JP5085011B2 - Active matrix display device - Google Patents

Description

  The present invention relates to an active matrix display device, and more particularly to an active matrix display device that performs signal writing using a current signal.

  In recent years, the demand for flat display devices typified by liquid crystal display devices has been rapidly increased by taking advantage of the features of thinness, light weight, and low power consumption. In particular, an active matrix display device in which a pixel switch having a function of electrically separating an on pixel and an off pixel and holding a video signal to the on pixel is provided in each pixel has crosstalk between adjacent pixels. Since a good display quality without any problem can be obtained, it has come to be used for various displays including portable information devices.

  As such a flat-type active matrix display device, an organic electroluminescence (EL) display device using a self-luminous element has attracted attention, and research and development has been actively conducted. This organic EL display device does not require a backlight that obstructs the reduction in thickness and weight, is suitable for moving image reproduction because of its high-speed response, and further has a feature that it can be used even in cold regions because the luminance does not decrease at low temperatures. .

  The organic EL display device includes an organic EL element as a display element for each pixel and a pixel circuit that supplies a drive current to the display element, and performs a display operation by controlling the light emission luminance of the display element. The pixel circuit includes, for example, a drive transistor and an output switch connected in series to the organic EL element, a diode connection switch connected between the gate and drain of the drive transistor, and holding a gate potential corresponding to a video signal. These drive transistors, output switches, and diode connection switches are composed of thin film transistors, for example. As such an organic EL display device, a method of supplying image information to a pixel circuit by a current signal is known.

  In the case of a display device that supplies a signal using a current signal, there is a risk that sufficient signal supply may not be possible due to the wiring capacity of the wiring that supplies the signal. In particular, there is a problem that a display defect due to insufficient writing occurs when the current value to be written is small. Further, when performing multi-grayscale display, writing becomes difficult on the low-grayscale side where the set current amount is small, resulting in display problems.

  In order to prevent insufficient writing due to such wiring capacity, an organic EL display device is provided that supplies two systems of current signals from a video signal driver and writes the difference current to a pixel as a video signal (for example, Patent Document 1). The display device supplies a base current to the pixel via the first video signal line and also supplies a gray scale current to the pixel via the second video signal line, and a difference current between the base current and the gray scale current. Is written into the pixel circuit. Then, the display element is driven by the differential current.

According to such a configuration, the current value supplied to the first and second video signal lines can be freely set, and the base current and the gradation current are set to current values sufficiently larger than the wiring capacity. be able to. As a result, it is possible to write a small current that is a difference current with a large write current that is not affected by the wiring capacitance.
JP 2004-341023 A

  However, since the display device configured as described above takes a differential current when writing a video signal, it is easily affected by the penetration current of the transistor during light emission. Since the punch-through current changes according to the characteristics of the transistor, it is visually recognized as display unevenness, and the display quality is deteriorated.

  As described above, the thin film transistor used in the pixel circuit has a current value that is not constant even in a saturation region in which the current value is originally constant due to the Early effect or the kink effect. The rate of increase varies according to the process variation that occurs when manufacturing the thin film transistor. The increase rate increases as the current value flowing through the thin film transistor decreases. Therefore, for example, in a circuit configuration in which a P-channel thin film transistor is used as a drive transistor, the variation in the emission current supplied to the organic EL element decreases as the screen brightness decreases, that is, the current flowing through the drive transistor decreases. growing. As a result, the display is defective and the display quality is lowered.

  The present invention has been made in view of the above problems, and an object thereof is to provide an active matrix display device with improved display quality.

