. In the case of R (off) >> Rp >> R (on), it is preferable that the off current of the driving transistor 14 can flow to the pass element 16. As a result, a current can be supplied to the pass element 16 only when the light emitting element is not lit.

 In order to satisfy the resistance value, for example, one end of the pass element that is not P1 is preferably connected to the counter electrode of the light emitting element. This is because the counter electrode of the light emitting element serves as a low potential power source, and current flows to the counter electrode.

 FIG. 2 shows a specific example of the pass element 16. As shown in FIG. 2A, a resistance element 19 can be used as the pass element 16.

 A semiconductor element can be used as the pass element 16. For example, as shown in FIG. 2B, a p-type thin film transistor (TFT) 20 can be used. The gate electrode of the thin film transistor only needs to be connected to a potential at which the thin film transistor is turned off. When a thin film transistor is used, the resistance value R (TFT) of the thin film transistor is R (TFT)> R (on), preferably R (TFT) >> R (on) when the light emitting element is turned on. That is, the thin film transistor is preferably off when the light emitting element is turned on. When the light-emitting element is not lit, the resistance value R (TFT) of the thin film transistor is R (off)> R (TFT), preferably R (off) >> R (TFT). That is, the thin film transistor is preferably off when the light-emitting element is not lit.

 As shown in FIG. 2C, a diode element 81 in which a gate electrode and a source electrode of a p-type thin film transistor are connected can be used. As shown in FIG. 2C, a diode element 82 in which a gate electrode and a drain electrode of an n-type thin film transistor are connected can be used.

 In such a resistive element 19, the thin film transistor 20, and the diode elements 81 and 82, one end that is not P1 may be connected to the counter electrode of the light emitting element. This is because the counter electrode is connected to a low potential power source.

 The operation of the thin film transistor 20 illustrated in FIG. 2B will be described. When the light emitting element is turned on, a current is supplied from the driving transistor 14. At this time, the thin film transistor 20 is turned off because the gate electrode is held at a high potential. That is, the resistance value of the thin film transistor 20 is set so as to satisfy Rp >> R (on), and no current flows through the thin film transistor 20 that is turned off.

 Next, the thin film transistor 20 when the light emitting element is not lit has one end which is not P1 connected to the counter electrode of the light emitting element. In this state, when an off current flows from the driving transistor 14, R (off)> Rp is satisfied, and the off current flows to the thin film transistor 20. As a result, the off current of the driving transistor 14 does not flow to the light emitting element.

 In this embodiment mode, the pixel structure is such that the counter electrode of the light-emitting element serves as a low-potential power source. Therefore, the pass element is described using a p-type thin film transistor. Such a pixel configuration may be used. In that case, an n-type thin film transistor may be used as the pass element. At this time, the driving transistor is preferably n-type.

 As described above, by providing the pass element 16, the off current of the driving transistor does not flow to the light emitting element when not lit, and the display device can perform accurate black display. In particular, when a reverse voltage is applied to the light-emitting element 15, when the light-emitting element 15 is not lit, an off-current of the driving transistor does not flow through the light-emitting element, and the display device can perform accurate black display. it can.

 With such a pass element, the structure of the driving transistor does not need to be a double gate structure or an LDD structure, so that a manufacturing process is not unnecessarily increased. However, the present invention is characterized in that a pass element is provided, and the structure of the driving transistor may be an LDD structure, a GOLD structure, or a double gate structure. By further combining the pass element of the present invention with a driving transistor having an LDD structure, a GOLD structure, or a double gate structure, a further reduction in off current can be expected.

 The pixel configuration in which such a pass element is provided is not limited to this embodiment mode. That is, a current input method in which a current is input as a video signal, a voltage input method in which a voltage is input as a video signal, a digital driving method in which a video signal is input as a digital value, and an analog driving in which a video signal is input as an analog value Method, a constant voltage driving method in which the driving transistor is operated in the linear region, a constant current driving method in which the driving transistor is operated in the saturation region, or a pixel configuration corresponding to the driving method. An element can be applied.

(Embodiment 2)
In this embodiment mode, a pixel structure including at least a switching transistor, a driving transistor, a light emitting element, and a pass element will be described.

 The pixel shown in FIG. 3 includes a signal line 10, a scanning line 11, a switching transistor 21, a driving transistor 14, a light emitting element 15, a pass element 16, a power supply line 12, and a capacitor element 22. The capacitive element 22 is connected between the gate electrode of the driving transistor 14 and one of the source electrode and the drain electrode. Other configurations are the same as those in FIG.

Note that the present invention is characterized in that the pass element is connected to the drive transistor 14 and the light emitting element 15 so that the off-current from the drive transistor can flow to the outside. It is not limited to.

 In this embodiment mode, an n-type transistor is used as a switching transistor and a p-type transistor is used as a driving transistor.

 When the switching transistor 21 is selected by the scanning line 11, a video signal is input from the signal line 10. The video signal may be a digital value or an analog value.

 In the case of a digital video signal, information of the video signal, that is, electric charge is accumulated in the capacitor 22. When this accumulated charge exceeds Vth of the driving transistor, the driving transistor 14 is turned on. Then, a current from the power supply line 12 is supplied to the light emitting element 15, and the light emitting element 15 is turned on with a predetermined luminance.

 Next, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when the driving transistor 14 has an off-current, the off-current is supplied to the light-emitting element 15. Therefore, a pass element 16 that satisfies R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on) is provided. By supplying an off current of the driving transistor 14 to the pass element 16, the off current can be prevented from being supplied to the light emitting element. In the case of R (off) >> Rp >> R (on), it is preferable that the off-state current of the driving transistor 14 can be supplied to the pass element 16. As a result, a clear black display can be performed. As the pass element, an element as shown in FIG. 2 can be used.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the light-emitting element 15 is not lit, when the present invention is applied, an off-current of a driving transistor does not flow through the light-emitting element, and the display device is accurate. It is preferable that a black display can be performed.

 As described above, when a digital video signal is input, multi-grayscale display cannot be performed as it is. However, multi-grayscale display can be performed using time grayscale display in which the lighting time of the light emitting element is controlled. it can.

 FIG. 17 shows a timing chart in which time gradation display is performed using the pixel configuration shown in FIG. The vertical axis indicates the scanning line from the first line to the last line, and the horizontal axis indicates time. As shown in FIG. 17, by providing three subframe periods (SF1 to SF3), 8-gradation display can be performed. In each subframe period, writing periods Ta1 to Ta3 are provided. In the writing period, in addition to a period in which a video signal is input from the signal line 10, periods (erase periods) Te1 to Te3 in which an erasing signal is input from the signal line 10 are provided. In this way, the lighting time of the light-emitting element can be controlled to perform 8-gradation display.

 As described above, the gradation is controlled by providing the erasing period Te in the writing period Ta by inputting the video signal or the erasing signal in one writing period, that is, one gate selection period. Therefore, it is not necessary to provide an erasing transistor, and the number of transistors in the pixel is not increased unnecessarily, so that a high aperture ratio can be achieved.

 Note that although this embodiment has been described with the case where three subframe periods are provided, the present invention is not limited to this. Two subframe periods and four or more subframe periods may be provided.

 Further, it is preferable to provide a period during which a reverse voltage is applied to the light-emitting element in addition to the writing period and the lighting period. This is because by applying a reverse voltage to the light emitting element, the state of the light emitting element can be improved and the life can be extended. FIG. 20 shows a timing chart when a reverse voltage is applied. Note that the period Tr for applying the reverse voltage need not be provided at the end of one frame period, and need not be provided for each frame. In the period in which the reverse voltage is applied, the potential of the power supply line 12 and the potential (Vca) of the cathode of the light emitting element are reversed. As a result, a reverse voltage can be applied to the light emitting element 15.

 Although the case where the subframe periods are provided in order has been described with reference to FIG. 17, the subframe periods may be provided randomly. As a result, pseudo contour can be prevented.

 In the present embodiment, the driving transistor 14 may be operated in either a linear region or a saturation region. Note that when the driving transistor 14 is operated in a linear region, it is not necessary to increase the driving voltage, so that power consumption can be reduced.

 Note that in this embodiment mode, the structure of the driving transistor may be an LDD structure, a GOLD structure, or a double gate structure. Further, by combining a pass element and a driving transistor having an LDD structure, a GOLD structure, or a double gate structure, a further reduction in off-current can be expected.

(Embodiment 3)
In this embodiment mode, a pixel configuration in which an erasing transistor is provided in addition to the pixel configuration in Embodiment Mode 2 will be described.

 4 includes a signal line 10, a scanning line 11, an erasing scanning line 23, a switching transistor 21, a driving transistor 14, an erasing transistor 24, a light emitting element 15, a pass element 16, a power supply line 12, and a capacitor. The element 22 is included. The erasing transistor 24 is connected to both ends of the capacitive element 22 so that the potential accumulated in the capacitive element 22 can be discharged. Other configurations are the same as those in FIG.

 Note that the present invention is characterized in that the pass element is connected to the drive transistor 14 and the light emitting element 15 so that the off-current from the drive transistor can flow to the outside. It is not limited to.

 In this embodiment mode, a case where an n-type transistor is used as a switching transistor and an erasing transistor and a p-type transistor is used as a driving transistor will be described.

