JP2010002794A - Display device, driving method of display device, and electronic equipment - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a display image with excellent quality without spoiling uniformity of an image, even if a bootstrap gain is not in an ideal status. <P>SOLUTION: A material of high resistance value such as aluminum is used as a wiring line 81 between a scan line 31 and a gate electrode of a writing transistor 23, and its wiring length is extended, or its wiring width is narrowed, thereby resistance component R is inserted. The resistance component R permits capacitance coupling to the gate electrode of the writing transistor 23 when a source potential of a driving transistor rises. A falling speed of a writing scan signal WS is delayed by the capacitance coupling, and a mobility correction period is adjusted for each pixel. Uniformity of brightness among pixels is achieved by the adjustment of the mobility correction period. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a display device, a display device driving method, and an electronic apparatus, and more particularly to a flat panel display device in which pixels are two-dimensionally arranged in a matrix (matrix shape), and a driving method of the display device. The present invention also relates to an electronic device having the display device.

  In recent years, in the field of display devices that perform image display, flat display devices in which pixels (pixel circuits) are arranged in a matrix are rapidly spreading. As one of flat-type display devices, there is a display device using a so-called current-driven electro-optical element whose light emission luminance changes according to a current value flowing through the device as a light-emitting element of a pixel. As a current-driven electro-optical element, an organic EL (Electro Luminescence) element that utilizes a phenomenon of light emission when an electric field is applied to an organic thin film is known.

  An organic EL display device using an organic EL element as an electro-optical element of a pixel has the following features. That is, since the organic EL element can be driven with an applied voltage of 10 V or less, the power consumption is low. Since the organic EL element is a self-luminous element, the visibility of the image is higher than that of a liquid crystal display device that displays an image by controlling the light intensity from the light source with a liquid crystal for each pixel, and a backlight. Therefore, it is easy to reduce the weight and thickness. Furthermore, since the response speed of the organic EL element is as high as about several μsec, an afterimage at the time of displaying a moving image does not occur.

  As in the liquid crystal display device, the organic EL display device can adopt a simple (passive) matrix method and an active matrix method as its driving method. However, although the simple matrix display device has a simple structure, the light-emission period of the electro-optic element decreases with an increase in the number of scanning lines (that is, the number of pixels), thereby realizing a large-sized and high-definition display device. There are problems such as difficult.

  For this reason, in recent years, active matrix display devices in which the current flowing through the electro-optical element is controlled by an active element provided in the same pixel as the electro-optical element, for example, an insulated gate field effect transistor, have been actively developed. Yes. As the insulated gate field effect transistor, a TFT (Thin Film Transistor) is generally used. An active matrix display device can easily realize a large-sized and high-definition display device because the electro-optic element continues to emit light over a period of one frame.

  By the way, it is generally known that the IV characteristic (current-voltage characteristic) of the organic EL element is deteriorated with time (so-called deterioration with time). Particularly in a pixel circuit using an N-channel TFT as a transistor for driving an organic EL element with current (hereinafter referred to as “driving transistor”), if the IV characteristic of the organic EL element deteriorates with time, the gate of the driving transistor -The source voltage Vgs changes. As a result, the light emission luminance of the organic EL element changes. This is because the organic EL element is connected to the source electrode side of the driving transistor.

  This will be described more specifically. The source potential of the drive transistor is determined by the operating points of the drive transistor and the organic EL element. When the IV characteristic of the organic EL element deteriorates, the operating point of the driving transistor and the organic EL element fluctuates. Therefore, even if the same voltage is applied to the gate electrode of the driving transistor, the source potential of the driving transistor is Change. As a result, since the source-gate voltage Vgs of the drive transistor changes, the value of the current flowing through the drive transistor changes. As a result, since the value of the current flowing through the organic EL element also changes, the light emission luminance of the organic EL element changes.

  In particular, in a pixel circuit using a polysilicon TFT, in addition to deterioration of the IV characteristics of the organic EL element over time, the transistor characteristics of the drive transistor change over time, or the transistor characteristics vary depending on manufacturing processes. It is different for each. That is, the transistor characteristics of the drive transistor vary from pixel to pixel. The transistor characteristics include the threshold voltage Vth of the driving transistor, the mobility μ of the semiconductor thin film constituting the channel of the driving transistor (hereinafter simply referred to as “mobility μ of the driving transistor”), and the like.

  When the transistor characteristics of the driving transistor differ from pixel to pixel, the current value flowing through the driving transistor varies from pixel to pixel. Therefore, even if the same voltage is applied between the pixels to the gate electrode of the driving transistor, the light emission of the organic EL element The luminance varies among pixels. As a result, the uniformity (uniformity) of the screen is impaired.

  Therefore, various corrections (compensations) are made to maintain the light emission luminance of the organic EL element constant without being affected by the deterioration of the IV characteristic of the organic EL element over time or the change in the transistor characteristic of the driving transistor over time. ) A function is given to the pixel circuit (for example, see Patent Document 1).

  Examples of the correction function include a compensation function for characteristic variation of the organic EL element, a correction function for variation in the threshold voltage Vth of the drive transistor, and a correction function for variation in mobility μ of the drive transistor. Hereinafter, the correction for the variation of the threshold voltage Vth of the driving transistor is referred to as “threshold correction”, and the correction for the variation of the mobility μ of the driving transistor is referred to as “mobility correction”.

  In this way, by providing each pixel circuit with various correction functions, the organic EL element is not affected by the deterioration of the IV characteristics of the organic EL element over time or the change of the transistor characteristics of the driving transistor over time. The light emission luminance of the element can be kept constant. As a result, the display quality of the organic EL display device can be improved.

  And the compensation function with respect to the characteristic fluctuation | variation of an organic EL element is performed by the following series of circuit operations. First, the video signal supplied through the signal line is written by the writing transistor and held in the holding capacitor connected between the gate and the source of the driving transistor. After that, the writing transistor is turned off, whereby the gate electrode of the driving transistor is electrically disconnected from the signal line to be in a floating state.

  When the gate electrode of the driving transistor is in a floating state, the storage capacitor is connected between the gate and the source of the driving transistor, so that the gate of the driving transistor is interlocked with (following) the fluctuation of the source potential Vs of the driving transistor. The potential Vg also varies. Thus, the operation in which the gate potential Vg fluctuates in conjunction with the source potential Vs of the driving transistor is referred to as a bootstrap operation in this specification. By this bootstrap operation, the gate-source voltage Vgs of the driving transistor can be kept constant. As a result, even if the IV characteristic of the organic EL element changes with time, the light emission luminance of the organic EL element can be kept constant.

JP 2006-133542 A

  In the bootstrap operation described above, the ratio (= ΔVg / ΔVs) of the variation ΔVg of the gate potential Vg to the variation ΔVs of the source potential Vs of the driving transistor becomes the bootstrap gain Gb. The bootstrap gain Gb is determined by the capacitance value of the storage capacitor, the capacitance value of the parasitic capacitance attached to the gate of the driving transistor, and the like.

  Details of the bootstrap operation will be described with reference to FIG. In FIG. 20, the operation of the pixel A in which the threshold voltage Vth of the driving transistor is relatively small and the mobility μ is relatively large is indicated by a broken line. Further, the operation of the pixel B in which the threshold voltage Vth of the driving transistor is relatively large and the mobility μ is relatively small is indicated by a one-dot chain line.

  Immediately before the light emission of the organic EL element, that is, when threshold correction and mobility correction are completed, the gate-source voltage Vgs of the driving transistor differs between the pixels A and B by ΔVth which is the difference between the threshold voltages Vth. In other words, by performing threshold correction and mobility correction, the gate-source voltage Vgs of the driving transistor is made different between the pixels A and B by the difference ΔVth.

  Here, if the bootstrap gain Gb is an ideal state, that is, if Gb = 100%, the light emission state can be maintained while maintaining the difference ΔVth of the threshold voltage Vth. Kept equal. However, since the parasitic capacitance exists in the write transistor, the actual bootstrap gain Gb is not in an ideal state due to the influence of the parasitic capacitance, that is, less than 100%. As a result, the difference between the gate-source voltage Vgs of the driving transistor between the pixels A and B during light emission is ΔVth × Gb.