In order to solve the above problems, an active matrix display device according to an aspect of the present invention includes a display element and a plurality of pixel circuits arranged in a matrix on a substrate, the pixel circuit supplying a driving current to the display element. A first signal current is supplied to the pixel circuit via the video signal line, and the video signal line is connected to the pixel unit via the video signal line. A signal line driving circuit for supplying a second signal current to the pixel circuit in a direction different from the first signal current,
Each of the pixel circuits is connected between the video signal line and the pixel circuit, and controls a selection and non-selection of the pixel portion, and a first switch corresponding to the first signal current when the pixel portion is selected. A P-channel first driving transistor that supplies one driving current; an N-channel second driving transistor that supplies a second driving current according to the second signal current; a source of the first driving transistor; the source of the second driver transistor, wherein a plurality of voltage power supply connected respectively to the display element, a first holding capacitor for holding an electric potential between the source and gate of pre-Symbol first driving transistor, is formed by the transistor, the A first holding switch connected between the gate of the first driving transistor and the pixel switch and controlling conduction and non-conduction of the first driving transistor; A first switch formed by a transistor and connected to a drain of the first holding switch and a drain of the first driving transistor; and a drain of the first holding switch and a drain of the second driving transistor formed by a transistor. A connected second switch; an output switch formed by a transistor and connected between a connection point of the first switch and the second switch and the display element; and a source and a gate of the second drive transistor. A second holding capacitor that holds the potential of the second driving transistor and is connected to a gate and a drain of the second driving transistor and controls conduction and non-conduction of the second driving transistor.
A first signal current is written from the signal line driving circuit to the first driving transistor of the pixel circuit via the video signal line, and then a second signal is supplied to the second driving transistor of the pixel circuit via the video signal line. When a current is written and the pixel portion is not selected, the first drive current and the second drive current are output from the first drive transistor and the second drive transistor, and a differential current between the first and second drive currents is output. A drive current is output to the display element.

  According to the present invention, it is possible to provide an active matrix display device capable of performing a good display operation without being affected by the wiring capacity and having improved display quality.

Hereinafter, an organic EL display device will be described in detail as a first embodiment of the present invention with reference to the drawings.
FIG. 1 is a plan view schematically showing an organic EL display device. As shown in FIG. 1, the organic EL display device is configured as, for example, a large active matrix display device of 10 type or more, and includes an organic EL panel 10 and a controller 12 that controls the organic EL panel 10.

  The organic EL panel 10 includes a light-transmitting insulating substrate 8 such as a glass plate, m × n display pixels PX arranged in a matrix on the insulating substrate and constituting a display region 11, and each display pixel row. The first scanning line Sga (1 to m), the second scanning line Sgb (1 to m), the third scanning line Sgc (1 to m), and the fourth scanning line Sga (1 to m), which are connected and provided independently by m. Scanning line Sgd (1 to m), fifth scanning line Sge (1 to m), sixth scanning line Sgf (1 to m), and n video signal lines X (1) connected to each column of display pixels PX To n), first, second, third, fourth, fifth and sixth scanning lines Sga (1 to m), Sgb (1 to m), Sgc (1 to m), Sgd (1 to m), Scan line driving circuits 14a and 14b for sequentially driving Sge (1 to m) and Sgf (1 to m) for each row of display pixels, and a plurality of images Line and a signal line driver circuit 15 for driving the X (1 to n). The scanning line driving circuits 14 a and 14 b and the signal line driving circuit 15 are integrally formed on the insulating substrate 8 outside the display area 11.

  Each display pixel PX that functions as a pixel portion includes a display element having a photoactive layer between opposing electrodes, and a pixel circuit 18 that supplies a drive current to the display element. The display element is, for example, a self-luminous element. In this embodiment, the organic EL element 16 including at least an organic light-emitting layer is used as a photoactive layer.

  FIG. 2 shows an equivalent circuit of the display pixel PX. The pixel circuit 18 is a current signal type pixel circuit that controls light emission of the organic EL element 16 in accordance with a video signal composed of a current signal, and includes a pixel switch 20, a first drive transistor 22a, a second drive transistor 22b, and a first holding circuit. A switch 23a, a second holding switch 23b, a first switch 24a, a second switch 24b, an output switch 26, and a first holding capacitor Cs1 and a second holding capacitor Cs2 as capacitors are provided.

  Except for the second driving transistor 22b, the pixel switch 20, the first driving transistor 22a, the first holding switch 23a, the second holding switch 23b, the first switch 24a, the second switch 24b, and the output switch 26 are the same in this case. A thin film transistor of a type, for example, a P channel type. The second drive transistor 22b is configured by an N-channel thin film transistor.

  In the present embodiment, the thin film transistors constituting the pixel switch 20, the first driving transistor 22a, the first holding switch 23a, the second holding switch 23b, the first switch 24a, the second switch 24b, and the output switch 26 are all in the same process, It is a thin film transistor having a top gate structure which is formed in the same layer structure and uses polysilicon as a semiconductor layer. The second driving transistor 22b is a thin film transistor having a top gate structure using polysilicon as a semiconductor layer, and is formed in the same process and the same layer structure as the pixel switch 20 and the like. They are created by implanting impurities of different conductivity types into the drain region. Each of the pixel switch 20, the first driving transistor 22a, the second driving transistor 22b, the first holding switch 23a, the second holding switch 23b, the first switch 24a, the second switch 24b, and the output switch 26 has a first terminal, There are two terminals and a control terminal, and in this embodiment, these are the first terminal, the second terminal, and the control terminal as the source, drain, and gate, respectively.