 When the switching transistor 21 is selected by the scanning line 11, a video signal is input from the signal line 10. The video signal may be a digital value or an analog value. For example, in the case of a digital value video signal, information of the video signal, that is, electric charge is accumulated in the capacitor 22. When this accumulated charge exceeds Vgs of the driving transistor, the driving transistor 14 is turned on. Then, a current from the power supply line 12 is supplied to the light emitting element 15, and the light emitting element 15 is turned on with a predetermined luminance.

 Next, when the light emitting element 15 is not turned on, the erasing transistor is turned on from the erasing scanning line 23 and the charge accumulated in the capacitor 22 is discharged. Then, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, the light-emitting element is caused to flow by passing an off-current of the driving transistor 14 through the pass element 16 that satisfies R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). 15 so that the off-current is not supplied. In the case of R (off) >> Rp >> R (on), it is preferable that the off current of the driving transistor 14 can flow to the pass element 16. As a result, a clear black display can be performed. As the pass element, an element as shown in FIG. 2 can be used.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the light-emitting element 15 is not lit, when the present invention is applied, the off-current of the driving transistor does not flow through the light-emitting element, and the display device is accurate. A black display can be performed, which is preferable.

 Note that when a digital video signal is input as described above, multi-grayscale display cannot be performed as it is. Therefore, multi-grayscale display may be performed using time grayscale display in which the lighting time of the light emitting element is controlled. .

 FIG. 18 shows a timing chart in the case of performing time gray scale display using the pixel configuration shown in FIG. The vertical axis indicates the scanning line from the first line to the last line, and the horizontal axis indicates time. As shown in FIG. 18, by providing three subframe periods (SF1 to SF3), 8-gradation display can be performed. In each subframe period, writing periods Ta1 to Ta3 are provided. After the writing period, the lighting periods are Ts1 to Ts3, respectively. In this way, 8-gradation display can be performed. Further, in a short lighting period such as Ts3, in order to start the first writing period Ta1 of the next frame, the charge of the capacitive element 22 is discharged by the erasing transistor 24 and the light emitting element 15 is forcibly turned off. A period SE may be provided. As a result, the duty can be increased.

 Although the case where the subframe periods are provided in order has been described with reference to FIG. 18, the subframe periods may be provided randomly. As a result, pseudo contour can be prevented.

 Note that although this embodiment has been described with the case where three subframe periods are provided, the present invention is not limited to this. Two subframe periods and four or more subframe periods may be provided.

 Further, a period for applying a reverse voltage to the light-emitting element may be provided in addition to the writing period and the lighting period. By applying a reverse voltage to the light-emitting element, the state of the light-emitting element can be improved or the life can be extended. For example, similarly to the timing chart shown in FIG. 20, a period Tr for applying a reverse voltage can be provided at the end of one frame.

 In the present embodiment, the driving transistor 14 may be operated in either a linear region or a saturation region. Note that when the driving transistor 14 is operated in a linear region, it is not necessary to increase the driving voltage, so that power consumption can be reduced.

 Note that in this embodiment mode, the structure of the driving transistor and the erasing transistor may be an LDD structure, a GOLD structure, or a double gate structure. Further, by combining a pass element and a driving transistor having an LDD structure, a GOLD structure, or a double gate structure, a further reduction in off-current can be expected.

(Embodiment 4)
In this embodiment, a pixel structure in which a current control transistor is further provided in the pixel structures of Embodiments 2 and 3 will be described.

 5A includes a signal line 10, a scanning line 11, a fixed potential line 26, a switching transistor 21, a driving transistor 14, a current control transistor 25, a light emitting element 15, a pass element 16, and a power supply line. 12 and a capacitor 22. The current control transistor 25 is connected between the driving transistor 14 and the power supply line 12 so that the gate potential of the driving transistor can be fixed. Other configurations are the same as those in FIG.

 The present invention is characterized in that the pass element is connected to the drive transistor 14 and the light emitting element 15 so that an off-current from the drive transistor can flow to the outside. It is not limited to.

 In this embodiment mode, an n-type transistor is used as a switching transistor, and a p-type transistor is used as a driving transistor and a current control transistor.

 When the switching transistor 21 is selected by the scanning line 11, a video signal is input from the signal line 10. The video signal may be a digital value or an analog value. For example, in the case of a digital value video signal, information of the video signal, that is, electric charge is accumulated in the capacitor 22. When the accumulated charge exceeds Vgs of the current control transistor 25, the current control transistor 25 is turned on. At this time, the driving transistor 14 is turned on simultaneously with the current control transistor 25. Then, a current from the power supply line 12 is supplied to the light emitting element 15, and the light emitting element 15 is turned on with a predetermined luminance. Since the gate electrode of the driving transistor 14 is connected to the fixed potential line 26, the voltage Vgs between the gate and the source of the transistor is always constant. By setting the gate potential of the driving transistor to a fixed potential, it is possible to operate so that the gate-source voltage Vgs due to parasitic capacitance or wiring capacitance does not change. For this reason, luminance unevenness due to variation in characteristics of the driving transistor can be suppressed. Therefore, the cause of display unevenness is further reduced, and the image quality of the display device can be greatly improved.

 Next, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light emitting element. Therefore, the light-emitting element is caused to flow by passing an off-current of the driving transistor 14 through the pass element 16 that satisfies R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). It is possible to prevent the off-current from being supplied. As a result, a clear black display can be performed. As the pass element, an element as shown in FIG. 2 can be used.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the light-emitting element 15 is not lit, the off-state current of the driving transistor does not flow to the light-emitting element when the pixel illustrated in FIG. 5A is applied. The display device is preferable because it can perform accurate black display.

 Note that when a digital video signal is input as described above, gradation display cannot be performed as it is, and therefore gradation display may be performed using time gradation display in which the lighting time of the light emitting element is controlled.

 FIG. 6 shows an example of an equivalent circuit having the same function as the equivalent circuit shown in FIG. A case where a p-type thin film transistor 20 is used as a pass element and one electrode of the p-type thin film transistor is connected to a counter electrode of the light-emitting element will be described.

 In the equivalent circuit shown in FIG. 6A, a p-type thin film transistor 20 is connected to the light emitting element 15, and a gate electrode of the thin film transistor is connected to the power supply line 12. The other structures are the same as those in FIG. In such a pixel circuit, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, light emission is caused by flowing the off-current through a p-type thin film transistor 20 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). The off current is not supplied to the element. In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor 14 can be supplied to the p-type thin film transistor 20. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the pixel illustrated in FIG. 6A is applied when the light-emitting element 15 is not lit, an off-state current of the driving transistor flows in the light-emitting element. The display device is preferable because it can perform accurate black display.

 The equivalent circuit diagram shown in FIG. 6B is different from FIG. 6A in that the gate electrode of the driving transistor 14 is connected to the control scanning line 30 formed of the same layer as the scanning line 11, that is, the same material. Has been. Therefore, the number of power supply lines can be reduced. The other structures are the same as those in FIG. 6A, so that FIG. 5A can be referred to. In such a pixel circuit, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, light emission is caused by flowing the off-current through a p-type thin film transistor 20 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). The off-state current of the driving transistor is prevented from being supplied to the element. In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor can be supplied to the p-type thin film transistor 20. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the pixel illustrated in FIG. 6B is applied when the light-emitting element 15 is not lit, an off-state current of the driving transistor flows in the light-emitting element. The display device is preferable because it can perform accurate black display.

 The equivalent circuit diagram shown in FIG. 6C is different from FIG. 6A in that the gate electrode of the current control transistor 25 is connected to the gate electrode of the driving transistor 14. Therefore, the number of control scanning lines and power supply lines can be reduced. The other structures are the same as those in FIG. 6A, so that FIG. 5A can be referred to. In such a pixel circuit, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, light emission is caused by flowing the off-current through a p-type thin film transistor 20 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). The off current of the driving transistor 14 is not supplied to the element. When R (off) >> Rp >> R (on), the off-current of the driving transistor 14 can be supplied to the p-type thin film transistor 20. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the pixel illustrated in FIG. 6C is applied when the light-emitting element 15 is not lit, an off-state current of the driving transistor flows through the light-emitting element. The display device is preferable because it can perform accurate black display.

 In FIG. 6, a p-type thin film transistor is used as the pass element, but the element shown in FIG. 2 may be used.

 Next, the pixel illustrated in FIG. 5B is described. The pixel includes a signal line 10, a scanning line 11, an erasing scanning line 23, a fixed potential line 26, a switching transistor 21, a driving transistor 14, an erasing transistor 24, a current control transistor 25, a light emitting element 15, and a pass element. The element 16, the power supply line 12, and the capacitor 22 are included. Note that in this embodiment mode, an n-type transistor is used as a switching transistor and an erasing transistor, and a p-type transistor is used as a driving transistor and a current control transistor.

 When the switching transistor 21 is selected by the scanning line 11, a video signal is input from the signal line 10. The video signal may be a digital value or an analog value. For example, in the case of a digital value video signal, information of the video signal, that is, electric charge is accumulated in the capacitor 22. When the accumulated charge exceeds Vgs of the current control transistor 25, the current control transistor 25 is turned on. At this time, the driving transistor 14 is turned on simultaneously with the current control transistor 25. Then, a current from the power supply line 12 is supplied to the light emitting element 15, and the light emitting element 15 is turned on with a predetermined luminance. Since the gate electrode of the driving transistor 14 is connected to the fixed potential line 26, the voltage Vgs between the gate and the source of the transistor is always constant. By setting the gate potential of the driving transistor to a fixed potential, it is possible to operate so that the gate-source voltage Vgs due to parasitic capacitance or wiring capacitance does not change. For this reason, luminance unevenness due to variation in characteristics of the driving transistor can be suppressed. Therefore, the cause of display unevenness is further reduced, and the image quality of the display device can be greatly improved.

