JP2009237426A - Display device, method for driving display device, and electronic device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To further improve display quality by surely performing threshold correction processing. <P>SOLUTION: The potential DS of a power supply line is switched to low potential Vini from high potential Vccp in an inactive period of potential (a write scanning signal) WS of a scanning line. The supply of a current from the power supply line to a driving transistor is thereby cut off to suppress the rise of source potential Vs of the driving transistor. If the source potential Vs of the driving transistor does not rise, gate potential Vg does not rise even if a gate electrode is in a floating state. As a result, threshold correction processing based on the gate potential Vg can be surely performed. <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. On the other hand, in the prior art described in Patent Document 1, the number of elements constituting the pixel circuit is increased, which hinders miniaturization of the pixel size and hence high definition of the display device.

  On the other hand, for example, a pixel circuit has been proposed in which the power supply potential supplied to the drive transistor is switchable, and the transistor that controls light emission / non-light emission of the organic EL element by switching the power supply potential is omitted ( For example, see Patent Document 2).

  By omitting the transistor for controlling the light emission / non-light emission of the organic EL element, the number of elements and the number of wirings constituting the pixel circuit can be reduced. Specifically, a pixel circuit can be configured with the minimum number of constituent elements necessary for a drive transistor for driving an organic EL element, a write transistor for writing a video signal, and a storage capacitor for holding the written video signal (see FIG. 2). .

  Here, an example of the operation of the organic EL device having the pixel configuration described in Patent Document 2 will be described with reference to the timing waveform diagram of FIG. Here, a case where the threshold value correction process is divided into 2H (H is a horizontal period) and executed twice is taken as an example.

  When the write scan signal WS becomes active ("H" level) in the first part of the 1H and 2H, the write transistor becomes conductive, and the reference potential Vofs supplied through the signal line is written into the pixel. As a result, the gate potential VG of the driving transistor is initialized to the reference potential Vofs.

  The power supply potential DS supplied to the driving transistor selectively takes a power supply potential Vccp on the high potential side and a power supply potential Vini on the low potential side. Prior to the threshold correction process, the power supply potential DS becomes the power supply potential Vini, so that the source potential Vs of the drive transistor is initialized to the power supply potential Vini.

  After the initialization of the gate potential Vg and the source potential Vs of the driving transistor, threshold correction processing is started. After the start of the threshold correction processing, the power supply potential DS is switched from the power supply potential Vini to the power supply potential Vccp. Then, after the threshold correction process is completed, the writing scan signal WS is activated, so that the video signal writing process and the mobility correction process by the writing transistor are performed in parallel.

JP 2006-133542 A JP 2007-310311 A

  As described above, in the conventional technique described in Patent Document 2, the write scanning signal is in an inactive (“L” level) state during the period from the start of the threshold correction processing to the writing of the video signal. The writing transistor becomes non-conductive. As a result, the gate electrode of the drive transistor is electrically disconnected from the signal line, so that the gate electrode of the drive transistor is in a floating state.

  Since the power supply potential Vccp is applied to the drive transistor after the start of the threshold correction process, a current flows through the drive transistor. When a current flows through the driving transistor, the source potential Vs of the driving transistor rises as shown in FIG. At this time, since the gate electrode of the driving transistor is in a floating state and a storage capacitor is connected between the gate electrode and the source electrode, when the source potential Vs rises as shown in FIG. In conjunction with this, the gate potential Vg also rises.

  The threshold correction processing is originally processing for changing the source potential Vg toward a potential obtained by subtracting the threshold voltage Vth of the drive transistor from the initialization potential with reference to the initialization potential Vofs of the gate potential Vg of the drive transistor. By this process, the gate-source voltage Vgs of the driving transistor finally converges to the threshold voltage Vth.

  Therefore, if the gate potential Vg rises in conjunction with the rise of the source potential Vs, the gate-source voltage Vgs of the drive transistor can be converged to the threshold voltage Vth, so that the threshold correction processing is incomplete. Become. As a result, the variation of the threshold voltage Vth for each pixel remains at the time of light emission, so that the effect of improving the display quality (image quality) associated with the threshold correction process cannot be obtained sufficiently.