  The first drive transistor 22a is connected in series with the organic EL element 16 between the voltage power supply line Vdd and the reference voltage power supply Vss, and outputs a current amount corresponding to the video signal to the organic EL element. The reference voltage power supply Vss and the voltage power supply line Vdd are set to potentials of −9 V and +6 V, for example. The first storage capacitor Cs1 is connected between the source and gate of the first drive transistor 22a and holds the gate control potential of the first drive transistor 22a determined by the video signal. The pixel switch 20 is connected between the corresponding video signal line X (1 to n) and the drain of the first drive transistor 22a, and its gate is connected to the corresponding fourth scanning line Sgd (1 to m). . The pixel switch 20 captures the video signal from the corresponding video signal line X (1 to n) in response to the control signal Sd (1 to m) supplied from the fourth scanning line Sgd (1 to m).

  The first holding switch 23a is connected between the drain and gate of the first drive transistor 22a, and the gate thereof is connected to the third scanning line Sgc (1 to m). The first holding switch 23a is turned on (conductive state) and turned off (non-conductive state) in response to the control signal Sc (1-m) from the third scanning line Sgc (1-m), and the first driving transistor 22a Controls connection and disconnection between the gate and the drain, and regulates current leakage from the first storage capacitor Cs1.

  The first switch 24a is connected in series between the drain of the first drive transistor 22a and the display element, and is connected to the drain of the first holding switch 23a and the drain of the first drive transistor 22a, and the gate thereof is connected to the second scan. It is connected to the line Sgb (1 to m). The first switch 24a is turned on (conductive state) and turned off (non-conductive state) in response to the control signal Sb (1-m) from the second scanning line Sgb (1-m), and the gate of the first drive transistor 22a. , And connection / disconnection between drains, and connection / disconnection of the current path to the first drive transistor 22a and the second drive transistor 22b are controlled.

  The second drive transistor 22b is connected in series with the organic EL element 16 between the two reference voltage power sources Vss, and outputs a current amount corresponding to the video signal to the organic EL element. The second storage capacitor Cs2 is connected between the source and gate of the second drive transistor 22b and holds the gate control potential of the second drive transistor 22b determined by the video signal.

  The second holding switch 23b is connected between the drain and gate of the second drive transistor 22b, and the gate thereof is connected to the first scanning line Sga (1 to m). The second holding switch 23b is turned on (conductive state) and turned off (non-conductive state) in response to the control signal Sa (1 to m) from the first scanning line Sga (1 to m), and the second driving transistor 22b Controls connection and disconnection between the gate and drain, and regulates current leakage from the second storage capacitor Cs2.

  The second switch 24b is connected to the drain of the first holding switch 23a and the drains of the second driving transistor 22b and the second holding switch 23b, and the gate thereof is connected to the fifth scanning line Sge (1 to m). Yes. The second switch 24b is turned on (conductive state) and turned off (non-conductive state) in response to the control signal Se (1-m) from the fifth scanning line Sge (1-m), and the drain of the second drive transistor 22b. And connection between the output switch 26 and the output switch 26 are controlled. That is, the second switch 24b controls connection / disconnection of the second drive transistor 22b and the current path to the first drive transistor 22a and connection / disconnection of the drain of the second drive transistor 22b and the video signal line X. At the same time, the connection / disconnection of the second drive transistor 22b and the display element is controlled.

  The output switch 26 is connected between the drains of the first and second drive transistors 22a and 22b and one electrode of the organic EL element 16, here the anode, and the gate thereof is connected to the sixth scanning line Sgf (1 to m). )It is connected to the. The output switch 26 is ON / OFF controlled by a control signal Sf (1-m) from the sixth scanning line Sgf (1-m), and the output switch 26 is connected to the organic EL element 16 between the first driving transistor 22a and the second driving transistor 22b. Control connection / disconnection. That is, the output switch 26 controls connection / disconnection between the current path on the first drive transistor 22a side and the current path on the second drive transistor 22b side and the display element.