  That is, the gate-source voltage Vgs of the drive transistor is a loss of ΔVth × (1−Gb) compared to the case where the bootstrap gain Gb is in an ideal state (details will be described later). Therefore, the influence of the variation between the pixels of the threshold voltage Vth corrected by the threshold correction processing occurs again.

  Thus, if the bootstrap gain Gb is not in an ideal state, the light emission state cannot be maintained while maintaining the difference ΔVth of the threshold voltage Vth between the pixels with respect to the gate-source voltage Vgs of the driving transistor. Variation in luminance occurs. The luminance variation between the pixels is visually recognized as vertical stripes, horizontal stripes, and luminance unevenness. For example, as shown in FIG. 20, in the pixel region P where the threshold voltage Vth is large and the mobility μ is small, the luminance is lower than that of the surrounding region O, and the luminance unevenness is visually recognized. As a result, the uniformity of the screen is impaired.

  Therefore, the present invention provides a display device capable of obtaining a high-quality display image without impairing the uniformity of the screen even when the bootstrap gain is not in an ideal state, a method for driving the display device, and the display device. An object is to provide an electronic device used.

In order to achieve the above object, the present invention provides:
An electro-optical element; a writing transistor for writing a video signal; a driving transistor for driving the electro-optical element in accordance with the video signal written by the writing transistor; and a gate electrode and a source electrode of the driving transistor. A storage capacitor connected to hold the video signal written by the write transistor;
In a display device in which pixels that perform mobility correction processing that applies negative feedback to the potential difference between the gate and source of the driving transistor with a correction amount corresponding to the current flowing through the driving transistor are arranged in a matrix,
The period of the mobility correction process is adjusted according to the transition speed of the source potential of the driving transistor determined by the mobility of the driving transistor.

  When the writing transistor is turned on, a video signal is written, and mobility correction processing is performed in parallel with the writing of the video signal. Then, when the writing transistor becomes non-conductive and the mobility correction process ends, a current flows through the driving transistor, so that the source potential of the driving transistor starts transition, for example, rises. At this time, since the current flowing through the driving transistor is proportional to the mobility, the rising speed of the source potential of the driving transistor is fast when the mobility is large, and is slow when the mobility is small.

  Therefore, by adjusting the period of mobility correction processing (hereinafter referred to as mobility correction period) according to the rising speed (transition speed) of the source potential of the driving transistor determined by the mobility of the driving transistor, the pixel The mobility correction period can be controlled every time. Then, by controlling the mobility correction period, the mobility correction period, which is set to be constant regardless of the pixel in the initial setting, is changed to the pixel B having a relatively low mobility in the pixel A having a relatively high mobility. Longer than that. Thereby, in the pixel A having a relatively small threshold voltage and a relatively high mobility, the amount of feedback in the mobility correction process is larger than that in the initial mobility correction period. Mobility correction processing is performed in the direction of decreasing the brightness of the.

  On the other hand, in the pixel B having a relatively large threshold voltage and a relatively small mobility, if the bootstrap gain is not in an ideal state, a loss corresponding to the bootstrap gain occurs in the gate-source voltage of the driving transistor. In addition, the luminance is reduced, resulting in luminance unevenness. On the other hand, in the pixel A having a small threshold voltage and a high mobility, as described above, the mobility correction process is performed in the direction of decreasing the brightness, so that the brightness between the pixels A and B is made uniform. .

  According to the present invention, by controlling the mobility correction period for each pixel, it is possible to achieve uniform luminance between pixels even when the bootstrap gain Gb is not in an ideal state. A high-quality display image can be obtained without impairing the image quality.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[System configuration]
FIG. 1 is a system configuration diagram showing an outline of the configuration of an active matrix display device to which the present invention is applied. Here, as an example, an active matrix organic EL display device using, as an example, a current-driven electro-optical element whose emission luminance changes according to a current value flowing through the device, for example, an organic EL element as a light-emitting element of a pixel (pixel circuit) This case will be described as an example.

  As shown in FIG. 1, an organic EL display device 10 according to this application example includes a plurality of pixels 20 including light emitting elements, a pixel array unit 30 in which the pixels 20 are two-dimensionally arranged in a matrix, and the pixel array. The drive unit is disposed around the unit 30. The drive unit drives each pixel 20 of the pixel array unit 30. As the drive unit, for example, a write scanning circuit 40, a power supply scanning circuit 50, and a signal output circuit 60 are provided.

  Here, when the organic EL display device 10 supports monochrome display, one pixel serving as a unit for forming a monochrome image corresponds to the pixel 20. On the other hand, when the organic EL display device 10 supports color display, one pixel as a unit for forming a color image is composed of a plurality of sub-pixels (sub-pixels), and this sub-pixel corresponds to the pixel 20. More specifically, in a display device for color display, one pixel includes a sub-pixel that emits red light (R), a sub-pixel that emits green light (G), and a sub-pixel that emits blue light (B). It consists of three sub-pixels of a pixel.

  However, one pixel is not limited to the combination of RGB three primary color subpixels, and one pixel may be configured by adding one or more color subpixels to the three primary color subpixels. Is possible. More specifically, for example, at least one sub-pixel that emits white light (W) is added to improve luminance to form one pixel, or at least one that emits complementary color light to expand the color reproduction range. It is also possible to configure one pixel by adding subpixels.

  The pixel array unit 30 includes scanning lines 31-1 to 31-m and a power supply line 32-1 along the row direction (pixel arrangement direction of pixels in the pixel row) with respect to the arrangement of the pixels 20 in m rows and n columns. ˜32-m are wired for each pixel row. Furthermore, signal lines 33-1 to 33-n are wired for each pixel column along the column direction (pixel arrangement direction of the pixel column).

  The scanning lines 31-1 to 31 -m are connected to the output ends of the corresponding rows of the writing scanning circuit 40, respectively. The power supply lines 32-1 to 32-m are connected to the output terminals of the corresponding rows of the power supply scanning circuit 50, respectively. The signal lines 33-1 to 33-n are connected to the output ends of the corresponding columns of the signal output circuit 60, respectively.

  The pixel array unit 30 is usually formed on a transparent insulating substrate such as a glass substrate. Thereby, the organic EL display device 10 has a flat panel structure. The drive circuit for each pixel 20 in the pixel array section 30 can be formed using an amorphous silicon TFT or a low-temperature polysilicon TFT. When the low-temperature polysilicon TFT is used, the write scanning circuit 40, the power supply scanning circuit 50, and the signal output circuit 60 can also be mounted on the display panel (substrate) 70 that forms the pixel array unit 30.

  The write scanning circuit 40 is configured by a shift register or the like that sequentially shifts (transfers) the start pulse sp in synchronization with the clock pulse ck. The writing scanning circuit 40 sequentially supplies writing scanning signals WS (WS1 to WSm) to the scanning lines 31-1 to 31-m when writing video signals to the respective pixels 20 of the pixel array section 30. Each pixel 20 of the unit 30 is scanned in order in a row unit (line-sequential scanning).

  The power supply scanning circuit 50 includes a shift register that sequentially shifts the start pulse sp in synchronization with the clock pulse ck. The power supply scanning circuit 50 synchronizes with the line sequential scanning by the write scanning circuit 40 and switches between a first power supply potential Vccp and a second power supply potential Vini lower than the first power supply potential Vccp. ) To the power supply lines 32-1 to 32-m. The light emission / non-light emission of the pixel 20 is controlled by switching the power supply potential DS to Vccp / Vini.

  The signal output circuit 60 has either a signal voltage (hereinafter also simply referred to as “signal voltage”) Vsig or a reference potential Vofs of a video signal corresponding to luminance information supplied from a signal supply source (not shown). Either one is selected as appropriate and output. The signal voltage Vsig / reference potential Vofs output from the signal output circuit 60 is written in units of rows to each pixel 20 of the pixel array unit 30 via the signal lines 33-1 to 33-n. In other words, the signal output circuit 60 employs a line-sequential writing drive configuration in which the signal voltage Vsig is written in units of rows (lines).

(Pixel circuit)
FIG. 2 is a circuit diagram showing a specific circuit configuration of the pixel (pixel circuit) 20.