  Accordingly, an object of the present invention is to provide a display device capable of further improving display quality by reliably performing threshold correction processing, a method for driving the display device, and an electronic apparatus using the display device. .

In order to achieve the above object, the present invention provides:
An electro-optic element;
A write transistor having a gate electrode connected to the scan line and one electrode connected to the signal line;
A drive transistor connected in series to the electro-optic element and having a gate electrode connected to the other electrode of the writing transistor;
In a display device including a pixel array section in which pixels having one electrode connected to the gate electrode of the drive transistor and the other electrode connected to the other electrode of the drive transistor are arranged in a matrix The following drive is performed.

That is, when driving each pixel of the pixel array unit,
While providing each pixel of the pixel array unit in a row unit while giving a write scan signal to the write transistor,
While selectively supplying a drive current through a power supply line capable of switching a power supply potential to the drive transistor,
During the inactive period of the write scan signal, supply of current from the power supply line to the drive transistor is cut off.

  In the inactive period of the write scan signal, the write transistor is turned off, so that the gate electrode of the drive transistor is electrically disconnected from the signal line and is in a floating state. During the inactive period of the write scan signal, the current supply from the power supply line to the drive transistor is cut off, so that no current flows through the drive transistor. If no current flows through the driving transistor, the source potential of the driving transistor does not rise.

  If the source potential of the driving transistor does not increase, even if the gate electrode of the driving transistor is in a floating state, the gate potential of the driving transistor initialized in the active period of the write scan signal does not increase. Thus, the threshold correction process for changing the source potential toward the potential obtained by subtracting the threshold voltage of the drive transistor from the initialization potential with reference to the initialization potential of the gate potential of the drive transistor can be reliably performed.

  According to the present invention, the threshold value correction process can be performed reliably, so that the display quality can be further improved.

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

[First Embodiment]
FIG. 1 is a system configuration diagram showing an outline of the configuration of the active matrix display device according to the first embodiment of the present invention. 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 10A according to this embodiment 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 color display, one pixel 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 according to the first embodiment.

  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 10A. 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 shown in the components of the drive circuit, and the 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 in an ideal operating state of an organic EL display device)
Next, with respect to the circuit operation in an ideal operation state in the organic EL display device 10 in which the pixels 20 having the above-described configuration are two-dimensionally arranged in a matrix, FIG. 5 and FIG. This will be described with reference to an operation explanatory diagram. 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 write transistor 23 is in a non-conductive 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 driving 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 driving 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, assuming that the bootstrap gain is 1 (ideal value), the amount of increase in the gate potential Vg is equal to the amount of increase in 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.

(Problems of conventional technology)
Subsequently, problems of the conventional technology will be described. Here, the threshold correction processing is described by taking as an example a case where the threshold correction processing is executed a plurality of times divided into a plurality of horizontal periods preceding 1H in addition to 1H (one horizontal period) for performing signal writing and mobility correction. It shall be. Hereinafter, performing the threshold correction processing a plurality of times by dividing into a plurality of H is referred to as “divided Vth correction”.

  Here, as an example, consider the case of divided Vth correction in which threshold correction processing is divided and executed twice over a total of 2H, which is 1H preceding 1H that performs signal writing and mobility correction. In the case of this divided Vth correction, as shown in the timing waveform diagram of FIG. 21, the first threshold correction processing is performed 1H before the 1H period in which signal writing and mobility correction are performed, that is, 1H of the pixel row one row before The period is t13-t14. The second threshold correction process is performed in a period from t15 to t16 in a 1H period in which signal writing and mobility correction are performed.

  As described above, the threshold value correction period is divided into a horizontal period in which signal writing and mobility correction are performed and a horizontal period preceding the horizontal period is provided, and the threshold value correction process is executed a plurality of times. A sufficient time can be secured as the correction period, that is, the gate-source voltage Vgs of the drive transistor 22 converges to the threshold voltage Vth even if the time allocated to the 1H period is shortened by the increase in the number of pixels due to high definition. Since sufficient time can be secured, the threshold cancellation process can be performed reliably.