Next, the configuration of the drive transistor 22 and the organic EL element 16 will be described in detail with reference to FIG. FIG. 3 shows a cross section of the display pixel Px including the organic EL element 16.
The P-channel type thin film transistor constituting the first drive transistor 22a includes a semiconductor layer 50 made of polysilicon formed on the insulating substrate 8. The semiconductor layer includes a source region 50a, a drain region 50b, and a source / drain region. It has a channel region 50c located between them. A gate insulating film 52 is formed over the semiconductor layer 50, and a gate electrode G is provided on the gate insulating film so as to face the channel region 50c. An interlayer insulating film 54 is formed over the gate electrode G, and a source electrode (source) S and a drain electrode (drain) D are provided on the interlayer insulating film. The source electrode S and the drain electrode D are respectively connected to the source region 50a and the drain region 50b of the semiconductor layer 50 through contacts formed through the interlayer insulating film 54 and the gate insulating film 52, respectively. The drain electrode D of the first drive transistor 22a is connected to the output switch 26 via a wiring formed on the interlayer insulating film 54 and the first switch 24a. Note that the thin film transistors constituting the pixel switch 20, the first holding switch 23a, the second holding switch 23b, the first switch 24a, the second switch 24b, and the output switch 26 are also formed in the same structure as described above. The second drive transistor 22b is also formed in the same structure as described above, but an LDD region may be further added.

  A plurality of wirings including the video signal lines X (1 to n) are provided on the interlayer insulating film 54. A protective film 56 is formed on the interlayer insulating film 54 so as to cover the source electrode S, the drain electrode D, and the wiring. On the protective film 56, a hydrophilic film 58 and a partition film 60 are laminated in this order.

  The organic EL element 16 has a structure in which an organic light emitting layer 64 containing a luminescent organic compound is sandwiched between an anode 62 and a cathode 66. The anode 62 is made of a transparent electrode material such as ITO (indium tin oxide) and is provided on the protective film 56. Of the hydrophilic film 58 and the partition wall film 60, the part facing the anode 62 is removed by etching. An anode buffer layer 63 and an organic light emitting layer 64 are formed on the anode 62, and a cathode 66 made of silver / aluminum alloy is laminated on the organic light emitting layer 64 and the partition wall film 60.

  In the organic EL element 16 having such a structure, when the holes injected from the anode 62 and the electrons injected from the cathode 66 recombine inside the organic light emitting layer 64, organic molecules constituting the organic light emitting layer are formed. Is excited to generate excitons. The excitons emit light in the process of radiation deactivation, and the light is emitted from the organic light emitting layer 64 to the outside through the transparent anode 62 and the insulating substrate 8.

  Here, the cathode 66 may be made light transmissive, and light may be taken out from the surface facing the insulating substrate 8. Further, a reverse lamination type in which the anode 62 is disposed on the insulating substrate 8 side with respect to the cathode 66 may be employed. In either case, it is necessary to form the light emitting surface side with a transparent conductive material. For example, when the cathode 66 is disposed on the light emitting surface side, the alkaline earth metal and the rare earth metal are thin enough to have light transmittance. This can be achieved by forming.

  On the other hand, the controller 12 shown in FIG. 1 is formed on a printed circuit board disposed outside the organic EL panel 10 and controls the scanning line driving circuits 14 a and 14 b and the signal line driving circuit 15. The controller 12 receives a digital video signal and a synchronization signal supplied from the outside, and generates a vertical scanning control signal for controlling the vertical scanning timing and a horizontal scanning control signal for controlling the horizontal scanning timing based on the synchronizing signal. The controller 12 supplies the vertical scanning control signal and the horizontal scanning control signal to the scanning line driving circuits 14a and 14b and the signal line driving circuit 15, respectively, and outputs a digital video signal in synchronization with the horizontal and vertical scanning timings. This is supplied to the line drive circuit 15.

  The scanning line driving circuits 14a and 14b include a shift register, an output buffer, and the like, and sequentially transfer a horizontal scanning start pulse supplied from the outside to the next stage, as shown in FIGS. 1 and 2, via the output buffer. Six types of control signals, that is, control signals Sa (1 to m), Sb (1 to m), Sc (1 to m), Sd (1 to m), and Se (1 to m) are applied to the display pixels PX in each row. , Sf (1 to m) are supplied. Thereby, each 1st, 2nd, 3rd, 4th, 5th, 6th scanning line Sga (1-m), Sgb (1-m), Sgc (1-m), Sgd (1-m) , Sge (1 to m) and Sgf (1 to m) are respectively the control signal Sa (1 to m), control signal Sb (1 to m), Sc (1 to m), Driven by the control signals Sd (1 to m), Se (1 to m), and the control signal Sf (1 to m).