  As shown in FIG. 2, the pixel 20 includes a current-driven electro-optical element whose emission luminance changes according to a current value flowing through the device, for example, an organic EL element 21, and a drive circuit that drives the organic EL element 21. It is constituted by. The organic EL element 21 has a cathode electrode connected to a common power supply line 34 that is wired in common to all the pixels 20 (so-called solid wiring).

  The drive circuit that drives the organic EL element 21 has a configuration including a drive transistor 22, a write transistor 23, a storage capacitor 24, and an auxiliary capacitor 25. Here, N-channel TFTs are used as the drive transistor 22 and the write transistor 23. However, the combination of conductivity types of the drive transistor 22 and the write transistor 23 is merely an example, and is not limited to these combinations.

  Note that when an N-channel TFT is used as the driving transistor 22 and the writing transistor 23, an amorphous silicon (a-Si) process can be used. By using the a-Si process, it is possible to reduce the cost of the substrate on which the TFT is formed, and thus to reduce the cost of the organic EL display device 10. Further, when the drive transistor 22 and the write transistor 23 have the same conductivity type, both the transistors 22 and 23 can be formed by the same process, which can contribute to cost reduction.

  The drive transistor 22 has one electrode (source / drain electrode) connected to the anode electrode of the organic EL element 21 and the other electrode (drain / source electrode) connected to the power supply line 32 (32-1 to 32-m). It is connected.

  The write transistor 23 has one electrode (source / drain electrode) connected to the signal line 33 (33-1 to 33-n) and the other electrode (drain / source electrode) connected to the gate electrode of the drive transistor 22. ing. The gate electrode of the writing transistor 23 is connected to the scanning line 31 (31-1 to 31-m).

  In the drive transistor 22 and the write transistor 23, one electrode refers to a metal wiring electrically connected to the source / drain region, and the other electrode refers to a metal wiring electrically connected to the drain / source region. Say. Further, depending on the potential relationship between one electrode and the other electrode, if one electrode becomes a source electrode, it becomes a drain electrode, and if the other electrode also becomes a drain electrode, it becomes a source electrode.

  The storage capacitor 24 has one electrode connected to the gate electrode of the drive transistor 22 and the other electrode connected to the other electrode of the drive transistor 22 and the anode electrode of the organic EL element 21.

  The auxiliary capacitor 25 has one electrode connected to the anode electrode of the organic EL element 21 and the other electrode connected to the common power supply line 34. The auxiliary capacitor 25 is provided as necessary in order to compensate for the insufficient capacity of the organic EL element 21 and to increase the video signal write gain to the storage capacitor 24. That is, the auxiliary capacitor 25 is not an essential component and can be omitted when the equivalent capacitance of the organic EL element 21 is sufficiently large.

  Here, the other electrode of the auxiliary capacitor 25 is connected to the common power supply line 34. However, the connection destination of the other electrode is not limited to the common power supply line 34, and any node having a fixed potential may be used. Good. By connecting the other electrode of the auxiliary capacitor 25 to a fixed potential, the intended purpose of compensating the shortage of the capacity of the organic EL element 21 and increasing the video signal writing gain to the holding capacitor 24 can be achieved.

  In the pixel 20 configured as described above, the writing transistor 23 becomes conductive in response to a high active writing scanning signal WS applied to the gate electrode from the writing scanning circuit 40 through the scanning line 31. Thereby, the write transistor 23 samples the signal voltage Vsig or the reference potential Vofs of the video signal corresponding to the luminance information supplied from the signal output circuit 60 through the signal line 33 and writes the sampled voltage in the pixel 20. The written signal voltage Vsig or reference potential Vofs is applied to the gate electrode of the driving transistor 22 and held in the holding capacitor 24.

  When the potential DS of the power supply line 32 (32-1 to 32-m) is at the first power supply potential Vccp, the drive transistor 22 has one electrode as a drain electrode and the other electrode as a source electrode in a saturation region. Operate. As a result, the drive transistor 22 is supplied with current from the power supply line 32 and drives the organic EL element 21 to emit light by current drive. More specifically, the drive transistor 22 operates in the saturation region to supply a drive current having a current value corresponding to the voltage value of the signal voltage Vsig held in the holding capacitor 24 to the organic EL element 21. The organic EL element 21 is caused to emit light by current driving.

  Further, when the power supply potential DS is switched from the first power supply potential Vccp to the second power supply potential Vini, the drive transistor 22 operates as a switching transistor with one electrode serving as a source electrode and the other electrode serving as a drain electrode. As a result, the drive transistor 22 stops supplying the drive current to the organic EL element 21 and puts the organic EL element 21 into a non-light emitting state. That is, the drive transistor 22 also has a function as a transistor that controls light emission / non-light emission of the organic EL element 21.

  By the switching operation of the drive transistor 22, a period during which the organic EL element 21 is in a non-light emitting state (non-light emitting period) is provided, and the ratio (duty) between the light emitting period and the non-light emitting period of the organic EL element 21 is controlled. By this duty control, the afterimage blur caused by the light emission of the pixels over one frame period can be reduced, so that the quality of the moving image can be particularly improved.

  Here, the reference potential Vofs that is selectively supplied from the signal output circuit 60 through the signal line 33 corresponds to a potential that serves as a reference for the signal voltage Vsig of the video signal corresponding to the luminance information (for example, the black level of the video signal). Potential).

  Of the first and second power supply potentials Vccp and Vini selectively supplied from the power supply scanning circuit 50 through the power supply line 32, the first power supply potential Vccp generates a drive current for driving the organic EL element 21 to emit light. The power supply potential for supplying to The second power supply potential Vini is a power supply potential for applying a reverse bias to the organic EL element 21. The second power supply potential Vini is set to a potential lower than the reference potential Vofs, for example, a potential lower than Vofs−Vth, preferably sufficiently lower than Vofs−Vth when the threshold voltage of the driving transistor 22 is Vth. Is done.

(Pixel structure)
FIG. 3 is a cross-sectional view illustrating an example of the cross-sectional structure of the pixel 20. As shown in FIG. 3, a driving circuit including the driving transistor 22 and the like is formed on the glass substrate 201. The pixel 20 has a configuration in which an insulating film 202, an insulating planarizing film 203, and a window insulating film 204 are formed in this order on a glass substrate 201, and the organic EL element 21 is provided in the recess 204A of the window insulating film 204. It has become. Here, only the drive transistor 22 is illustrated among the components of the drive circuit, and other components are omitted.

  The organic EL element 21 includes an anode electrode 205, an organic layer (electron transport layer, light emitting layer, hole transport layer / hole injection layer) 206, and a cathode electrode 207. The anode electrode 205 is made of a metal or the like formed on the bottom of the recess 204A of the window insulating film 204. The organic layer 206 is formed on the anode electrode 205. The cathode electrode 207 is made of a transparent conductive film formed on the organic layer 206 in common for all pixels.

  In the organic EL element 21, the organic layer 206 is formed by sequentially depositing a hole transport layer / hole injection layer 2061, a light emitting layer 2062, an electron transport layer 2063 and an electron injection layer (not shown) on the anode electrode 205. It is formed. Then, current flows from the driving transistor 22 to the organic layer 206 through the anode electrode 205 under current driving by the driving transistor 22 in FIG. 2, so that electrons and holes are recombined in the light emitting layer 2062 in the organic layer 206. It is designed to emit light.

  The drive transistor 22 includes a gate electrode 221, source / drain regions 223 and 224 provided on both sides of the semiconductor layer 222, and a channel formation region 225 at a portion facing the gate electrode 221 of the semiconductor layer 222. . The source / drain region 223 is electrically connected to the anode electrode 205 of the organic EL element 21 through a contact hole.

  Then, as shown in FIG. 3, after the organic EL element 21 is formed on the glass substrate 201 through the insulating film 202, the insulating planarizing film 203, and the window insulating film 204, the passivation film 208 is formed. Then, the sealing substrate 209 is bonded by the adhesive 210. The display panel 70 is formed by sealing the organic EL element 21 with the sealing substrate 209.

(Circuit operation of organic EL display device)
Next, the circuit operation of the organic EL display device 10 in which the pixels 20 having the above-described configuration are two-dimensionally arranged in a matrix will be described with reference to the operation waveform diagrams of FIGS. 5 and 6 based on the timing waveform diagram of FIG. To do. In the operation explanatory diagrams of FIGS. 5 and 6, the write transistor 23 is illustrated by a switch symbol for simplification of the drawing.