 Next, when the light emitting element 15 is not turned on, the erasing transistor is turned on from the erasing scanning line 23 and the charge accumulated in the capacitor 22 is discharged. Then, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, the off current of the driving transistor is supplied to the pass element 16 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on), and light emission occurs. The off-state current is not supplied to the element. In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor can be supplied to the pass element 16. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the pixel illustrated in FIG. 5B is applied when the light-emitting element 15 is not lit, an off-state current of a driving transistor flows through the light-emitting element. The display device is preferable because it can perform accurate black display.

 Note that when a digital video signal is input as described above, multi-grayscale display cannot be performed as it is. Therefore, multi-grayscale display may be performed using time grayscale display in which the lighting time of the light emitting element is controlled. .

 FIG. 7 shows an equivalent circuit example having the same function as the equivalent circuit shown in FIG. A case where a p-type thin film transistor 20 is used as a pass element and one electrode of the p-type thin film transistor is connected to a counter electrode of the light-emitting element will be described.

 In the equivalent circuit shown in FIG. 7A, a p-type thin film transistor 20 is connected to the light emitting element 15, and a gate electrode of the thin film transistor is connected to the power supply line 12. The other structures are the same as those in FIG. In such a pixel circuit, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, light emission is caused by flowing the off-current through a p-type thin film transistor 20 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). The off-state current is not supplied to the element. In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor can be supplied to the p-type thin film transistor 20. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the light-emitting element 15 is not lit, when the pixel illustrated in FIG. 7A is applied, an off-state current of the driving transistor flows through the light-emitting element. This is preferable because the display device can perform accurate black display.

 The equivalent circuit diagram shown in FIG. 7B is different from FIG. 7A in that the gate electrode of the driving transistor 14 is connected to the control scanning line 30 formed of the same layer as the scanning line 11, that is, the same material. Has been. Therefore, the number of power supply lines can be reduced. The other structures are the same as those in FIG. 7A, so that FIG. 5B can be referred to. In such a pixel circuit, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, light emission is caused by flowing the off-current through a p-type thin film transistor 20 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). The off-state current of the driving transistor is prevented from being supplied to the element. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the light-emitting element 15 is not turned on, the off-state current of the driving transistor flows through the light-emitting element when the pixel illustrated in FIG. 7B is applied. This is preferable because the display device can perform accurate black display.

 The equivalent circuit diagram shown in FIG. 7C is different from FIG. 7A in that the gate electrode of the current control transistor 25 is connected to the gate electrode of the driving transistor 14. Therefore, the number of control scanning lines and power supply lines can be reduced. The other structures are the same as those in FIG. 7A, so that FIG. 5B can be referred to. In such a pixel circuit, when the light emitting element 15 is not turned on, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current is supplied to the light-emitting element. Therefore, light emission is caused by flowing the off-current through a p-type thin film transistor 20 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). The off-state current is not supplied to the element. In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor can be supplied to the p-type thin film transistor 20. As a result, a clear black display can be performed.

 In particular, when a reverse voltage is applied to the light-emitting element 15, when the pixel illustrated in FIG. 7C is applied when the light-emitting element 15 is not lit, an off-state current of the driving transistor flows through the light-emitting element. The display device is preferable because it can perform accurate black display.

 In FIG. 7, a p-type thin film transistor is used as the pass element, but it goes without saying that other elements as shown in FIG. 2 may be used.

 In the present embodiment, the driving transistor 14 may be operated in either a linear region or a saturation region. Note that when the driving transistor 14 is operated in a linear region, it is not necessary to increase the driving voltage, so that power consumption can be reduced.

 Note that in this embodiment mode, the structure of the driving transistor, the erasing transistor, and the current control transistor may be an LDD structure, a GOLD structure, or a double gate structure. Further, by combining a pass element and a driving transistor having an LDD structure, a GOLD structure, or a double gate structure, a further reduction in off-current can be expected.

(Embodiment 5)
In this embodiment mode, one mode of a display device including a pixel of the present invention will be described.

 As illustrated in FIG. 10, the display device including the pixel described in the above embodiment includes a pixel region 201 in which a plurality of pixels having the above-described structure are arranged in a matrix, a first gate driver 41, A second gate driver 42 and a source driver 43 are included. The first gate driver 41 and the second gate driver 42 are disposed so as to face each other with the pixel region 201 interposed therebetween, or are disposed in one of the upper, lower, left, and right sides of the pixel region 201.

 The source driver 43 includes a pulse output circuit 44, a latch 45, and a selection circuit 46. The latch 45 has a first latch 47 and a second latch 48. The selection circuit 46 includes a transistor 49 (hereinafter referred to as TFT 49) and an analog switch 50. The TFT 49 and the analog switch 50 are provided in each column corresponding to the signal lines (S1... Sm). The inverter 51 is for generating an inverted signal of a WE (Write Erase) signal, and may not be provided when an inverted signal of the WE signal is supplied from the outside. The gate electrode of the TFT 49 is connected to the selection signal line 52, one of the source electrode and the drain electrode is connected to the signal line (S 1... Sm), and the other is connected to the power source 53. The analog switch 50 is provided between the second latch 48 and the signal lines (S1... Sm). That is, the input node of the analog switch 50 is connected to the second latch 48, and the output node is connected to the signal line Sm. One of the two control nodes of the analog switch 50 is connected to the selection signal line 52, and the other is connected to the selection signal line 52 via the inverter 51. The potential of the power source 53 is a potential for turning off the driving transistor 14 included in the pixel. When the driving transistor 14 is n-type, the potential of the power source 53 is set to L (Low) level, and the driving transistor 14 is P-type. In this case, the potential of the power supply 53 is set to H (High) level.

 The first gate driver 41 has a pulse output circuit 54 and a selection circuit 55. The second gate driver 42 has a pulse output circuit 56 and a selection circuit 57. The selection circuits 55 and 57 are connected to the selection signal line 52. However, the selection circuit 57 included in the second gate driver 42 is connected to the selection signal line 52 via the inverter 58. That is, the WE signals input to the selection circuits 55 and 57 via the selection signal line 52 are in an inverted relationship with each other.

 Each of the selection circuits 55 and 57 has a tristate buffer. The input node of the tristate buffer is connected to the pulse output circuit 54 or the pulse output circuit 56, and the control node of the tristate buffer is connected to the selection signal line 52. The output nodes of the tristate buffer are connected to the scanning lines (G1... Gn), respectively. The tri-state buffer is in an operating state when a signal transmitted from the selection signal line 52 is at an H level, and is in an indefinite state when the signal is at an L level.

 The pulse output circuit 44 included in the source driver 43, the pulse output circuit 54 included in the first gate driver 41, and the pulse output circuit 56 included in the second gate driver 42 include a shift register or a decoder circuit including a plurality of flip-flop circuits. Have. If a decoder circuit is applied as the pulse output circuits 44, 54, and 56, the signal lines (S1... Sm) or the scanning lines (G1... Gn) can be selected at random. When the signal lines (S1... Sm) or the scanning lines (G1... Gn) are selected at random, it is possible to suppress the generation of a pseudo contour that occurs when the time gray scale method is applied.

 The configuration of the source driver 43 is not limited to the above description, and a level shifter and a buffer may be provided. The configurations of the first gate driver 41 and the second gate driver 42 are not limited to the above description, and a level shifter or a buffer may be provided. In addition, the source driver 43, the first gate driver 41, and the second gate driver 42 may include a protection circuit. The protection circuit can reduce electrostatic breakdown and the like.

 The display device of the present invention may include a power supply control circuit. The power supply control circuit includes a power supply circuit that supplies power to the light emitting element 15 and a controller. The power supply circuit is connected to the pixel electrode of the light emitting element 15 through the driving transistor 14 and the power supply line (V1... Vm). The power supply circuit is connected to the counter electrode of the light emitting element 15 through the power supply line.

 When such a power supply control circuit applies a forward bias voltage (forward voltage) to the light emitting element 15 to cause the light emitting element 15 to turn on by lighting the current, the potential of the first power supply 17 is opposite The potential difference between the first power supply 17 and the counter electrode 18 is set so as to be higher than the potential of the electrode 18. On the other hand, when a reverse voltage is applied to the light emitting element 15, the potentials of the first power source 17 and the counter electrode 18 are set so that the potential of the first power source 17 is lower than the potential of the counter electrode 18. . Such a power supply setting can be performed by supplying a predetermined signal from the controller to the power supply circuit.

 As a result of applying the reverse voltage in this manner, it is possible to suppress deterioration with time of the light emitting element 15 and improve reliability. In addition, the light emitting element 15 can reduce initial defects in which the anode and the cathode are short-circuited due to adhesion of foreign matters, pinholes due to fine protrusions on the anode or the cathode, and non-uniformity of the electroluminescent layer.

 The display device may include a monitor circuit and a control circuit. The monitor circuit operates based on the ambient temperature (hereinafter referred to as environmental temperature). The control circuit has a constant current source and a buffer. The monitor circuit has a monitor light emitting element. The monitor circuit and the control circuit are collectively referred to as a temperature compensation function circuit.