  The signal line driving circuit 15 converts the video signal sequentially obtained in each horizontal scanning period into an analog format under the control of the horizontal scanning control signal to obtain the first signal current Ip and the second signal current In, and the first and second signal currents. Are supplied in parallel to the plurality of video signal lines X (1 to n). As shown in FIG. 2, the signal line driving circuit 15 includes a plurality of IC units 40 connected to the video signal lines X (1 to n). Each IC section 40 includes an N-channel IC and a first constant current circuit 42a that supplies a first signal current Ip to the first drive transistor 22a, and a P-channel IC that supplies a second signal current In to the second drive transistor 22b. And a second constant current circuit 42b for supplying.

  The first constant current circuit 42a is connected in series to the video signal lines X (1 to n) via a switch 44a formed by a transistor, for example, and is connected to a constant voltage power supply 46a. The second constant current circuit 42b is connected in series to the video signal line X (1 to n) via the switch 44b formed by a transistor, for example, and in parallel with the first constant current circuit 42a. The second constant current circuit 42b is connected to the constant voltage power supply 46b.

  The current amounts of the first signal current Ip and the second signal current In are set to current amounts that do not cause insufficient writing. That is, the current amounts of the first signal current Ip and the second signal current In are the potential from the highest gradation display to the lowest gradation display in the wiring capacitance (Cp) of the video signal line X in one horizontal scanning period (t). A value larger than the amount of charge corresponding to the value obtained by multiplying the change (maximum voltage change ΔV) is set (Ip (In)> Cp × ΔV / t). For example, the first signal current Ip and the second signal current In are set to the same magnitude as the drive current for performing the highest gradation display of the organic EL display device. As an example, in the case of performing full color display, in the display pixel PX that emits red light, the driving current at the time of the highest gradation display is about 2 μA.

  In addition, one of the first signal current Ip and the second signal current In, for example, the first signal current Ip is a constant signal current, and the second signal current Ip is a signal current that varies according to the gradation. Note that the second signal current In may be a constant signal current, and the first signal current Ip may be a signal current that varies according to gradation. Alternatively, both the first signal current Ip and the second signal current In can be made variable signal currents.

  In the organic EL display device configured as described above, the operation of the pixel circuit 18 is divided into a first signal current (P channel) writing operation, a second signal current (N channel) writing operation, and a light emitting operation. As shown in FIG. 4, in the first signal current writing operation, for example, the first scanning line driving circuit 14 a to the first switch 24 a, the first holding switch 23 a, and the pixel switch 20 for the display pixel PX in the first row. Are turned on (on potential), here, low level control signals Sb1, Sc1, Sd1 are output. At the same time, from the first scanning line driving circuit 14a and the second scanning line driving circuit 14b, the second switch 24b, the second holding switch 23b, and the output switch 26 are turned off (off potential). Control signals Sa1, Se1, and Sf1 are output. As a result, the first switch 24a, the first holding switch 23a, and the pixel switch 20 are turned on (conductive state), and the second switch 24b, the second holding switch 23b, and the output switch 26 are turned off (non-conductive state). The first signal current write operation is started.

  In the first signal current writing period, the switch 44a of the corresponding IC unit 40 of the signal line drive circuit 15 is turned on and the switch 44b is turned off. Then, for example, the first signal current Ip set to a predetermined constant current is supplied from the first constant current circuit 42a to the video signal line X1, and is supplied to the selected display pixel PX via the pixel switch 20. The

  In the display pixel PX, the pixel switch 20, the first holding switch 23a, and the first switch 24a are in an on state, and the captured first signal current Ip is supplied to the first driving transistor 22a to write the first driving transistor 22a. State. As a result, a write current flows from the voltage power supply line Vdd to the video signal line X1 through the first drive transistor 22a, and the gate-source potential of the first drive transistor 22a corresponding to the current amount of the first signal current Ip is held first. It is written in the capacitor Cs1.

  Next, the control signals Sb1 and Sc1 are turned off (high level), and the first holding switch 23a and the first switch 24a are turned off. Thereby, the first signal current writing operation is completed. Subsequently, as shown in FIG. 5, the control signals Sa1 and Se1 are turned on (low level), and the second holding switch 23b and the second switch 24b are turned on. Thereby, the second signal current write operation starts.