  The timing waveform diagram of FIG. 4 shows changes in the potential (writing scanning signal) WS of the scanning lines 31 (31-1 to 31-m) and the potentials (power supply potentials) of the power supply lines 32 (32-1 to 32-m). ) Changes in DS and changes in the gate potential Vg and source potential Vs of the drive transistor 22 are shown. Further, the waveform of the gate potential Vg is indicated by a one-dot chain line, and the waveform of the source potential Vs is indicated by a dotted line so that the two can be identified.

<Light emission period of previous frame>
In the timing waveform diagram of FIG. 4, the period before time t1 is the light emission period of the organic EL element 21 in the previous frame (field). In the light emission period of the previous frame, the potential DS of the power supply line 32 is at the first power supply potential (hereinafter referred to as “high potential”) Vccp, and the writing transistor 23 is in a non-conduction state.

  At this time, the drive transistor 22 is set to operate in a saturation region. As a result, as shown in FIG. 5A, the drive current (drain-source current) Ids according to the gate-source voltage Vgs of the drive transistor 22 passes from the power supply line 32 through the drive transistor 22 to the organic EL element. 21 is supplied. Therefore, the organic EL element 21 emits light with a luminance corresponding to the current value of the drive current Ids.

<Threshold correction preparation period>
At time t1, a new frame (current frame) for line sequential scanning is entered. As shown in FIG. 5B, the second power supply potential (hereinafter, referred to as the potential DS of the power supply line 32 is sufficiently lower than Vofs−Vth with respect to the reference potential Vofs of the signal line 33 from the high potential Vccp. Switch to Vini) (described as “low potential”).

  Here, the threshold voltage of the organic EL element 21 is Vthel, and the potential of the common power supply line 34 (cathode potential) is Vcath. At this time, if the low potential Vini is Vini <Vthel + Vcath, the source potential Vs of the drive transistor 22 is substantially equal to the low potential Vini, so that the organic EL element 21 is in a reverse bias state and extinguished.

  Next, when the potential WS of the scanning line 31 transits from the low potential side to the high potential side at time t2, as shown in FIG. 5C, the writing transistor 23 becomes conductive. At this time, since the reference potential Vofs is supplied from the signal output circuit 60 to the signal line 33, the gate potential Vg of the drive transistor 22 becomes the reference potential Vofs. Further, the source potential Vs of the driving transistor 22 is at a potential Vini that is sufficiently lower than the reference potential Vofs.

  At this time, the gate-source voltage Vgs of the drive transistor 22 is Vofs-Vini. Here, if Vofs−Vini is not larger than the threshold voltage Vth of the drive transistor 22, threshold correction processing described later cannot be performed, and therefore it is necessary to set a potential relationship of Vofs−Vini> Vth.

  As described above, the process of fixing (initializing) the gate potential Vg of the drive transistor 22 to the reference potential Vofs and the source potential Vs to the low potential Vini is a preparation before performing a threshold correction process described later. (Threshold correction preparation) processing. Therefore, the reference potential Vofs and the low potential Vini become the initialization potentials of the gate potential Vg and the source potential Vs of the drive transistor 22, respectively.

<Threshold correction period>
Next, at time t3, as shown in FIG. 5D, when the potential DS of the power supply line 32 is switched from the low potential Vini to the high potential Vccp, the threshold value is maintained while the gate potential Vg of the drive transistor 22 is maintained. The correction process is started. That is, the source potential Vs of the drive transistor 22 starts to increase toward the potential obtained by subtracting the threshold voltage Vth of the drive transistor 22 from the gate potential Vg.

  Here, for convenience, processing for changing the source potential Vs toward the potential obtained by subtracting the threshold voltage Vth of the drive transistor 22 from the initialization potential Vofs with reference to the initialization potential Vofs of the gate electrode of the drive transistor 22 is corrected by the threshold value. This is called processing. As the threshold correction process proceeds, the gate-source voltage Vgs of the drive transistor 22 eventually converges to the threshold voltage Vth of the drive transistor 22. A voltage corresponding to the threshold voltage Vth is held in the holding capacitor 24.

  In the period for performing the threshold correction process (threshold correction period), the organic EL element 21 is cut off in order to prevent the current from flowing exclusively to the storage capacitor 24 and not to the organic EL element 21. As described above, the potential Vcath of the common power supply line 34 is set.

  Next, at time t4, the potential WS of the scanning line 31 transitions to the low potential side, so that the writing transistor 23 is turned off as illustrated in FIG. At this time, the gate electrode of the driving transistor 22 is electrically disconnected from the signal line 33 to be in a floating state. However, since the gate-source voltage Vgs is equal to the threshold voltage Vth of the drive transistor 22, the drive transistor 22 is in a cutoff state. Therefore, the drain-source current Ids does not flow through the driving transistor 22.

<Signal writing & mobility correction period>
Next, at time t5, as shown in FIG. 6B, the potential of the signal line 33 is switched from the reference potential Vofs to the signal voltage Vsig of the video signal. Subsequently, at time t6, the potential WS of the scanning line 31 transitions to the high potential side, so that the writing transistor 23 becomes conductive as shown in FIG. 6C, and the signal voltage Vsig of the video signal is sampled. To write in the pixel 20.

  By writing the signal voltage Vsig by the writing transistor 23, the gate potential Vg of the driving transistor 22 becomes the signal voltage Vsig. When the driving transistor 22 is driven by the signal voltage Vsig of the video signal, the threshold voltage Vth of the driving transistor 22 is canceled with a voltage corresponding to the threshold voltage Vth held in the holding capacitor 24. Details of the principle of threshold cancellation will be described later.

  At this time, the organic EL element 21 is in a cutoff state (high impedance state). Therefore, a current (drain-source current Ids) that flows from the power supply line 32 to the drive transistor 22 in accordance with the signal voltage Vsig of the video signal flows into the auxiliary capacitor 25. Therefore, charging of the auxiliary capacitor 25 is started.

  As the auxiliary capacitor 25 is charged, the source potential Vs of the drive transistor 22 rises with time. At this time, the pixel-to-pixel variation in the threshold voltage Vth of the drive transistor 22 has already been cancelled, and the drain-source current Ids of the drive transistor 22 depends on the mobility μ of the drive transistor 22.

  Here, it is assumed that the ratio of the holding voltage Vgs of the holding capacitor 24 to the signal voltage Vsig of the video signal, that is, the writing gain is 1 (ideal value). Then, the source potential Vs of the drive transistor 22 rises to the potential of Vofs−Vth + ΔV, so that the gate-source voltage Vgs of the drive transistor 22 becomes Vsig−Vofs + Vth−ΔV.

  That is, the increase ΔV of the source potential Vs of the drive transistor 22 is subtracted from the voltage (Vsig−Vofs + Vth) held in the holding capacitor 24, in other words, the charge of the holding capacitor 24 is discharged. And negative feedback was applied. Therefore, the increase ΔV of the source potential Vs becomes a feedback amount of negative feedback.

  In this way, by applying negative feedback to the gate-source voltage Vgs with a feedback amount ΔV corresponding to the drain-source current Ids flowing through the drive transistor 22, the mobility μ of the drain-source current Ids of the drive transistor 22. The dependence on can be negated. This canceling process is a mobility correction process for correcting the variation of the mobility μ of the driving transistor 22 for each pixel.

  More specifically, since the drain-source current Ids increases as the signal amplitude Vin (= Vsig−Vofs) of the video signal written to the gate electrode of the drive transistor 22 increases, the absolute value of the feedback amount ΔV of the negative feedback increases. The value also increases. Therefore, mobility correction processing according to the light emission luminance level is performed.

  Further, when the signal amplitude Vin of the video signal is constant, the absolute value of the feedback amount ΔV of the negative feedback increases as the mobility μ of the drive transistor 22 increases. Can do. Therefore, it can be said that the feedback amount ΔV of the negative feedback is a correction amount for mobility correction. Details of the principle of mobility correction will be described later.