 Such a control circuit can supply a signal for changing the power supply potential to the power supply control circuit based on the output of the monitor circuit. The power supply control circuit changes the power supply potential supplied to the pixel region 201 based on the signal supplied from the control circuit. As a result, it is possible to improve the reliability by suppressing the fluctuation of the current value caused by the change in the environmental temperature. Such a control circuit can be provided between a pixel region having a light-emitting element and a driver that generates a signal to be supplied to a transistor that drives the light-emitting element.

(Embodiment 6)
In this embodiment mode, signals for operating pixels are described. Note that the operation described in this embodiment mode includes a period for writing a video signal and a period for writing an erasing signal in one gate selection period as in the timing chart shown in FIG. Such an operation method can also be applied to a display device having any of the pixel structures described in the above embodiment modes. In addition, when such an operation method is used, in a pixel configuration having an erasing transistor, the erasing transistor can be eliminated, and a high aperture ratio can be achieved. Note that FIG. 10 can be referred to for the form of the display device.

 The operation of the display device will be described with reference to FIG. First, the operation of the source driver will be described with reference to FIG. The pulse output circuit 44 receives a clock signal (hereinafter referred to as SCK), a clock inverted signal (hereinafter referred to as SCKB), and a start pulse (hereinafter referred to as SSP), and according to the timing of these signals. The sampling pulse is output to the first latch 47. The first latch 47 to which data is input holds the video signal from the first column to the last column in accordance with the timing at which the sampling pulse is input. When a latch pulse is input, the second latch 48 transfers the video signals held in the first latch 47 to the second latch 48 all at once.

 Here, the operation of the selection circuit 46 in each period will be described with the period T1 when the WE signal transmitted from the selection signal line 52 is L level and the period T2 when the WE signal is H level. The periods T1 and T2 correspond to half the horizontal scanning period, and the period T1 can be referred to as a first sub-gate selection period, and the period T2 can be referred to as a second sub-gate selection period.

 In the period T1 (first sub-gate selection period), the WE signal transmitted from the selection signal line 52 is at the L level, the TFT 49 is turned on, and the analog switch 50 is turned off. Then, the plurality of signal lines (S1... Sm) are electrically connected to the power source 53 via the TFTs 49 arranged in each column. That is, the plurality of signal lines (S1... Sm) have the same potential as the power supply 53. At this time, the switching element 13 included in the pixel is turned on, and the potential of the power supply 53 is transmitted to the gate electrode of the driving transistor 14 via the switching element 13. Then, the driving transistor 14 is turned off, and the two electrodes included in the light emitting element 15 have the same potential. That is, no current flows between the two electrodes included in the light emitting element 15 and the lighting is not performed. In this manner, regardless of the state of the video signal, the potential of the power supply 53 is transmitted to the gate electrode of the driving transistor 14, the switching element 13 is turned off, and the potentials of the two electrodes of the light emitting element 15 are the same. An operation that becomes a potential is called an erase operation.

 In the period T2 (second sub-gate selection period), the WE signal transmitted from the selection signal line 52 is at the H level, the TFT 49 is turned off, and the analog switch 50 is turned on. Then, the video signal held in the second latch 48 is simultaneously transmitted to a plurality of signal lines (S1... Sm) for one row. At this time, the switching element 13 included in the pixel is turned on, and the video signal is transmitted to the gate electrode of the driving transistor 14 via the switching element 13. Then, in accordance with the input video signal, the driving transistor 14 is turned on or off, and the two electrodes included in the light emitting element 15 have different potentials or the same potential. More specifically, when the driving transistor 14 is turned on, two electrodes of the light emitting element 15 have different potentials, and a current flows through the light emitting element 15. Then, the light emitting element 15 is turned on. Note that the current flowing through the light emitting element 15 is the same as the current flowing between the source and drain of the driving transistor 14. On the other hand, when the driving transistor 14 is turned off, the two electrodes included in the light emitting element 15 have the same potential, and no current flows through the light emitting element 15. That is, the light emitting element 15 is not lit. In this manner, an operation in which the driving transistor 14 is turned on or off in accordance with the video signal and the potentials of the two electrodes of the light-emitting element 15 are different from each other or the same potential is referred to as a writing operation.

 Next, operations of the first gate driver 41 and the second gate driver 42 will be described with reference to FIG. G1CK (first gate driver clock signal), G1CKB (first gate driver clock inverted signal), and G1SP (first gate driver start pulse signal) are input to the pulse output circuit 54. Pulses are sequentially output to the selection circuit 55 in accordance with the signal timing. G2CK (second gate driver clock signal), G2CKB (second gate driver clock inverted signal), and G2SP (second gate driver start pulse signal) are input to the pulse output circuit 56. Pulses are sequentially output to the selection circuit 57 in accordance with the signal timing. FIG. 11B shows each column of the i-th row, j-th row, k-th row, and p-th row (i, j, k, p are natural numbers, 1 ≦ i, j, k, p ≦ n). The potential of the pulse supplied to the selection circuits 55 and 57 is shown.

 Here, similarly to the description of the operation of the source driver 43, the period T1 is when the WE signal transmitted from the selection signal line 52 is L level, and the period T2 is when the WE signal is H level. The operation of the selection circuit 55 included in the first gate driver 41 and the selection circuit 57 included in the second gate driver 42 will be described. Note that in the timing chart of FIG. 11B, the potential of the scanning line (G1... Gn) to which a signal is transmitted from the first gate driver 41 is denoted as Gn41, and a signal is transmitted from the second gate driver 42. The transmitted potential of the scanning lines (G1... Gn) is denoted as Gn42. Gn41 and Gn42 indicate the same wiring.

 In the period T1 (first sub-gate selection period), the WE signal transmitted from the selection signal line 52 is at the L level. Then, the L level WE signal is input to the selection circuit 55 included in the first gate driver 41, and the selection circuit 55 becomes indefinite. On the other hand, an H level signal obtained by inverting the WE signal is input to the selection circuit 57 included in the second gate driver 42, and the selection circuit 57 enters an operating state. That is, the selection circuit 57 transmits an H level signal (row selection signal) to the i-th scanning line Gi, and the scanning line Gi has the same potential as the H-level signal. That is, the second gate driver 42 selects the i-th scanning line Gi. As a result, the switching element 13 included in the pixel is turned on. Then, the potential of the power supply 53 included in the source driver 43 is transmitted to the gate electrode of the driving transistor 14, the driving transistor 14 is turned off, and the potentials of both electrodes of the light emitting element 15 become the same potential. That is, during this period, an erasing operation in which the light emitting element 15 is not lit is performed.

 In the period T2 (second sub-gate selection period), the WE signal transmitted from the selection signal line 52 is at the H level. Then, an H level WE signal is input to the selection circuit 55 included in the first gate driver 41, and the selection circuit 55 enters an operating state. That is, the selection circuit 55 transmits an H level signal to the i-th scanning line Gi, and the scanning line Gi has the same potential as the H level signal. Then, the first gate driver 41 selects the i-th scanning line Gi. As a result, the switching element 13 included in the pixel is turned on. Then, a video signal is transmitted from the second latch 48 included in the source driver 43 to the gate electrode of the driving transistor 14, the driving transistor 14 is turned on or off, and the potentials of the two electrodes included in the light emitting element 15 are Different potentials or the same potential. That is, in this period, the writing operation in which the light emitting element 15 is turned on or off is performed. On the other hand, an L level signal is input to the selection circuit 57 included in the second gate driver 42, and an indefinite state is set.

 Thus, the scanning line Gn is selected by the second gate driver 42 in the period T1 (first subgate selection period) and selected by the first gate driver 41 in the period T2 (second subgate selection period). The In other words, the scanning line is complementarily controlled by the first gate driver 41 and the second gate driver 42. In the first and second sub-gate selection periods, the erase operation is performed on one side and the write operation is performed on the other side.

 Note that in a period in which the first gate driver 41 selects the i-th scanning line Gi, the second gate driver 42 is not operating (the selection circuit 57 is in an indefinite state), or other than the i-th row. A row selection signal is transmitted to the scanning line of the row. Similarly, during a period in which the second gate driver 42 transmits a row selection signal to the i-th scanning line Gi, the first gate driver 41 is in an indefinite state, or the scanning lines of other rows excluding the i-th row A row selection signal is transmitted.

 Further, according to the present invention that performs the operation as described above, the light emitting element 15 can be forcibly turned off, so that the duty ratio can be improved even when the number of gradations increases. In addition, although the light emitting element 15 can be forcibly turned off, it is not necessary to provide a TFT for discharging the charge of the capacitor, so that a high aperture ratio is realized. When a high aperture ratio is realized, the luminance of the light-emitting element can be lowered with an increase in the area that emits light. That is, since the driving voltage can be lowered, power consumption can be reduced.

 Note that the present invention is not limited to the above-described form in which the gate selection period is divided into two. The gate selection period may be divided into three or more. This embodiment mode can be freely combined with the above embodiment modes.

 Note that an erasing signal is input to the pixel in the first half of the gate selection period (first sub-gate selection period), and an image (video) signal is input to the pixel in the second half of the gate selection period (second sub-gate selection period). However, it is not limited to this. A video signal may be input to the pixel in the first half of the gate selection period (first sub-gate selection period), and an erasure signal may be input to the pixel in the second half of the gate selection period (second sub-gate selection period).

  Alternatively, a video signal is input to the pixel also in the first half of the gate selection period (first sub-gate selection period), and a video signal is input to the pixel also in the second half of the gate selection period (second sub-gate selection period). Also good. A signal corresponding to a different subframe may be input to each. As a result, the subframe period can be provided so that the lighting period is continuously arranged without providing the erasing period. In this case, since it is not necessary to provide an erasing period, the duty ratio can be increased.