  In terms of circuit operation, times t11 to t13 and t17 to t20 in the timing waveform diagram of FIG. 21 correspond to times t1 to t3 and t5 to t8 in the timing waveform diagram of FIG. 4, and the timing waveform diagram of FIG. Times t12 and t15 and t14 and t16 in FIG. 4 correspond to times t2 and t4 in the timing waveform diagram of FIG.

  In the conventional technique described above, the write scanning signal WS is in an inactive (“L” level) state in a period from when the threshold correction process is started until the signal voltage Vsig of the video signal is written. Specifically, the write scan signal WS becomes inactive in the period from time t14 to time t15 and in the period from time t16 to time t18, so that the write transistor 23 becomes non-conductive. In these periods, since the gate electrode of the drive transistor 22 is electrically disconnected from the signal line 33, the gate electrode of the drive transistor 22 is in a floating state.

  On the other hand, since the high potential Vccp is applied to the drain electrode of the drive transistor 22 after the start of the threshold correction process (t13), a current corresponding to the gate-source voltage Vgs of the drive transistor 22 is supplied to the power supply line. The current flows from 32 to the driving transistor 22. When a current flows through the drive transistor 22, the source potential Vs of the drive transistor 22 rises as shown in FIG.

  At this time, since the gate electrode of the drive transistor 22 is in a floating state and the storage capacitor 24 is connected between the gate electrode and the source electrode, when the source potential Vs rises, the gate potential Vg is generated by the bootstrap operation. Also rises. The increase in the potentials Vs and Vg increases as the gate-source voltage Vgs of the drive transistor 22 increases. Therefore, the rise of the source potential Vs and the gate potential Vg becomes more problematic after the initial (first time in this example) threshold correction processing when the divided Vth correction is performed.

  As described above, the threshold correction process is a process for converging the gate-source voltage Vgs of the drive transistor 22 to the threshold voltage Vth with the initialization potential Vofs of the gate potential Vg of the drive transistor 22 as a reference. For this reason, if the reference gate potential Vg rises in conjunction with the rise of the source potential Vs, the gate-source voltage Vgs of the drive transistor 22 cannot be converged to the threshold voltage Vth. As a result, the threshold correction process (threshold cancellation) process becomes incomplete, and the display quality improvement effect associated with the threshold correction process cannot be sufficiently obtained.

(Characteristic part of the first embodiment)
In the organic EL display device 10A according to the first embodiment, as shown in the timing waveform diagram of FIG. 10, the potential DS of the power supply line 32 is increased during the inactive period of the write scan signal (potential of the scan line 31) WS. It is characterized by switching from the potential Vccp to the low potential Vini.

  Specifically, the potential DS of the power supply line 32 is lowered at the same timing as timings t14 and t16 at which the potential WS of the scanning line 31 transits from the “H” level to the “L” level or at timings t21 and t23 later than that. The potential falls to Vini. Further, the potential DS of the power supply line 32 is set to the high potential Vccp at the same timing t15 and t18 as the timing t15 and t18 at which the potential WS of the scanning line 31 transits from the “L” level to the “H” level. Launch.

  In this way, no current is supplied from the power supply line 32 to the drive transistor 22 by setting the potential DS of the power supply line 32 to the low potential Vini during the inactive period of the write scan signal WS. Therefore, the source potential Vs of the driving transistor 22 does not increase and becomes constant. If the source potential Vs does not rise, even if the gate electrode of the drive transistor 22 is in a floating state, the gate potential Vg of the drive transistor 22 does not rise and becomes constant. Thereby, particularly when the divided Vth correction is employed, it is possible to suppress the increase in the source potential Vs and the gate potential Vg after the initial (first time in this example) threshold correction processing.

  If the increase in the gate potential Vg of the drive transistor 22 can be suppressed, the gate-source voltage Vgs of the drive transistor 22 can be converged to the threshold voltage Vth by the threshold correction process with the gate potential Vg as a reference. Thereby, since the variation of the threshold voltage Vth for each pixel can be reliably corrected by the threshold cancel operation using the voltage corresponding to the threshold voltage Vth held in the holding capacitor 24, the display quality improvement effect accompanying the threshold correction processing You can get enough.