<Light emission period>
Next, at time t7, the potential WS of the scanning line 31 shifts to the low potential side, so that the writing transistor 23 is turned off as illustrated in FIG. 6D. As a result, the gate electrode of the drive transistor 22 is electrically disconnected from the signal line 33 and is in a floating state.

  Here, when the gate electrode of the driving transistor 22 is in a floating state, the storage capacitor 24 is connected between the gate and the source of the driving transistor 22, so that the driving transistor 22 is interlocked with the change in the source potential Vs. The gate potential Vg also varies. Thus, the operation in which the gate potential Vg of the drive transistor 22 varies in conjunction with the variation in the source potential Vs is a bootstrap operation by the storage capacitor 24.

  The gate electrode of the drive transistor 22 enters a floating state, and at the same time, the drain-source current Ids of the drive transistor 22 starts to flow into the organic EL element 21, whereby the anode potential of the organic EL element 21 is set according to the current Ids. To rise.

  When the anode potential of the organic EL element 21 exceeds Vthel + Vcath, the drive current starts to flow through the organic EL element 21, and the organic EL element 21 starts to emit light. The increase in the anode potential of the organic EL element 21 is nothing but the increase in the source potential Vs of the drive transistor 22. When the source potential Vs of the drive transistor 22 rises, the gate potential Vg of the drive transistor 22 also rises in conjunction with the bootstrap operation of the storage capacitor 24.

  At this time, when it is assumed that the bootstrap gain is 1 (ideal state), the increase amount of the gate potential Vg is equal to the increase amount of the source potential Vs. Therefore, the gate-source voltage Vgs of the drive transistor 22 is kept constant at Vsig−Vofs + Vth−ΔV during the light emission period. At time t8, the potential of the signal line 33 is switched from the signal voltage Vsig of the video signal to the reference potential Vofs.

  In the series of circuit operations described above, each processing operation of threshold correction preparation, threshold correction, signal voltage Vsig writing (signal writing), and mobility correction is executed in one horizontal scanning period (1H). Further, the signal writing and mobility correction processing operations are executed in parallel during the period from time t6 to time t7.

(Threshold cancellation principle)
Here, the principle of threshold cancellation (that is, threshold correction) of the drive transistor 22 will be described. The drive transistor 22 operates as a constant current source because it is designed to operate in the saturation region. As a result, a constant drain-source current (drive current) Ids given by the following equation (1) is supplied from the drive transistor 22 to the organic EL element 21.
Ids = (1/2) · μ (W / L) Cox (Vgs−Vth) ² (1)
Here, W is the channel width of the drive transistor 22, L is the channel length, and Cox is the gate capacitance per unit area.

  FIG. 7 shows characteristics of the drain-source current Ids of the drive transistor 22 versus the gate-source voltage Vgs.

  As shown in this characteristic diagram, if no cancellation process is performed for the variation of the threshold voltage Vth of the drive transistor 22 for each pixel, the drain-source current corresponding to the gate-source voltage Vgs when the threshold voltage Vth is Vth1. Ids becomes Ids1.

  On the other hand, when the threshold voltage Vth is Vth2 (Vth2> Vth1), the drain-source current Ids corresponding to the same gate-source voltage Vgs is Ids2 (Ids2 <Ids). That is, when the threshold voltage Vth of the drive transistor 22 varies, the drain-source current Ids varies even if the gate-source voltage Vgs is constant.

On the other hand, in the pixel (pixel circuit) 20 having the above configuration, as described above, the gate-source voltage Vgs of the drive transistor 22 at the time of light emission is Vsig−Vofs + Vth−ΔV. Therefore, when this is substituted into the equation (1), the drain-source current Ids is expressed by the following equation (2).
Ids = (1/2) · μ (W / L) Cox (Vsig−Vofs−ΔV) ²
(2)

  That is, the term of the threshold voltage Vth of the drive transistor 22 is canceled, and the drain-source current Ids supplied from the drive transistor 22 to the organic EL element 21 does not depend on the threshold voltage Vth of the drive transistor 22. As a result, even if the threshold voltage Vth of the drive transistor 22 varies from pixel to pixel due to variations in the manufacturing process of the drive transistor 22 and changes over time, the drain-source current Ids does not vary. The brightness can be kept constant.

(Principle of mobility correction)
Next, the principle of mobility correction of the drive transistor 22 will be described. FIG. 8 shows a characteristic curve in a state where a pixel A having a relatively high mobility μ of the driving transistor 22 and a pixel B having a relatively low mobility μ of the driving transistor 22 are compared. When the driving transistor 22 is composed of a polysilicon thin film transistor or the like, it is inevitable that the mobility μ varies between pixels like the pixel A and the pixel B.

  Consider a case where the signal amplitude Vin (= Vsig−Vofs) of the same level is written to both the pixels A and B, for example, in the gate electrode of the drive transistor 22 in a state where the mobility μ varies between the pixel A and the pixel B. In this case, if the mobility μ is not corrected at all, it is between the drain-source current Ids1 ′ flowing through the pixel A having a high mobility μ and the drain-source current Ids2 ′ flowing through the pixel B having a low mobility μ. There will be a big difference. Thus, when a large difference occurs between the pixels in the drain-source current Ids due to the variation in mobility μ from pixel to pixel, the uniformity of the screen is impaired.

  Here, as is clear from the transistor characteristic equation of Equation (1), the drain-source current Ids increases when the mobility μ is large. Therefore, the feedback amount ΔV in the negative feedback increases as the mobility μ increases. As shown in FIG. 8, the feedback amount ΔV1 of the pixel A having a high mobility μ is larger than the feedback amount ΔV2 of the pixel B having a low mobility.

  Therefore, by applying negative feedback to the gate-source voltage Vgs with the feedback amount ΔV corresponding to the drain-source current Ids of the drive transistor 22 by the mobility correction processing, the negative feedback is increased as the mobility μ is increased. become. As a result, variation in mobility μ for each pixel can be suppressed.

  Specifically, when the feedback amount ΔV1 is corrected in the pixel A having a high mobility μ, the drain-source current Ids greatly decreases from Ids1 ′ to Ids1. On the other hand, since the feedback amount ΔV2 of the pixel B having a low mobility μ is small, the drain-source current Ids decreases from Ids2 ′ to Ids2, and does not decrease that much. As a result, since the drain-source current Ids1 of the pixel A and the drain-source current Ids2 of the pixel B are substantially equal, the variation in mobility μ from pixel to pixel is corrected.

  In summary, when there are a pixel A and a pixel B having different mobility μ, the feedback amount ΔV1 of the pixel A having a high mobility μ is larger than the feedback amount ΔV2 of the pixel B having a low mobility μ. That is, the larger the mobility μ, the larger the feedback amount ΔV, and the larger the amount of decrease in the drain-source current Ids.

  Therefore, by applying negative feedback to the gate-source voltage Vgs with a feedback amount ΔV corresponding to the drain-source current Ids of the driving transistor 22, the current value of the drain-source current Ids of the pixels having different mobility μ. Is made uniform. As a result, variation in mobility μ for each pixel can be corrected. That is, the process for applying negative feedback to the gate-source voltage Vgs of the drive transistor 22 with the feedback amount ΔV corresponding to the current flowing through the drive transistor 22 (drain-source current Ids) is the mobility correction process.

  Here, in the pixel (pixel circuit) 20 shown in FIG. 2, the relationship between the signal voltage Vsig of the video signal and the drain-source current Ids of the drive transistor 22 depending on the presence or absence of threshold correction and mobility correction is shown in FIG. I will explain.

  In FIG. 9, (A) does not perform both threshold correction and mobility correction, (B) does not perform mobility correction, and performs only threshold correction, (C) performs threshold correction and mobility correction. Each case is shown. As shown in FIG. 9A, when neither threshold correction nor mobility correction is performed, the drain-source current Ids is caused by variations in the threshold voltage Vth and the mobility μ for each of the pixels A and B. A large difference occurs between the pixels A and B.

  On the other hand, when only the threshold correction is performed, as shown in FIG. 9B, although the variation in the drain-source current Ids can be reduced to some extent, it is caused by the variation in the mobility μ between the pixels A and B. The difference between the drain-source current Ids between the pixels A and B to be left remains. Then, by performing both the threshold correction and the mobility correction, as shown in FIG. 9C, the drain between the pixels A and B due to the variation of the threshold voltage Vth and the mobility μ for each of the pixels A and B. -The difference in the current Ids between the sources can be almost eliminated. Therefore, the luminance variation of the organic EL element 21 does not occur at any gradation, and a display image with good image quality can be obtained.