(Embodiment 7)
In this embodiment, a pixel structure which is different from that in the above embodiment is described.

 As shown in FIG. 8, the pixel includes a light emitting element 15, a switching transistor 103, a holding transistor 104, a driving transistor 14, a conversion transistor 102, a pass element 16, and a capacitor element 112. The gate electrode of the switching transistor 103 is connected to the first scanning line 107, one of the source electrode and the drain electrode is connected to the signal line 10, and the other is connected to the gate electrode of the conversion transistor 102. One of the source electrode and the drain electrode of the conversion transistor 102 is connected to the power supply line 110, and the other is connected to the gate electrode of the conversion transistor 102. The gate electrode of the holding transistor 104 is connected to the second scanning line 108, one of the source electrode and the drain electrode is connected to the gate electrode of the conversion transistor 102, and the other is connected to the gate electrode of the driving transistor 14. One of the source electrode and the drain electrode of the driving transistor 14 is connected to the power supply line 110, and the other is connected to the pixel electrode of the light emitting element 15. The counter electrode of the light emitting element 15 is connected to the second power source 114. The capacitive element 112 is connected between the gate electrode of the driving transistor 14 and the power supply line 110, and the pass element 16 is connected to the pixel electrode of the light emitting element 15. A current source 106 controlled in accordance with luminance information is connected to the signal line 10, and a first power supply 111 is connected to the power line 110.

 The conductivity types of the switching transistor 103 and the holding transistor 104 are not limited, and may be either N-type or P-type. Further, the conductivity types of the driving transistor 14 and the conversion transistor 102 are not limited, but both must be the same conductivity type. In the light emitting element 15, when a current flows from the pixel electrode to the counter electrode to emit light, the driving transistor 14 and the conversion transistor 102 are preferably P-type as shown in FIG. In addition, in the case where light is emitted in a direction in which current flows from the counter electrode to the pixel electrode, it is desirable that the driving transistor 14 and the conversion transistor 102 be N-type.

 The pass element 16 has a function of turning off when the light emitting element 15 is turned on and allowing an off current of the driving transistor to flow when the light emitting element 15 is not turned on. As the pass element 16, a p-type thin film transistor 20 as shown in FIG. 2 and other elements can be used.

 The pass element is set so that when the off-current is generated in the driving transistor 14 during the non-lighting period, the off-current flows to the pass element. Specifically, the pass element 16 is provided so as to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor can be supplied to the pass element 16. By passing an off current of the driving transistor through the pass element 16, the off current is not supplied to the light emitting element, so that a clear black display can be performed.

  The operation of the pixel configuration shown in FIG. 8 will be described. As shown in FIGS. 9A to 9C, the operation of the pixel can be divided into a programming period, a lighting period, and a non-lighting period.

 First, in the programming period shown in FIG. 9A, an H level signal is input to the first scan line 107 and the second scan line 108, the switching transistor 103 and the holding transistor 104 are turned on, and the current source By connecting 106 and the conversion transistor 102, a signal current Idata corresponding to luminance information flows between the source and drain of the conversion transistor 102. At this time, since the gate electrode and the drain electrode of the conversion transistor 102 are connected to each other, the conversion transistor 102 operates in a saturation region and is necessary for the signal current Idata to flow between the source and the drain of the conversion transistor 102. A large gate-source voltage is stored in the capacitive element 112. After that, an L level signal is input to the first scanning line 107 and the second scanning line 108, the switching transistor 103 and the holding transistor 104 are turned off, the programming period ends, and the lighting period starts. At this time, it is preferable that an L-level signal is output to the second scanning line 108 before the first scanning line 107 and the holding transistor 104 is turned off before the switching transistor 103.

  In the lighting period shown in FIG. 9B, the current Idriv is supplied to the light emitting element 15 by the driving transistor 14 in accordance with the potential difference stored in the capacitor 112 in the programming period. However, it is necessary to control the second power supply 114 so that the driving transistor 14 operates in the saturation region. At this time, the current value Idriv supplied to the light emitting element 15 is equal to the signal current Idata, the driving transistor 14 and the conversion transistor 102 if the mobility and threshold of the driving transistor 14 and the conversion transistor 102 are the same. The current supplied to the light-emitting element 15 is determined by the ratio between the channel width and the channel length, where the channel length of the driving transistor 14 is L1, the channel width is W1, the channel length of the conversion transistor 102 is L2, and the channel width is W2. The value Idriv is expressed by the formula (1).

  Idriv = (W1 / L1) / (W2 / L2) × Idata (1)

  As described above, even when there is a variation in transistor characteristics between pixels, if there is no variation in mobility and threshold value between adjacent transistors (the driving transistor 14 and the conversion transistor 102), the light emission of each pixel 404 is performed. Since the current supplied to the element depends only on the signal current Idata supplied from the current source 106, high-quality display with no variation in light emission luminance is possible.

  Next, in the non-lighting period illustrated in FIG. 9C, the driving transistor 14 is turned off. At this time, when an off-current is generated in the driving transistor 14, the off-current flows to the light-emitting element. Therefore, the off-current of the driving transistor 14 is caused to flow through the pass element 16 provided to satisfy R (off)> Rp >> R (on), more preferably R (off) >> Rp >> R (on). Thus, the off-state current is not supplied to the light-emitting element. In the case of R (off) >> Rp >> R (on), the off-state current of the driving transistor can be supplied to the pass element 16. As a result, a clear black display can be performed.

 Note that in the pixel circuit illustrated in FIG. 8, a configuration in which a reverse voltage is applied to the light emitting element 15 may be provided. Usually, even if a reverse voltage is applied to the light emitting element 15, no current flows. However, if there is a short part in the light emitting element 15, the current concentrates in the short part. The reliability can be improved. By applying such a reverse voltage, not only the initial short-circuited portion but also the progressive short-circuited portion can be burned out, the deterioration of the light emitting element 15 can be reduced, and the reliability can be improved.

 In the pixel structure described in this embodiment mode, the off-state current of the driving transistor is not supplied to the light-emitting element in the non-lighting period by the pass element, so that clear black display can be performed. In addition, the pixel structure described in this embodiment can provide a display device with high image quality regardless of transistor variations and high reliability.

(Embodiment 8)
In this embodiment, a cross-sectional structure of the pixel described in the above embodiment will be described.

 FIG. 12 is a cross-sectional view of the switching element 13, the driving transistor 14, and the light emitting element 15. On the base insulating film 61 provided on the insulating substrate 60, a thin film transistor is provided as the switching element 13 and a thin film transistor is provided as the driving transistor 14. In the present embodiment, the conductivity type of the thin film transistor 13 is p-type, and the conductivity type of the thin film transistor 14 is n-type.

  Examples of the insulating substrate 60 include glass substrates such as barium borosilicate glass and alumino borosilicate glass, and quartz substrates. Other substrates having an insulating surface include plastics typified by polyethylene-terephthalate (PET), polyethylene naphthalate (PEN), and polyethersulfone (PES), and flexible synthetic resins such as acrylic. There is a substrate that becomes.

  The thin film transistors 13 and 14 each include a semiconductor film serving as an active layer, a gate insulating film 62 provided over the semiconductor film, and a gate electrode.

 The semiconductor film is an amorphous semiconductor, semi-amorphous silicon (SAS) in which an amorphous state and a crystalline state are mixed, and a microcrystalline semiconductor in which crystal grains of 0.5 nm to 20 nm can be observed in the amorphous semiconductor. And any state selected from crystalline semiconductors.

 In this embodiment, an amorphous semiconductor film is formed and a crystalline semiconductor film crystallized by heat treatment is used. The heat treatment can be a heating furnace, laser irradiation, irradiation of light emitted from a lamp instead of laser light (hereinafter referred to as lamp annealing), or a combination thereof.

In the case of using laser irradiation, a continuous wave laser beam (CW laser beam) or a pulsed laser beam (pulse laser beam) can be used. As the laser beam, Ar laser, Kr laser, excimer laser, YAG laser, Y ₂ O ₃ laser, YVO ₄ laser, YLF laser, YAlO ₃ laser, glass laser, ruby laser, alexandrite laser, Ti: sapphire laser, copper vapor A laser or a gold vapor laser oscillated from one or a plurality of types can be used. By irradiating the fundamental wave of such a laser beam and the second, third, or fourth harmonic of the fundamental wave, a crystal with a large grain size can be obtained. For example, a second harmonic (562 nm) or a third harmonic (655 nm) of an Nd: YVO ₄ laser (fundamental wave 1064 nm) can be used. Energy density of the laser is about 0.01 to 100 MW / cm ² (preferably 0.1 to 10 MW / cm ²⁾ is required. Then, irradiation is performed at a scanning speed of about 10 to 2000 cm / sec.

 The continuous wave fundamental laser beam and the continuous wave harmonic laser beam may be irradiated, or the continuous wave fundamental laser beam and the pulsed harmonic laser beam may be irradiated. You may do it. By irradiating a plurality of laser beams, energy can be supplemented.

 It is also possible to use a pulse oscillation type laser beam that oscillates the laser at an oscillation frequency that allows irradiation of the next pulse of laser light after the semiconductor film is melted by the laser light and solidifies. it can. By oscillating the laser beam at such a frequency, crystal grains continuously grown in the scanning direction can be obtained. A specific oscillation frequency of the laser beam is 10 MHz or more, and a frequency band that is significantly higher than a frequency band of several tens to several hundreds Hz that is normally used is used.