  In the second signal current writing period, the switch 44a of the corresponding IC unit 40 of the signal line driving circuit 15 is turned off and the switch 44b is turned on. Then, the second signal current In corresponding to the desired gradation is supplied from the second constant current circuit 42b to the video signal line X1, and is supplied to the selected display pixel PX via the pixel switch 20.

  In the display pixel PX, the pixel switch 20, the second switch 24b, and the second holding switch 23b are in an on state, and the captured second signal current In is supplied to the second drive transistor 22b to write the second drive transistor 22b. State. As a result, the second signal current In flows from the second constant current circuit 42b through the second drive transistor 22b, and the gate-source potential of the second drive transistor 22b corresponding to the amount of the second signal current In is held second. It is written in the capacity Cs2.

  Subsequently, the control signals Sa1 and Sd1 are turned off (high level), and the second holding switch 23b and the pixel switch 20 are turned off. Thereby, the second signal current writing operation is completed. Next, as shown in FIG. 6, the control signals Sb1 and Sf1 are turned on (low level) while the control signal Se1 is kept on, and the first and second switches 24a and 24b and the output switch 26 are turned on. It becomes. Other control signals are turned off (high level), and the other switches are turned off. Thereby, the light emission operation starts.

In the light emission period, the first drive transistor 22a outputs a first drive current Ip having a current amount corresponding to the first signal current Ip by the gate control voltage written in the first storage capacitor Cs1. The second drive transistor 22b outputs a second drive current In having a current amount corresponding to the second signal current In by the gate control voltage written in the second storage capacitor Cs2. The current directions of the first drive current Ip and the second drive current In are set to be opposite to each other. For this reason, the second drive current In corresponding to the second signal current In of the first drive current Ip supplied through the first drive transistor 22a is supplied to the voltage power source through the second switch 24b and the second drive transistor 22b. Vss is supplied. Then, a drive current Ie that is a difference current (Ip−In) between the first drive current Ip and the second drive current In is supplied to the organic EL element 16 through the output switch 26. Thereby, the organic EL element 16 emits light, and the light emission operation is started. The organic EL element 16 maintains the light emitting state until the control signal Sf1 becomes the off potential again after one frame period.
According to the organic EL display device configured as described above, in the writing of the video signal current, the first signal current is supplied to the pixel circuit through the video signal line and then written, and then the pixel circuit through the video signal line. The second signal current is supplied to and written to, and at the time of light emission, the difference current between the first drive current corresponding to the first signal current and the second drive current corresponding to the second signal current is output to the display element as a drive current. It is configured to do. Therefore, even when low gradation light emission is performed, it is possible to freely set the current values of the first and second signal currents supplied to the video signal line, which are sufficiently larger than the wiring capacity of the video signal line. Can be set to Therefore, even when displaying at low brightness, the signal current can be written sufficiently and in a short time without being affected by the wiring capacity, and the display failure at low brightness, unevenness, and the feeling of roughness are eliminated. High-quality image display can be realized.

  Even when a low current write is performed after a high current write to the video signal line, a shortage of a low current video signal can be solved. For example, conventionally, when writing a video signal with the highest gradation display (white display) and then writing with the lowest gradation display (black display), the latter lack of video signal writing causes the higher gradation display (white display) to be written. There is a possibility that the image is in a writing state and the white display has a tail on display. According to the present embodiment, it is possible to eliminate such display defects caused by insufficient writing.

  In each pixel circuit, a signal current or a drive current flowing in other transistors except for the output switch 26 at the time of signal current writing and light emission operation is several times to several tens of times the drive current supplied to the organic EL element 16. Can be large. The variation in the current increase rate due to the Early effect and the kink effect of the first and second drive transistors 22a and 22b and other thin film transistors constituting the switches is smaller as the current flowing through the transistors is larger. Therefore, as in this embodiment, by increasing the current flowing through the transistor to several times to several tens of times the light emission current supplied to the organic EL element 16, variation in the current increase rate of the transistor is suppressed, and the organic EL A driving current without variation can be supplied to the element. As a result, it is possible to suppress the luminance variation between the display pixels PX and to display a good image with improved display quality.

  In particular, since the difference between the first drive current and the second drive current is taken at the time of light emission, potential fluctuations due to the Early effect of the first drive transistor and the second drive transistor can be offset, and a favorable display operation can be performed. It becomes.