  Further, the pixel 20 shown in FIG. 2 has the function of bootstrap operation by the holding capacitor 24 described above in addition to the correction functions of threshold correction and mobility correction. Obtainable.

  That is, even if the source potential Vs of the drive transistor 22 changes with time-dependent changes in the IV characteristics of the organic EL element 21, the gate-source potential Vgs of the drive transistor 22 is set by the bootstrap operation by the storage capacitor 24. Can be kept constant. Therefore, the current flowing through the organic EL element 21 does not change and is constant. As a result, since the light emission luminance of the organic EL element 21 is kept constant, even if the IV characteristic of the organic EL element 21 changes with time, it is possible to realize image display without luminance deterioration associated therewith.

(About bootstrap gain Gb)
In the description so far, it is assumed that the bootstrap gain Gb is in an ideal state (Gb = 100%). However, due to the presence of the parasitic capacitance in the driving transistor 22, the actual bootstrap gain Gb is not in an ideal state, that is, less than 100% due to the influence of the regulation capacitance.

Here, assuming that the capacitance values of the parasitic capacitance between the gate and the source of the driving transistor 22 are Cgs, Cgd, the capacitance value of the parasitic capacitance of the write transistor 23 is Cws, and the capacitance value of the storage capacitor 24 is Cs. The bootstrap gain Gb is given by the following equation (3).
Gb = (Cs + Cgs) / (Cs + Cgs + Cgd + Cws) (3)
As is apparent from this equation (3), the presence of the parasitic capacitance attached to the gate electrode of the driving transistor 22, particularly the parasitic capacitance between the gate and drain of the driving transistor 22 and the parasitic capacitance of the writing transistor 23, The strap gain Gb is not in an ideal state.

  As described above, the current value that determines the light emission luminance of the organic EL element 21, that is, the drain-source current Ids of the drive transistor 21 is given by the equation (1). Here, consider the gate-source voltage Vgs of the drive transistor 22 in the equation (1). In FIG. 20, a pixel having a relatively small threshold voltage Vth and a relatively high mobility μ is referred to as a pixel A, and a pixel having a relatively large threshold voltage Vth and a mobility μ of a driving transistor is a pixel. B.

Then, the gate-source voltage Vgs of the drive transistor 22 immediately before the bootstrap operation, that is, immediately after the end of the mobility correction process is set to pixel A: Vgs1 and pixel B: Vgs2, respectively. Further, it is assumed that the source potential Vs of the driving transistor 22 is increased by ΔVs1 in the pixel A and increased by ΔVs2 in the pixel B in the bootstrap operation. When the difference between the threshold voltages Vth of the pixels A and B is ΔVth,
Vgs1 = Vgs2 + ΔVth, ΔVs1 = ΔVs2 + ΔVth (4)
It is.

The gate-source voltage Vgs of the drive transistor 22 immediately after the bootstrap operation is respectively
Pixel A: VgsA = Vgs1−ΔVs1 × (1−Gb)
Pixel B: VgsB = Vgs2−ΔVs2 × (1−Gb)
It is represented by

Here, from equation (4),
VgsA = Vgs1−ΔVs1 × (1−Gb)
= Vgs2 + ΔVth− (ΔVs2 + ΔVth) × (1-Gb)
= Vgs2 + ΔVth−ΔVs2 × (1−Gb) −ΔVth × (1−Gb)
It becomes.

Therefore, the difference (VgsA−VgsB) in the gate-source voltage Vgs between the pixel A and the pixel B immediately after the bootstrap operation is
VgsA−VgsB = ΔVth−ΔVth × (1−Gb)
= ΔVth × Gb
It becomes.

  That is, after the threshold correction process, the gate-source voltage Vgs difference (VgsA−VgsB) is intentionally set to “Vth1−Vth2 = ΔVth”, but after the bootstrap operation, “ΔVth × Gb ”. Then, the difference “ΔVth × (1−Gb)” of the gate-source voltage Vgs between the pixel A and the pixel B becomes a variation remaining in the final light emission luminance. Specifically, in FIG. 20, in the region P of the pixel B where the threshold voltage Vth is large and the mobility μ is small, the luminance is lower than that of the region O of the surrounding pixel A, and the luminance unevenness is visually recognized.

  If mobility correction is possible and there is virtually no difference in the constant β between the pixels A and B, the difference in drive current (light emission current) of the organic EL elements 21 of the pixels A and B is as follows. . Here, the drive current of the organic EL element 21 of the pixel A is IdsA, and the drive current of the organic EL element 21 of the pixel B is IdsB. The constant β is μ (W / L) Cox in the equation (1).

In other words, the difference | IdsA−IdsB |
| IdsA−IdsB | = | (β / 2) (ΔVth × Gb−ΔVth) ² |
= | (Β / 2) [ΔVth × (1-Gb) ² ] |
It becomes. Therefore, unless the bootstrap gain Gb is in an ideal state (100%), a loss occurs in the gate-source voltage Vgs of the drive transistor 22 between the pixels A and B, resulting in luminance unevenness and the like, thereby degrading the image quality.

[Characteristics of this embodiment]
In the present embodiment, even if the bootstrap gain Gb is not in an ideal state, the following configuration is adopted in order to obtain a high-quality display image without impairing the screen uniformity. That is, the mobility correction period (mobility correction processing period) is adjusted in accordance with the transition speed (rising speed in this example) of the source potential Vs of the drive transistor 22 determined by the mobility μ of the drive transistor 22. To do.

First, in the initial setting, the mobility correction period t is set based on the following equation (5).
t = C / (kμVsig) (5)
Here, the constant k is k = (1/2) (W / L) Cox. Further, C is a capacity of a node that is discharged when the mobility correction is performed. In the circuit example of FIG. 2, C is an equivalent capacity of the organic EL element 21, a combined capacity of the holding capacity 24 and the auxiliary capacity 25.

  This initial mobility correction period t is set in common for all pixels. In the present embodiment, the mobility correction period t is adjusted for each pixel in accordance with the rising speed of the source potential Vs of the drive transistor 22. As apparent from the equation (1), since the drain-source current Ids of the drive transistor 22 is proportional to the mobility μ, the rising speed of the source potential Vs of the drive transistor 22 is fast when the mobility μ is large. Slow when degree μ is small.

  Therefore, the mobility correction period can be controlled for each pixel by adjusting the mobility correction period according to the rising speed of the source potential Vs of the drive transistor 22. By controlling the mobility correction period, the mobility correction period t, which is set to be constant regardless of the pixel by default, is relatively small in the pixel A where the mobility μ is relatively large. It becomes longer than the pixel B. Thereby, in the pixel A having a relatively small threshold voltage Vth and a relatively large mobility μ, the feedback amount ΔV in the mobility correction process is larger than in the case of the mobility correction period t set as an initial setting. Therefore, the mobility correction process is performed in the direction of decreasing the luminance of the pixel.

  On the other hand, in the pixel B where the threshold voltage Vth is relatively large and the mobility μ is relatively small, if the bootstrap gain Gb is not in an ideal state, the gate-source voltage Vgs of the drive transistor 22 is set to the gate transistor voltage Vgs as described above. A loss corresponding to the bootstrap gain Gb occurs. As a result, the luminance is reduced and luminance unevenness is caused, and the image quality is deteriorated.

  On the other hand, as described above, by adjusting the mobility correction period according to the rising speed of the source potential Vs of the driving transistor 22, the mobility correction processing is performed in the pixel A in the direction of decreasing the luminance. In addition, the luminance between the pixels A and B is made uniform. As a result, a high quality display image can be obtained without impairing the uniformity of the screen. Hereinafter, a specific example for adjusting the mobility correction period according to the rising speed of the source potential Vs of the drive transistor 22 will be described.

(Example)
FIG. 10 is a plan pattern diagram showing the wiring structure of the main part of the pixel portion according to one embodiment of the present invention. In FIG. 10, the same parts as those in FIG. 1 and FIG. .