 Further, the laser beam may be irradiated in an inert gas atmosphere such as a rare gas or nitrogen. Thereby, roughness of the semiconductor surface due to laser beam irradiation can be suppressed, flatness can be increased, and variations in threshold values caused by variations in interface state density can be suppressed.

Alternatively, a microcrystalline semiconductor film may be formed using SiH ₄ and F ₂ , or SiH ₄ and H ₂ , and then crystallized by performing laser irradiation as described above.

 As another heat treatment, when a heating furnace is used, the amorphous semiconductor film is heated at 500 to 550 ° C. for 2 to 20 hours. At this time, the temperature may be set in multiple stages in the range of 500 to 550 ° C. so that the temperature gradually increases. In the first low-temperature heating step, hydrogen or the like of the amorphous semiconductor film comes out, so that it is possible to perform so-called hydrogen extraction that reduces film roughness during crystallization. Furthermore, it is preferable to form a metal element that promotes crystallization, such as nickel (Ni), over the amorphous semiconductor film because the heating temperature can be reduced. Even in the case of crystallization using such a metal element, heat treatment at 600 to 950 ° C. may be performed.

  However, when a metal element is formed, there is a concern that the electrical characteristics of the semiconductor element may be adversely affected. Therefore, it is necessary to perform a gettering step for reducing or removing the metal element. For example, as a gettering step, a step of capturing a metal element using an amorphous semiconductor film as a gettering sink may be performed.

Alternatively, a crystalline semiconductor film may be formed directly over the base insulating film. In this case, a crystalline semiconductor film is directly formed using heat or plasma using a fluorine-based gas such as GeF ₄ or F ₂ and a silane-based gas such as SiH ₄ or Si ₂ H _6. Can do.

 In such a method for manufacturing a semiconductor film, when high temperature treatment is required, a quartz substrate with high heat resistance is preferably used.

 A gate insulating film and a gate electrode are sequentially formed on the semiconductor film thus formed. As the gate insulating film, an oxide film containing silicon or a nitride film containing silicon can be used.

 Thereafter, an impurity element is added in a self-aligning manner using the gate electrode as a mask. Then, a channel formation region is formed below the source and drain regions to which the impurity element is added and the gate electrode. At this time, a low concentration impurity region (LDD region) can be formed by tapering the end face of the gate electrode. A structure having a low concentration impurity region is an LDD structure. The LDD structure has a feature that resistance to hot carrier deterioration can be increased and off-leakage current can be reduced. A structure having a region overlapping with the gate electrode in the low concentration impurity region is a GOLD structure. The GOLD structure has a high current driving force and a very excellent feature against hot carrier deterioration resistance. For example, an LDD structure or a GOLD structure can be formed by using a stacked structure of gate electrodes and making the tapered shape of the first gate electrode different from the tapered shape of the second gate electrode. By forming a driving transistor using such an LDD structure or a GOLD structure and further providing a pass element, a further reduction in off-current can be expected.

 The gate electrode is formed by combining the first conductive film with tantalum nitride (TaN), the second conductive film with tungsten (W), the first conductive film with tantalum nitride (TaN), A combination of two conductive films made of titanium (Ti), a first conductive film made of tantalum nitride (TaN), a second conductive film made of aluminum (Al), and the first conductive film made of tantalum nitride. (TaN) and the second conductive film is preferably formed of a combination of copper (Cu). As the first conductive film and the second conductive film, a semiconductor film typified by a polycrystalline silicon film doped with an impurity element such as phosphorus (P), or an AgPdCu alloy may be used. In order to prevent the short channel effect that occurs as the channel formation region is miniaturized, an insulator is formed on the side surface of the gate electrode, and a low-concentration impurity region is formed below the insulator. It is preferable.

 After that, the gate insulating film is opened, wirings connected to the source region and the drain region (respectively referred to as source wiring and drain wiring) are formed, and the thin film transistor can be completed.

 Further, in this embodiment mode, a passivation film 63 is formed so as to cover the gate electrode and the semiconductor film. The passivation film 63 can prevent the gate electrode surface from being oxidized. In addition, the defects (dangling bonds) of the semiconductor film can be terminated by hydrogen contained in the passivation film. As the passivation film 63, an oxide film containing silicon or a nitride film containing silicon, specifically, silicon oxide (SiOx), silicon nitride (SiNx), silicon oxynitride (SiOxNy) (x> y), silicon nitride oxide ( SiNxOy) (x> y) (x, y = 1, 2,...) Can be used. Further, this embodiment mode is characterized in that an interlayer insulating film is provided to improve flatness. An organic material or an inorganic material can be used for the interlayer insulating film. As the organic material, polyimide, acrylic, polyamide, polyimide amide, resist, benzocyclobutene, siloxane, or polysilazane can be used. Siloxane is formed using a polymer material having a skeleton structure of a bond of silicon (Si) and oxygen (O) and having a compound (for example, an alkyl group or an aromatic hydrocarbon) as a starting material. As a substituent, fluorine may be used, and a compound containing at least hydrogen in addition to fluorine may be used in combination. Polysilazane is formed using a liquid material containing a polymer material having a bond of silicon (Si) and nitrogen (N) as a starting material. As the inorganic material, silicon oxide (SiOx), silicon nitride (SiNx), silicon oxynitride (SiOxNy) (x> y), silicon nitride oxide (SiNxOy) (x> y) (x, y = 1, 2,... An insulating film containing oxygen or nitrogen such as ()) can be used. Further, a stacked structure of these insulating films may be used as the interlayer insulating film. For example, when an interlayer insulating film is formed using an organic material, flatness is improved, but moisture and oxygen are easily absorbed. In order to prevent this, an insulating film containing an inorganic material is preferably formed over the organic material. When an insulating film containing nitrogen is used as the inorganic material, intrusion of alkali ions such as Na in addition to moisture can be prevented. In the present embodiment, a colored organic material is used for the first interlayer insulating film 64 and a light-transmitting organic material is used for the second interlayer insulating film 65. Color can be obtained by dispersing particles such as carbon black in an organic material. The colored organic material can suppress the wraparound of light due to wiring or the like. A function as a so-called black matrix can be achieved.

 Thereafter, openings are formed in the first and second interlayer insulating films 64 and 65, the passivation film 63, and the gate insulating film 62, and source wirings and drain wirings 66 are formed. The source wiring and the drain wiring are formed with a single layer or a stacked layer using a conductive material. For example, a laminated structure of titanium (Ti) / aluminum silicon (Al—Si) / Ti, Mo / Al—Si / Mo, and MoN / Al—Si / MoN can be used. Alternatively, an aluminum alloy (referred to as Al (C + Ni)) film containing carbon and nickel (1 to 20 wt%) may be used as the conductive material. The (Al (C + Ni)) film has high heat resistance even after energization or heat treatment, and has a redox potential close to that of the pixel electrode (ITO or ITSO) shown below. It is a material that does not change greatly.

After that, the pixel electrode (first electrode) 73 is connected to the source wiring and drain wiring 66 that connect the thin film transistors 13 and 14. The pixel electrode 73 is formed from a light-transmitting or non-light-transmitting material. For example, in the case of translucency, indium tin oxide (ITO), IZO (indium zinc oxide) in which 2 to 20% zinc oxide (ZnO) is mixed with indium oxide, and 2 to 20 in indium oxide. It is also possible to use ITO-SiOx (referred to as ITSO for convenience), organic indium, organic tin, or the like mixed with% silicon oxide (SiO ₂ ). In addition to silver (Ag), an element selected from tantalum, tungsten, titanium, molybdenum, aluminum, and copper, or an alloy material or a compound material containing the element as a main component is used as the non-translucent material. it can.

 An insulating film 67 is formed so as to cover the end of the pixel electrode 73. The insulating film 67 functions as a partition wall (bank) when forming the electroluminescent layer. The insulating film 67 may be formed using either an inorganic material or an organic material, like the interlayer insulating film.

 Next, an opening is formed in the insulating film 67, and an electroluminescent layer 74 is formed in the opening. At this time, since the electroluminescent layer is formed so as to be in contact with the insulating film 67, the insulating film 67 preferably has a shape in which the radius of curvature continuously changes so that pinholes and the like are not generated in the electroluminescent layer. In addition, the heat treatment of the insulating film 67 and the formation of the electroluminescent layer 74 are preferably performed continuously without being exposed to the atmosphere.

 The material of the electroluminescent layer can be used as an organic material (including a low molecule or a polymer) or a composite material of an organic material and an inorganic material. The electroluminescent layer can be formed by a droplet discharge method, a coating method, or a vapor deposition method. The polymer material is preferably a droplet discharge method or a coating method, and the low molecular material is preferably an evaporation method, particularly a vacuum evaporation method. In this embodiment mode, a low molecular material is formed by a vacuum evaporation method as the electroluminescent layer.

 Note that the type of molecular excitons formed by the electroluminescent layer can be a singlet excited state or a triplet excited state. The ground state is usually a singlet state, and light emission from the singlet excited state is called fluorescence. In addition, light emission from the triplet excited state is called phosphorescence. The light emission from the electroluminescent layer includes the case where either excited state contributes. Furthermore, fluorescence and phosphorescence may be used in combination, and either fluorescence or phosphorescence can be selected according to the emission characteristics of each RGB (emission luminance, lifetime, etc.). For example, a material in a triplet excited state can be used for the electroluminescent layer for R, and a material in a singlet excited state can be used for G and B.