  Incidentally, a parasitic capacitance Cgd is generally interposed between the gate electrode and the drain electrode of the drive transistor 22. When the parasitic capacitance Cgd is interposed between the gate electrode and the drain electrode of the drive transistor 22, the gate potential Vg of the drive transistor 22 is coupled by the parasitic capacitance Cgd at the timing t21 and t23 of the fall of the potential DS of the power supply line 32. Decreases. When the gate potential Vg decreases, the gate-source voltage Vgs of the drive transistor 22 decreases.

  In the above description, the current DS is not supplied from the power supply line 32 to the drive transistor 22 because the potential DS of the power supply line 32 becomes the low potential Vini. However, in reality, the power supply line 32 is not in a floating state, so that a current flows from the power supply line 32 to the drive transistor 22 in the process in which the potential DS of the power supply line 32 transitions from the high potential Vccp to the low potential Vini. Flows.

  On the other hand, the gate potential Vg of the drive transistor 22 is lowered by the coupling at the time of the fall of the potential DS of the power supply line 32, and the gate-source voltage Vgs of the drive transistor 22 is reduced. The flowing current can be suppressed. In particular, if the gate-source voltage Vgs is reduced below the threshold voltage Vth, no current flows through the drive transistor 22. If no current flows through the dynamic transistor 22, the source potential Vs of the drive transistor 22 does not rise, and therefore the gate potential Vg does not rise.

  As described above, the gate-source voltage Vgs of the drive transistor 22 is preferably reduced to the threshold voltage Vth or less by coupling when the power supply potential DS falls, so that the source potential Vs and the gate potential Vg of the drive transistor 22 are reduced. The rise can be prevented. As a result, the threshold value correction process, in particular, the divided Vth correction process can be stably realized, so that a display image with good image quality can be obtained.

  The timing at which the power supply potential DS falls may be the same as the timings t14 and t16 at which the potential WS of the scanning line 31 transitions from the “H” level to the “L” level, but is later than that. t21 and t23 are preferable. The reason is as follows. This is because there is a concern that the power supply potential DS may fall at a timing earlier than the timings t14 and t16 when the timing WS is the same as the falling timing t14 and t16 of the scanning line 31 when there is a difference between both timings. .

  At timings earlier than timings t14 and t16, since the write transistor 23 is in a conductive state, the gate electrode of the drive transistor 22 is not in a floating state. Therefore, if the power supply potential DS falls at a timing earlier than the timings t14 and t16, even if coupling occurs, the gate potential Vg of the drive transistor 22 does not fall, and the gate-source voltage Vgs cannot be reduced. become.

  In this example, the coupling by the parasitic capacitance Cgd interposed between the gate electrode and the drain electrode of the driving transistor 22 is used. However, when the capacitance value of the parasitic capacitance Cgd is small and a sufficient coupling amount cannot be obtained. You can do as follows. That is, as shown in FIG. 11, a coupling capacitor 26 having a capacitance value capable of obtaining a sufficient coupling amount is connected between the gate electrode and the drain electrode of the driving transistor 22 to promote the coupling. You can do it. Here, a sufficient coupling amount is a cup that can reduce the gate-source voltage Vgs to such an extent that the rise of the source potential Vs and the gate potential Vg of the driving transistor 22 can be suppressed, and preferably can be reduced below the threshold voltage Vth. Say the ring amount.

[Second Embodiment]
FIG. 12 is a system configuration diagram showing an outline of the configuration of the active matrix display device according to the second embodiment of the present invention. In FIG. 12, the same parts as those in FIG. Again, the case of an active matrix organic EL display device is taken as an example.

  The organic EL display device 10B according to the present embodiment includes an auxiliary scanning circuit 80 as a driving unit disposed around the pixel array unit 30 in addition to the writing scanning circuit 40, the power supply scanning circuit 50, and the signal output circuit 60. It is the composition which has. The configurations and operations of the write scanning circuit 40, the power supply scanning circuit 50, and the signal output circuit 60 are basically the same as those in the first embodiment.