  Further, in the pixel circuit, the gate potentials of the first and second switches 24a and 24b located in the current path during the light emission operation are adjusted, and the first and second switches 24a and 24b are configured to operate in the saturation region, respectively. can do. For this reason, it is possible to suppress a variation in the current increase rate due to the Early effect and the kink effect of the first and second switches 24a and 24b, and to display a good image with further improved display quality.

  According to the present embodiment, the second switch 24b in the pixel circuit 18 is provided between the drain of the first switch 24a and the drain of the second holding switch 23b. In this case, the second switch 24b can act to prevent current leakage from the second storage capacitor Cs2. Thereby, a more stable image display is possible. Further, this may be applied to the output switch 26.

  According to the present embodiment, the writing operation can be performed by two operations, and it is possible to sufficiently cope with the shortening of the writing time to the pixels for one row accompanying the increase in the screen size.

Next, an organic EL display device according to a second embodiment of the present invention will be described with reference to FIG.
According to the second embodiment, in the pixel circuit 18 constituting the display pixel PX, the second switch 24b is provided between the drain of the second drive transistor 22b and the train of the second holding switch 23b. Yes.

  In the second embodiment, the other configurations and operations of the organic EL display device are the same as those of the first embodiment described above, and the same parts are denoted by the same reference numerals and detailed description thereof is omitted. . And also in 2nd Embodiment, the effect similar to 1st Embodiment can be acquired.

Next, an organic EL display device according to a third embodiment of the present invention will be described with reference to FIG.
According to the third embodiment, in the pixel circuit 18 constituting the display pixel PX, the second drive transistor 22b and the second holding switch 23b connected between the gate and the train of the second drive transistor 22b are: Each is formed by an N channel type thin film transistor. The second holding switch 23b and the first holding switch 23a formed of a P-channel type thin film transistor are formed in the same size.

  In the third embodiment, other configurations and operations of the organic EL display device are the same as those of the first embodiment described above, and the same parts are denoted by the same reference numerals and detailed description thereof is omitted. . And also in 3rd Embodiment, the effect similar to 1st Embodiment can be acquired. Furthermore, according to the third embodiment, the fluctuation of the drive current due to the punch-through voltage generated when the first and second holding switches 23a and 23b are turned on and off is suppressed, and a good image display with improved display quality is achieved. It becomes possible.

  That is, at the time of light emission, the first and second holding switches 23a and 23b are turned off. At this time, the first drive current Ip and the second drive current In vary by ΔI due to the punch-through voltage. According to this embodiment, the first holding switch 23a is formed of a P-channel thin film transistor, and the second holding switch 23b is formed of an N-channel thin film transistor. Therefore, the polarity of the fluctuation current ΔI caused by the punch-through voltage is reversed between the first holding switch 23a and the second holding switch 23b. According to the present embodiment, the differential current between the first drive current Ip and the second drive current In is taken at the time of light emission operation, not at the time of writing the signal current, and is supplied to the organic EL element 16 as the drive current. Yes. Therefore, the drive current Ie, which is a differential current, becomes (Ip−ΔI) − (In−ΔI), and the fluctuation current ΔI is canceled out, so that the influence of the punch-through voltage can be eliminated.

According to the organic EL display device according to the fourth embodiment of the present invention shown in FIG. 9, in the pixel circuit 18 constituting the display pixel PX, the second switch 24b is connected to the drain of the second drive transistor 22b and the second 2 between the holding switch 23b and the train.
In the fourth embodiment, the configuration and operation of the organic EL display device are the same as those of the above-described third embodiment except that the second holding switch 23b is configured by an N-channel thin film transistor. The same reference numerals are assigned and detailed description thereof is omitted. And also in 4th Embodiment, the effect similar to said embodiment can be acquired.

  The present invention is not limited to the above-described embodiments as they are, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  In the above-described embodiments, the semiconductor layer of the thin film transistor is not limited to polysilicon, but can be composed of amorphous silicon. The self-luminous elements constituting the display pixels are not limited to organic EL elements, and various display elements capable of self-luminance are applicable.