 The detailed electroluminescent layer is laminated in the order of HIL (hole injection layer), HTL (hole transport layer), EML (light emitting layer), ETL (electron transport layer), and EIL (electron injection layer) in this order from the pixel electrode 73 side. Has been. Note that the electroluminescent layer can have a single-layer structure or a mixed structure in addition to the stacked structure.

Specifically, CuPc or PEDOT is used as HIL, α-NPD is used as HTL, BCP or Alq _{3 is used} as ETL, and BCP: Li or CaF ₂ is used as EIL. Further, for example, EML may be Alq ₃ doped with a dopant corresponding to each emission color of R, G, and B (DCM in the case of R, DMQD in the case of G).

 Note that the electroluminescent layer is not limited to the above materials. For example, instead of CuPc or PEDOT, an oxide such as molybdenum oxide (MoOx: x = 2 to 3) and α-NPD or rubrene can be co-evaporated to improve the hole injection property. A benzoxazole derivative (shown as BzOS) may be used for the electron injection layer.

 Furthermore, when each RGB electroluminescent layer is formed, high-definition display can be performed using a color filter. This is because the color filter can correct a broad peak in the emission spectrum of each RGB so as to be sharp.

 The case where the RGB electroluminescent layers are formed has been described above, but an electroluminescent layer exhibiting monochromatic light emission may be formed. In this case, full color display can be performed by combining a color filter and a color conversion layer. For example, when an electroluminescent layer that emits white or orange light is formed, a full color display can be performed by providing a color filter or a combination of a color filter and a color conversion layer.

 Needless to say, a monochromatic display may be performed by forming an electroluminescent layer that emits monochromatic light. For example, an area color type display can be performed using monochromatic light emission. The area color type is suitable mainly for displaying characters and symbols.

 Thereafter, a counter electrode (second electrode) 75 of the light emitting element 15 is formed so as to cover the electroluminescent layer 74 and the insulating film 67.

 Note that the material of the pixel electrode 73 and the counter electrode 75 needs to be selected in consideration of the work function. The pixel electrode 73 and the counter electrode 75 can both be an anode and a cathode depending on the pixel configuration. Below, the electrode material used for an anode and a cathode is demonstrated.

 As an electrode material used as the anode, it is preferable to use a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like having a high work function (work function of 4.0 eV or more). As specific examples, ITO, ZnO, IZO, ITSO, gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or a nitride of a metal material (for example, titanium nitride) is used. be able to.

As an electrode material used as the cathode, it is preferable to use a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like having a low work function (work function of 3.8 eV or less). Specific materials include elements belonging to Group 1 or Group 2 of the periodic table, that is, alkali metals such as lithium and cesium, and magnesium, calcium, strontium, and the like, and alloys containing them (Mg: Ag, Al: Li ) And compounds (LiF, CsF, CaF ₂ ), and transition metals including rare earth metals.

 By making such a pixel electrode or a counter electrode light-transmitting or non-light-transmitting, the light emission direction from the electroluminescent layer can be selected. For example, in the case where the pixel electrode and the counter electrode are formed using a light-transmitting material, a dual emission display in which light from the electroluminescent layer is emitted to the insulating substrate 60 side and the sealing substrate side can be performed.

 When light from the electroluminescent layer is emitted to the substrate 60, the pixel electrode may be light-transmitting and the counter electrode may be non-light-transmitting. As a result, a bottom emission display device can be provided. In addition, when light from the electroluminescent layer is emitted to the sealing substrate side, the pixel electrode may be non-light-transmitting and the counter electrode may be light-transmitting. As a result, a top emission display device can be provided. Light can be effectively used by using a highly reflective conductive film for the non-translucent electrode provided on the side that does not correspond to the light emission direction.

  In the present embodiment, a colored organic material can be used for the first interlayer insulating film 64, and this can be used as a black matrix. Then, a non-light-transmitting material is used for the pixel electrode and a light-transmitting material such as ITO is used for the counter electrode, so that a top emission type is obtained. Further, by using a light-transmitting material such as ITO for the pixel electrode without using a colored organic material for the first interlayer insulating film 64, a bottom emission type can be obtained.

 In this embodiment mode, in the case where the pixel electrode and the counter electrode are required to have a light-transmitting property, a metal or an alloy containing these metals may be formed to be very thin. Further, it can be formed by laminating ITO, IZO, ITSO or other transparent conductive films (including alloys) on a thin metal.

 The pixel portion can be formed as described above.

 In order to prevent crosstalk between the signal line and the scanning line, it is preferable to increase the thickness of the interlayer insulating film. At this time, in order to secure a film thickness that does not cause crosstalk, an organic material may be used for part of the interlayer insulating film, and a stacked structure may be used. When an inorganic material is used for the interlayer insulating film, it is preferable to use a low dielectric constant material (low-k material).

 In addition, when an interlayer insulating film is stacked and light from a light emitting element is emitted downward, it is preferable to prevent light from being refracted at the interface between different materials. For example, an opening is formed in the first interlayer insulating film, and a second interlayer insulating film is formed so as to fill the opening. As a result, light refraction at the interface between the first interlayer insulating film and the second interlayer insulating film can be prevented, and the light extraction efficiency can be increased.

 A structural example in which such interlayer insulating films are stacked is shown in FIG.

 FIG. 12B is different from FIG. 12A in that an interlayer insulating film is stacked and an opening is provided in the first interlayer insulating film. The opening is formed in a region where the electroluminescent layer is provided. The switching element 13 is a thin film transistor having a multi-gate structure in which a plurality of gate electrodes are formed over a semiconductor film. The description of other structures is omitted because it is similar to that of FIG. As a result, even when a colored organic material is used for the first interlayer insulating film, bottom emission can be performed. Even when a colored organic material is not used, the refraction of light at the interface of the interlayer insulating film or the like can be reduced by providing an opening in the first interlayer insulating film.

 Similarly to FIG. 12A, a double-sided emission type can be obtained by using a light-transmitting material for the pixel electrode and the counter electrode. Needless to say, top emission can be performed by using a non-light-transmitting material for the pixel electrode and a light-transmitting material for the counter electrode.

 FIG. 13A is different from FIG. 12A in that the wiring 66 is formed after the pixel electrode 73 is formed. The description of other structures is omitted because it is similar to that of FIG.

 FIG. 13B is different from FIG. 13A in that an opening is provided in the first interlayer insulating film. In the region where the electroluminescent layer is provided, an opening is provided in the first interlayer insulating film. The switching element 13 is a thin film transistor having a multi-gate structure in which a plurality of gate electrodes are formed over a semiconductor film. The other structures are the same as those in FIG. 13A, so that FIG. 12A can be referred to. As a result, even when a colored organic material is used for the first interlayer insulating film, bottom emission can be performed. Even when a colored organic material is not used, the refraction of light at the interface of the interlayer insulating film or the like can be reduced by providing an opening in the first interlayer insulating film.

 Further, as in FIG. 12A, by using a light-transmitting material for the pixel electrode and the counter electrode, a dual emission type can be obtained as shown in FIG. 13B. Needless to say, top emission can be performed by using a non-light-transmitting material for the pixel electrode and a light-transmitting material for the counter electrode.

 14A differs from FIG. 12A in that the passivation film has a stacked structure, wiring 66 is formed before the interlayer insulating film is formed, an opening is formed in the interlayer insulating film 64, and the wiring 66 is connected. In short, the pixel electrode 73 is formed. As the passivation film, a silicon oxynitride (SiNO) film can be used for the first layer, and a silicon nitride oxide (SiON) film can be used for the second layer. In the pixel shown in FIG. 14A, a structure in which the first interlayer insulating film 64 and the second interlayer insulating film 65 are stacked may be used. The description of other structures is omitted because it is similar to that of FIG.

 FIG. 14B is different from FIG. 14A in that an opening is provided in the first interlayer insulating film. In the region where the electroluminescent layer is provided, an opening is provided in the first interlayer insulating film. The switching element 13 is a thin film transistor having a multi-gate structure in which a plurality of gate electrodes are formed over a semiconductor film. The other structures are the same as those in FIG. 14A, so that FIG. 12A can be referred to. As a result, even when a colored organic material is used for the first interlayer insulating film, bottom emission can be performed. Even when a colored organic material is not used, the refraction of light at the interface of the interlayer insulating film or the like can be reduced by providing an opening in the first interlayer insulating film.

 Further, as in FIG. 12A, by using a light-transmitting material for the pixel electrode and the counter electrode, a dual emission type can be obtained as shown in FIG. 14B. Needless to say, top emission can be performed by using a non-light-transmitting material for the pixel electrode and a light-transmitting material for the counter electrode.

 FIG. 15A is different from FIG. 12A in that the wiring 66 is provided in two layers. That is, an opening is provided in the first interlayer insulating film 64 to form the wiring 66a, then a second interlayer insulating film 65 is formed, and an opening is provided in the second interlayer insulating film 65 to form the wiring 66b. Form. For example, an aluminum alloy (Al (C + Ni)) containing carbon and nickel can be used as the wiring 66a, and a Ti / Al—Si / Ti stacked structure can be used as the wiring 66b. The other structures are the same as those in FIG. 14A; therefore, the description in FIG. 12A can be referred to.