  In FIG. 10, the scanning line 31 is wired by a material generally having a low resistance value per unit, such as aluminum (Al). Further, between the scanning line 31 and the gate electrode of the writing transistor 23, a wiring 81 made of a high resistance material generally having a high resistance value per unit such as molybdenum (Mo) or polysilicon (ps). Are electrically connected. The wiring 81 is electrically connected to the scanning line 31 by the contact portion 82.

  Here, the wiring 81 generally connects the scanning line 31 and the gate electrode of the writing transistor 23 with the shortest distance, as indicated by a broken line. On the other hand, in this embodiment, the wiring 81 is bent so that the wiring resistance of the wiring 81 between the scanning line 31 and the gate electrode of the writing transistor 23 is increased. As the wiring length of the wiring 81 is increased, the wiring resistance of the wiring 81 can be increased.

  Note that here, the wiring 81 is bent and the wiring 81 has a long wiring length, thereby increasing the wiring resistance of the wiring 81. However, by reducing the wiring width of the wiring 81, the wiring 81 The wiring resistance can also be increased. That is, the resistance component between the scanning line 31 and the gate electrode of the writing transistor 23 is determined by the material of the wiring 81 (high resistance material), and is determined by the wiring length and wiring width of the wiring 81.

  The signal line 33 is basically wired by a material such as aluminum, but is electrically connected by a contact portion 83 to a wiring such as molybdenum that is wired in a different layer at the intersection with the scanning line 31. The write transistor 23 is formed by aligning a semiconductor layer 84 made of polysilicon or the like and a gate electrode, and the source region / drain region on one side of the gate electrode is electrically connected to the signal line 33 by the contact portion 85. Connected.

  FIG. 11 shows an equivalent circuit of the pixel 20A according to this embodiment. In FIG. 11, the same parts as those in FIG. As described above, by using a high resistance material such as aluminum as the wiring 81 between the scanning line 31 and the gate electrode of the writing transistor 23, a resistance is provided between the scanning line 31 and the gate electrode of the writing transistor 23. Component R can be interposed. At this time, the resistance value of the resistance component R can be set higher by adopting a technique in which the wiring length of the wiring 81 is set longer, the wiring width is set narrower, or both.

  In the following, the operational effects associated with interposing the resistance component R between the scanning line 31 and the gate electrode of the writing transistor 23 will be described.

  First, when the source potential Vs of the drive transistor 22 rises due to the presence of the resistance component R, the gate of the write transistor 23 is passed through the storage capacitor 24 and the parasitic capacitance Cws of the write transistor 23 as shown by a broken line in FIG. Capacitive coupling enters the electrode. That is, the resistance component R is applied to the gate electrode of the write transistor 23 due to the transition of the gate potential Vg at the transition of the gate potential Vg in conjunction with the transition of the source potential Vs of the driving transistor 22 when the mobility correction process is completed. Acts to allow capacitive coupling.

  Specifically, when the mobility correction process is completed at time t7 in FIG. 4, a current flows through the drive transistor 22 so that the source potential Vs of the drive transistor 22 rises. At this time, the drain-source voltage Vds of the drive transistor 22 is proportional to the mobility μ of the drive transistor 22 as is apparent from the equation (1). Therefore, the rising speed of the source potential Vs of the driving transistor 22 is fast when the mobility μ is large, and is slow when the mobility μ is small.

  Here, in order to facilitate understanding, as in the case described above, the threshold voltage Vth is relatively small and the mobility μ is relatively large, and the threshold voltage Vth is relatively large. A description will be given by taking a pixel B having a relatively small μ as an example.

  When the mobility correction process is completed and light emission starts, the pixel A having a small threshold voltage Vth and a large mobility μ increases the source potential Vs of the drive transistor 22 as compared with the pixel B having a small mobility μ. Therefore, more coupling enters the gate electrode of the write transistor 23 through the storage capacitor 24 and the parasitic capacitance Cws of the write transistor 23. The timing at which this coupling enters is time t7 in FIG. 4, that is, the falling timing of the write scanning signal WS.

  Then, the gate potential of the write transistor 23 changes in a direction that increases due to coupling due to the rise of the gate potential Vs of the drive transistor 22. That is, the change in the gate potential of the write transistor 23 is in a direction against the falling edge of the write scan signal WS. Accordingly, as shown in FIG. 12, in the pixel A (solid line) with a small threshold voltage Vth and a high mobility μ, the falling edge of the write scanning signal WS is lower than that of the pixel B (broken line) with a small mobility μ. The speed is delayed.

  Here, the mobility correction period is determined by the rising speed and falling speed of the write scanning signal WS with respect to the threshold voltage Vth of the driving transistor 22. Therefore, the mobility correction period of the pixel A becomes longer than that of the pixel B because the falling speed of the writing scanning signal WS is delayed in the pixel A as compared with the pixel B. As a result, the correction works in the direction of decreasing the luminance of the pixel A having a small threshold voltage Vth and a high mobility μ, so that the luminance between the pixels A and B is made uniform. As described above, the pixel B having a large threshold voltage Vth and a small mobility μ is a pixel B whose luminance is reduced due to the bootstrap gain Gb being not in an ideal state, resulting in luminance unevenness.

  It should be noted that when the coupling due to the rise of the gate potential Vs of the drive transistor 22 is applied to the gate electrode of the write transistor 23, the parasitic capacitance Cws of the write transistor 23 may be insufficient. In such a case, a capacitor 90 may be added between the source electrode of the drive transistor 22 and the gate electrode of the write transistor 23 as shown in FIG. As a result, the capacity shortage of the parasitic capacitance Cws of the write transistor 23 can be compensated, so that the effect of coupling due to the rise of the gate potential Vs of the drive transistor 22 can be obtained more reliably.

[Modification of Embodiment]
In the above embodiment, the driving circuit of the organic EL element 21 is basically described as an example of the pixel configuration including the two transistors of the driving transistor 22 and the writing transistor 23. However, the present invention is not limited to this pixel configuration. The application is not limited to. That is, the present invention can be applied to a pixel configuration in which light emission / non-light emission control of the organic EL element 21 is controlled by switching the potential (power supply potential) DS of the power supply line 32 that supplies a drive current to the drive transistor 22. It is.

  As an example, as shown in FIG. 14, in addition to the drive transistor 22 and the write transistor 23, a pixel 20 ′ having a configuration including five transistors including a light emission control transistor 26 and two switching transistors 27 and 28 is known. (For example, refer to JP-A-2005-345722). Here, a Pch transistor is used as the light emission control transistor 26 and an Nch is used as the switching transistors 27 and 28, but the combination of these conductivity types is arbitrary.

  The light emission control transistor 26 is connected in series to the drive transistor 22, and selectively controls the light emission / non-light emission of the organic EL element 21 by selectively supplying the high potential Vccp to the drive transistor 22. The switching transistor 27 initializes the gate potential Vg to the reference potential Vofs by selectively applying the reference potential Vofs to the gate electrode of the drive transistor 22. The switching transistor 28 initializes the source potential Vs to the low potential Vini by selectively applying the low potential Vini to the source electrode of the drive transistor 22.

  Here, as another pixel configuration, a configuration including five transistors is taken as an example. For example, by supplying the reference potential Vofs through the signal line 33 and writing the reference potential Vofs by the write transistor 23. Various pixel configurations such as omitting the switching transistor 27 are conceivable.

  In the above embodiment, the case where the present invention is applied to an organic EL display device using an organic EL element as the electro-optical element of the pixel 20 has been described as an example. However, the present invention is not limited to this application example. . Specifically, the present invention relates to a display device using a current-driven electro-optical element (light-emitting element) such as an inorganic EL element, an LED element, or a semiconductor laser element whose emission luminance changes according to the current value flowing through the device. Applicable to all.

[Application example]
The display device according to the present invention described above can be applied to display devices of electronic devices in various fields that display video signals input to electronic devices or video signals generated in electronic devices as images or videos. Is possible. As an example, the present invention can be applied to various electronic devices shown in FIGS. 15 to 19 such as a digital camera, a notebook personal computer, a mobile terminal device such as a mobile phone, and a display device such as a video camera.