FIG. 1 is a plan view schematically showing an organic EL display device according to the first embodiment of the present invention. FIG. 2 is a plan view showing an equivalent circuit of display pixels in the organic EL display device. FIG. 3 is a cross-sectional view showing a driving transistor and an organic EL element of the organic EL display device. FIG. 4 is a plan view showing an equivalent circuit of a display pixel when writing the first signal current in the organic EL display device. FIG. 5 is a plan view showing an equivalent circuit of the display pixel at the time of writing the second signal current in the organic EL display device. FIG. 6 is a plan view showing an equivalent circuit of the display pixel during the light emitting operation of the organic EL display device. FIG. 7 is a plan view showing an equivalent circuit of the display pixel in the organic EL display device according to the second embodiment of the present invention. FIG. 8 is a plan view showing an equivalent circuit of a display pixel in an organic EL display device according to the third embodiment of the present invention. FIG. 9 is a plan view showing an equivalent circuit of a display pixel in an organic EL display device according to the fourth embodiment of the present invention.

Explanation of symbols

8 ... Insulating substrate, 10 ... Organic EL panel, 12 ... Controller,
14a, 14b ... scanning line drive circuit, 15 ... signal line drive circuit, 16 ... organic EL element,
18 ... Pixel circuit, 20 ... Pixel switch, 22a ... First drive transistor,
22b ... second drive transistor, 23a ... first holding switch,
23b ... second holding switch, 24a ... first switch, 24b ... second switch,
26 ... Output switch, 40 ... IC section, 42a ... First constant current circuit,
42b ... Second constant current circuit

Claims (10)

A plurality of pixel portions including a display element and a pixel circuit for supplying a driving current to the display element, the pixel parts being arranged in a matrix on the substrate;
A plurality of video signal lines connected to each column of the pixel portion;
A signal line drive that supplies a first signal current to the pixel circuit through the video signal line and then supplies a second signal current to the pixel circuit in a direction different from the first signal current through the video signal line. A circuit,
Each of the pixel circuits is connected between the video signal line and the pixel circuit, and controls a selection and non-selection of the pixel portion, and a first switch corresponding to the first signal current when the pixel portion is selected. A P-channel first driving transistor that supplies one driving current; an N-channel second driving transistor that supplies a second driving current according to the second signal current; a source of the first driving transistor; the source of the second driver transistor, wherein a plurality of voltage power supply connected respectively to the display element, a first holding capacitor for holding an electric potential between the source and gate of pre-Symbol first driving transistor, is formed by the transistor, the A first holding switch connected between the gate of the first driving transistor and the pixel switch and controlling conduction and non-conduction of the first driving transistor; A first switch formed by a transistor and connected to a drain of the first holding switch and a drain of the first driving transistor; and a drain of the first holding switch and a drain of the second driving transistor formed by a transistor. A connected second switch; an output switch formed by a transistor and connected between a connection point of the first switch and the second switch and the display element; and a source and a gate of the second drive transistor. A second holding capacitor that holds the potential of the second driving transistor and is connected to a gate and a drain of the second driving transistor and controls conduction and non-conduction of the second driving transistor.
A first signal current is written from the signal line driving circuit to the first driving transistor of the pixel circuit via the video signal line, and then a second signal is supplied to the second driving transistor of the pixel circuit via the video signal line. When a current is written and the pixel portion is not selected, the first drive current and the second drive current are output from the first drive transistor and the second drive transistor, and a differential current between the first and second drive currents is output. An active matrix display device that outputs a drive current to the display element. The signal line driving circuit includes a constant voltage power source, a first constant current circuit including an N-channel IC and supplying a first signal current to the first driving transistor, and a P-channel IC including a first constant current circuit. The active matrix display device according to claim 1, further comprising a second constant current circuit that supplies two signal currents. 2. The active matrix display device according to claim 1 , wherein the second switch is connected between a drain of the first holding switch and a drain of the second holding switch. 2. The active matrix display device according to claim 1 , wherein the second switch is connected between a drain of the second drive transistor and a drain of the second holding switch. It said first holding switch and the second holding switch, each active matrix display device according to claims 1 are formed by P-channel type thin film transistor any one of four. The first holding switch is formed by a P-channel thin film transistor, active matrix display device according to any one of the second holding switch claims 1 are formed by N-channel type thin film transistor 5 . The active matrix display device according to claim 6 , wherein the first holding switch and the second holding switch are formed in the same size. The active matrix display device according to any one of claims 1 to 7 , wherein the first switch, the second switch, and the output switch are set to a gate potential that operates in a saturation region. Said transistor, said first driving transistor, and the second driving transistor, active matrix display device according to claim 1, which consists of thin film transistor using a polysilicon semiconductor layer. The display device is an active matrix display device according to any one of claims 1 is a self-light emitting device having an organic luminescent layer between opposed electrodes 9.

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