 FIG. 15B is different from FIG. 15A in that an opening is provided in the first interlayer insulating film. In the region where the electroluminescent layer is provided, an opening is provided in the first interlayer insulating film. The switching element 13 is a thin film transistor having a multi-gate structure in which a plurality of gate electrodes are formed over a semiconductor film. The other structures are the same as those in FIG. 15A, so that FIG. 12A can be referred to. As a result, even when a colored organic material is used for the first interlayer insulating film, bottom emission can be performed. Even when a colored organic material is not used, the refraction of light at the interface of the interlayer insulating film or the like can be reduced by providing an opening in the first interlayer insulating film. Further, by using a light-transmitting material for the pixel electrode and the counter electrode, a dual emission type can be obtained as shown in FIG. Needless to say, top emission can be performed by using a non-light-transmitting material for the pixel electrode and a light-transmitting material for the counter electrode.

 When the pixel electrode 73 is thus formed on a flat surface such as an interlayer insulating film, a voltage can be applied uniformly. As a result, a good image display can be performed.

 In addition, the display device thus formed may be provided with a polarizing plate and a circular polarizing plate in order to improve contrast. In this case, when the light emission side of the light emitting element is provided with a film having a light emission wavelength band as a central wavelength and polarizing the wavelength region (polarizing film), the contrast is improved and the mirror surface is formed by wiring or the like ( (Reflection) can be prevented.

 In addition, a driver circuit portion such as a signal line driver circuit or a scan line driver circuit can be formed over the same substrate as the above pixel portion. In this case, it is preferable to use a polycrystalline silicon film as the semiconductor film.

 FIG. 16 is a cross-sectional view of the pixel portion, the first gate driver 41, and the second gate driver 42 shown in FIG. Although not shown in FIG. 14, the capacitor 22 can be formed using a material for the gate electrode, an insulating material such as the interlayer insulating film 64, and the wiring 66. A sealing material 408 is provided on part of the first and second gate driver regions. The counter substrate 406 can be attached to the substrate with the sealing material. A space formed when the counter substrate 406 is attached to the counter substrate 406 may be filled with an inert gas such as nitrogen or a resin material, or a desiccant may be provided. Deterioration of the light emitting element 15 due to moisture or oxygen can be prevented.

 In addition, as shown in FIG. 16, by providing the sealing material on the gate driver, it is possible to achieve a narrow frame of the display device. Further, a sealing material may be provided on the source driver. However, since many lead wires are provided, attention is required.

 Such a sealing structure is applied to any of the pixel structures shown in FIGS. 12A and 12B, FIGS. 13A and 13B, FIGS. 14B and 15A and 15B. can do.

 Note that this embodiment can be freely combined with any of the other embodiments.

(Embodiment 9)
As an electronic device having a pixel region including the pass element of the present invention, a portable information terminal such as a television device (TV, television receiver), a digital camera, a digital video camera, a mobile phone device (mobile phone), a PDA, etc. In addition, sound reproducing devices such as portable game machines, monitors, computers, car audio, and image reproducing devices equipped with a recording medium such as a home game machine. A specific example will be described with reference to FIG.

 A portable information terminal using the display device of the present invention illustrated in FIG. 19A includes a main body 9201, a display portion 9202, and the like, and can perform clean black display according to the present invention. In addition, power consumption can be reduced by operating the driving transistor in a linear region. A digital video camera using the display device of the present invention shown in FIG. 19B includes display portions 9701, 9702, and the like, and can perform clean black display according to the present invention. In addition, power consumption can be reduced by operating the driving transistor in a linear region. A portable terminal using the display device of the present invention illustrated in FIG. 19C includes a main body 9101, a display portion 9102, and the like, and can perform clean black display according to the present invention. In addition, power consumption can be reduced by operating the driving transistor in a linear region. A portable television device using the display device of the present invention illustrated in FIG. 19D includes a main body 9301, a display portion 9302, and the like, and can perform clean black display according to the present invention. In addition, power consumption can be reduced by operating the driving transistor in a linear region. A portable computer using the display device of the present invention illustrated in FIG. 19E includes a main body 9401, a display portion 9402, and the like, and can perform clean black display according to the present invention. In addition, power consumption can be reduced by operating the driving transistor in a linear region. A television set using the display device of the present invention illustrated in FIG. 19F includes a main body 9501, a display portion 9502, and the like, and can perform clean black display according to the present invention. In addition, power consumption can be reduced by operating the driving transistor in a linear region. In the electronic device described above, by providing the pass element, the off-state current of the driving transistor can flow to the outside through the pass element. As a result, it is possible to provide an electronic device that performs clean black display.

It is the circuit diagram which showed the pixel of this invention It is sectional drawing which showed the element for a path | pass of this invention. It is the circuit diagram which showed the pixel of this invention It is the circuit diagram which showed the pixel of this invention It is the circuit diagram which showed the pixel of this invention It is the circuit diagram which showed the pixel of this invention It is the circuit diagram which showed the pixel of this invention It is the circuit diagram which showed the pixel of this invention It is the figure which showed operation | movement of the pixel of this invention. It is the figure which showed the display apparatus of this invention It is the figure which showed the operation | movement waveform of the pixel of this invention. It is sectional drawing of the pixel of this invention. It is sectional drawing of the pixel of this invention. It is sectional drawing of the pixel of this invention. It is sectional drawing of the pixel of this invention. It is sectional drawing of the display apparatus of this invention. It is a figure which shows the timing of operation | movement of the pixel of invention. It is a figure which shows the timing of operation | movement of the pixel of invention. It is sectional drawing which showed the electronic device of this invention It is a figure which shows the timing of operation | movement of the pixel of invention.

Claims (17)

A light emitting element, a transistor for driving the light emitting element, the light emitting element and an element connected to the transistor,
The display device according to claim 1, wherein a resistance value of the element is smaller than a resistance value when the light emitting element is not turned on and larger than a resistance value when the light emitting element is turned on. A light emitting element, a transistor for driving the light emitting element, a light emitting element and a resistance element connected to the transistor,
The display device according to claim 1, wherein a resistance value of the resistance element is smaller than a resistance value when the light emitting element is not turned on and is larger than a resistance value when the light emitting element is turned on. A light emitting element; a transistor for driving the light emitting element; and the thin film transistor connected to the light emitting element and the transistor.
The resistance value when the thin film transistor is turned on is smaller than the resistance value when the light emitting element is not lit,
The display device is characterized in that a resistance value when the thin film transistor is turned off is larger than a resistance value when the light emitting element is turned on. A light emitting element, a transistor for driving the light emitting element, the light emitting element and a diode element connected to the transistor,
The display device characterized in that a resistance value of the diode element is smaller than a resistance value when the light emitting element is not lit and larger than a resistance value when the light emitting element is lit. A light emitting element, a first transistor for driving the light emitting element, and a second transistor connected to the light emitting element and the transistor,
The display device, wherein the second transistor is turned off when the first transistor is turned off, and turned off when the first transistor is turned on and the light emitting element is turned on. A light emitting element; a first transistor for driving the light emitting element; and a second transistor connected to the light emitting element and the first transistor;
The second transistor is turned off when the first transistor is turned off, and turned off when the first transistor is turned on and the light emitting element is turned on,
The display device according to claim 1, wherein a resistance value of the second transistor is smaller than a resistance value when the light emitting element is not lit and larger than a resistance value when the light emitting element is lit. A light emitting element; a first transistor for driving the light emitting element; and a second transistor connected to the light emitting element and the first transistor;
The polarity of the second transistor is p-type,
The display device, wherein a gate electrode of the second transistor is connected to a power supply line, and one of the other electrodes of the p-type transistor is connected to a counter electrode of the light emitting element. A light emitting element; a first transistor for driving the light emitting element; and a second transistor connected to the light emitting element and the first transistor;
The polarity of the second transistor is p-type,
The gate electrode of the p-type transistor is connected to a power supply line, and one of the other electrodes of the p-type transistor is connected to the counter electrode of the light emitting element,
In addition, the display device is characterized in that a resistance value of the transistor is smaller than a resistance value when the light emitting element is not turned on and larger than a resistance value when the light emitting element is turned on. A light emitting element, and a transistor for driving the light emitting element,
While applying a reverse voltage to the light emitting element, when the light emitting element is not lit, the transistor is turned on,
While the forward voltage is applied to the light emitting element, the transistor is turned on when the light emitting element is turned on. In any one of Claims 1 thru | or 9,
A switching element connected to a first transistor for driving the light emitting element;
The switching element is selected by a scanning line;
When the switching element is selected, a video signal is input from a signal line to the switching element. In any one of Claims 1 thru | or 10,
A display device comprising: a first transistor for driving the light emitting element; and a current control transistor provided between the light emitting element. In claim 11,
A display device, wherein a gate electrode of a first transistor for driving the light emitting element has a fixed potential. In any one of Claims 1 thru | or 12,
A display device comprising: a capacitor element provided between a gate electrode of a transistor for driving the light emitting element and one of the other electrode. In claim 13,
A display device comprising an erasing transistor for discharging the charge of the capacitor. In any one of Claims 1 thru | or 14,
A display device, wherein an element having a temperature compensation function is provided between a pixel region having the light-emitting element and a driver that generates a signal to be supplied to a transistor that drives the light-emitting element. In any one of Claims 1 thru | or 15,
A display device, wherein a protection circuit is provided between a pixel region including the light-emitting element and a driver that generates a signal to be supplied to a transistor that drives the light-emitting element. In any one of Claims 1 thru | or 16,
A display device, wherein a counter substrate is provided through a sealing material provided on the driver.

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