  In this manner, by using the display device according to the present invention as a display device for electronic devices in all fields, high-quality image display can be performed in various electronic devices. That is, as is apparent from the above description of the embodiment, the display device according to the present invention can obtain a high-quality display image without impairing the uniformity of the screen even when the bootstrap gain is not in an ideal state. Therefore, a high-quality display image can be obtained.

  The display device according to the present invention includes a module-shaped one having a sealed configuration. For example, a display module formed by attaching a facing portion such as transparent glass to the pixel array portion 30 is applicable. The transparent facing portion may be provided with a color filter, a protective film, and the like, and further the above-described light shielding film. Note that the display module may be provided with a circuit unit for inputting / outputting a signal to the pixel array unit from the outside, an FPC (flexible printed circuit), and the like.

  Specific examples of electronic devices to which the present invention is applied will be described below.

  FIG. 15 is a perspective view showing an appearance of a television set to which the present invention is applied. The television set according to this application example includes a video display screen unit 101 including a front panel 102, a filter glass 103, and the like, and is created by using the display device according to the present invention as the video display screen unit 101.

  16A and 16B are perspective views showing the external appearance of a digital camera to which the present invention is applied. FIG. 16A is a perspective view seen from the front side, and FIG. 16B is a perspective view seen from the back side. The digital camera according to this application example includes a light emitting unit 111 for flash, a display unit 112, a menu switch 113, a shutter button 114, and the like, and is manufactured by using the display device according to the present invention as the display unit 112.

  FIG. 17 is a perspective view showing the appearance of a notebook personal computer to which the present invention is applied. A notebook personal computer according to this application example includes a main body 121 including a keyboard 122 that is operated when characters and the like are input, a display unit 123 that displays an image, and the like, and the display device according to the present invention is used as the display unit 123. It is produced by this.

  FIG. 18 is a perspective view showing the appearance of a video camera to which the present invention is applied. The video camera according to this application example includes a main body part 131, a lens 132 for photographing an object on the side facing forward, a start / stop switch 133 at the time of photographing, a display part 134, etc., and the display part 134 according to the present invention. It is manufactured by using a display device.

  FIG. 19 is an external view showing a mobile terminal device to which the present invention is applied, for example, a mobile phone, (A) is a front view in an open state, (B) is a side view thereof, and (C) is closed. (D) is a left side view, (E) is a right side view, (F) is a top view, and (G) is a bottom view. A cellular phone according to this application example includes an upper casing 141, a lower casing 142, a connecting portion (here, a hinge portion) 143, a display 144, a sub-display 145, a picture light 146, a camera 147, and the like. Then, by using the display device according to the present invention as the display 144 or the sub display 145, the mobile phone according to this application example is manufactured.

1 is a system configuration diagram showing an outline of a configuration of an organic EL display device to which the present invention is applied. It is a circuit diagram which shows the circuit structure of a pixel. It is sectional drawing which shows an example of the cross-sectional structure of a pixel. It is a timing waveform diagram with which it uses for description of the circuit operation | movement of the organic electroluminescence display which concerns on this application example. It is explanatory drawing (the 1) of the circuit operation | movement of the organic electroluminescence display which concerns on this application example. It is explanatory drawing (the 2) of the circuit operation | movement of the organic electroluminescence display which concerns on this application example. It is a characteristic view with which it uses for description of the subject resulting from the dispersion | variation in the threshold voltage Vth of a drive transistor. It is a characteristic view with which it uses for description of the subject resulting from the dispersion | variation in the mobility (mu) of a drive transistor. FIG. 10 is a characteristic diagram for explaining the relationship between the signal voltage Vsig of the video signal and the drain-source current Ids of the drive transistor depending on whether threshold correction and mobility correction are performed. It is a plane pattern figure which shows the wiring structure of the principal part of the pixel part which concerns on one Example of this invention. It is a circuit diagram which shows the equivalent circuit of the pixel which concerns on a present Example. It is a wave form diagram which shows the rising waveform of the write scanning signal WS when it receives capacitive coupling. It is a circuit diagram which shows the equivalent circuit of the pixel which concerns on the modification of a present Example. It is a circuit diagram which shows the circuit structure of the pixel of another structure. It is a perspective view which shows the external appearance of the television set to which this invention is applied. It is a perspective view which shows the external appearance of the digital camera to which this invention is applied, (A) is the perspective view seen from the front side, (B) is the perspective view seen from the back side. 1 is a perspective view illustrating an appearance of a notebook personal computer to which the present invention is applied. It is a perspective view which shows the external appearance of the video camera to which this invention is applied. BRIEF DESCRIPTION OF THE DRAWINGS It is an external view which shows the mobile telephone to which this invention is applied, (A) is the front view in the open state, (B) is the side view, (C) is the front view in the closed state, (D) Is a left side view, (E) is a right side view, (F) is a top view, and (G) is a bottom view. It is explanatory drawing about the detail of bootstrap operation | movement.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Organic EL display device 20, 20A, 20 '... Pixel (pixel circuit), 21 ... Organic EL element, 22 ... Drive transistor, 23 ... Write transistor, 24 ... Retention capacity, 25 ... Auxiliary capacity, 26 ... Light emission control Transistors 27 and 28... Switching transistor 30... Pixel array section 31 (31-1 to 31 -m) Scan line 32 (32-1 to 32-m) Power supply line 33 (33-1 to 33-3 33-n) ... signal line, 34 ... common power supply line, 40 ... write scanning circuit, 50 ... power supply scanning circuit, 60 ... signal output circuit, 70 ... display panel

Claims (6)

An electro-optical element; a writing transistor for writing a video signal; a driving transistor for driving the electro-optical element in accordance with the video signal written by the writing transistor; and a gate electrode and a source electrode of the driving transistor. A storage capacitor connected to hold the video signal written by the write transistor;
Pixels that perform mobility correction processing that applies negative feedback to the potential difference between the gate and source of the drive transistor with a correction amount corresponding to the current flowing through the drive transistor are arranged in a matrix,
The display device according to claim 1, wherein the pixel includes an adjustment unit that adjusts a period of the mobility correction processing according to a transition speed of a source potential of the driving transistor determined by the mobility of the driving transistor. The adjusting means includes a resistance component that is wired for each pixel row and exists between a scan line that supplies a write scan signal to the gate electrode of the write transistor and the gate electrode of the write transistor,
The resistance component causes capacitive coupling to the gate electrode of the write transistor due to the transition of the gate potential at the transition of the gate potential in conjunction with the transition of the source potential of the driving transistor when the mobility correction process is completed. The display device according to claim 1. The wiring between the scanning line and the gate electrode of the writing transistor is made of a high resistance material having a higher resistance value than the wiring material of the scanning line,
The display device according to claim 2, wherein the resistance value of the resistance component is determined by a wiring length and a wiring width between the scanning line and a gate electrode of the writing transistor and a resistance value of the high-resistance material. The display device according to claim 2, wherein the adjustment unit includes a capacitor connected between a source electrode of the driving transistor and a gate electrode of the writing transistor. An electro-optical element; a writing transistor for writing a video signal; a driving transistor for driving the electro-optical element in accordance with the video signal written by the writing transistor; and a gate electrode and a source electrode of the driving transistor. A storage capacitor connected to hold the video signal written by the write transistor;
When driving a display device in which pixels are arranged in a matrix, a mobility correction process is performed to apply negative feedback to the potential difference between the gate and source of the drive transistor with a correction amount corresponding to the current flowing through the drive transistor.
A method for driving a display device, wherein the mobility correction processing period is adjusted in accordance with a transition speed of a source potential of the driving transistor determined by the mobility of the driving transistor. An electro-optical element; a writing transistor for writing a video signal; a driving transistor for driving the electro-optical element in accordance with the video signal written by the writing transistor; and a gate electrode and a source electrode of the driving transistor. A storage capacitor connected to hold the video signal written by the write transistor;
Pixels that perform mobility correction processing that applies negative feedback to the potential difference between the gate and source of the drive transistor with a correction amount corresponding to the current flowing through the drive transistor are arranged in a matrix,
The electronic apparatus including a display device, wherein the pixel includes an adjustment unit that adjusts a period of the mobility correction processing according to a transition speed of a source potential of the driving transistor determined by the mobility of the driving transistor.

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