  The auxiliary scanning circuit 80 includes a shift register that sequentially shifts the start pulse sp in synchronization with the clock pulse ck. The auxiliary scanning circuit 80 performs auxiliary scanning in which the control signals CT (CT1 to CTm) are sequentially supplied to the auxiliary scanning lines 35-1 to 35-m in synchronization with the line sequential scanning performed by the writing scanning circuit 40. By the scanning by the auxiliary scanning circuit 80, the current supply from the power supply line 32 to the driving transistor 22 is selectively cut off.

(Pixel circuit)
FIG. 13 is a circuit diagram showing a specific circuit configuration of a pixel (pixel circuit) 20 ′ according to the second embodiment. In FIG. 13, the same parts as those in FIG.

  In the pixel 20 ′ according to the second embodiment, the drive circuit that drives the organic EL element 21 includes, for example, a control transistor 27 as a switch element in addition to the drive transistor 22, the write transistor 23, the storage capacitor 24, and the auxiliary capacitor 25. It is the composition which has. The control transistor 27 is connected in series to the drive transistor 22 and functions as a switch element that selectively cuts off supply of drive current to the drive transistor 22. Here, an N-channel transistor is used as the control transistor 27, but a P-channel transistor can also be used.

  The functions of the drive transistor 22, the write transistor 23, the storage capacitor 24, and the auxiliary capacitor 25 are the same as those in the first embodiment. The control transistor 27 is connected in series with the drive transistor 22. Specifically, the control transistor 27 has a drain electrode connected to the power supply line 32, a source electrode connected to the drain electrode of the drive transistor 22, and a gate electrode connected to the auxiliary scanning line 35.

(Explanation of operation in actual operation)
Next, the operation in the actual operation state of the organic EL display device 10B according to the second embodiment will be described with reference to the timing waveform diagram of FIG.

  In the timing waveform diagram of FIG. 14, the potential of the scanning line 31 (write scanning signal) WS changes, the potential of the power supply line 32 (power supply potential) DS, the change of the control signal CT, the gate potential Vg of the driving transistor 22. And the change of the source potential Vs is 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.

  In the timing waveform diagram of FIG. 14, the timings of time t11 to time t20 and time t21 to time t24 are the time t11 to time t20 and time t21 to time t24 in the timing waveform diagram of FIG. 10 according to the first embodiment. It corresponds to the timing.

  As is apparent from this timing waveform diagram, the control signal CT is in an active (“H” level in this example) state in the period from time t21, the period from time t22 to time t23, and the period after time t24. . Control signal CT is inactive ("L" level) in the period from time t21 to time t22 and in the period from time t23 to time t24.

  That is, the control signal CT changes from the “H” level to the “L” at the same timing as the timings t14 and t16 at which the potential WS of the scanning line 31 transits from the “H” level to the “L” level or at later timings t21 and t23. Fall to the level. The falling timings t21 and t23 are preferably later than the times t14 and t16 for the same reason as in the first embodiment.

  In the inactive period of the write scan signal WS, the control signal CT becomes inactive, whereby the control transistor 27 becomes non-conductive and cuts off the current path from the power supply line 32 to the drive transistor 22. As a result, the drain electrode of the drive transistor 22 is in a floating state, and no current is supplied to the drive transistor 22 from the power supply line 32. Therefore, the source potential Vs of the drive transistor 22 does not increase and becomes constant.

  If the source potential Vs of the driving transistor 22 does not increase, even if the gate electrode of the driving transistor 22 is in a floating state, the gate potential Vg of the driving transistor 22 does not increase and becomes constant. In particular, when the divided Vth correction is employed, it is possible to suppress an increase in the source potential Vs and the gate potential Vg after the initial (first time in this example) threshold correction processing.

  If the increase of the gate potential Vg of the drive transistor 22 can be suppressed, the gate-source voltage Vgs of the drive transistor 22 can be converged to the threshold voltage Vth by the threshold correction process with the gate potential Vg as a reference. Thereby, since the variation of the threshold voltage Vth for each pixel can be surely corrected by the threshold cancel operation, the display quality improvement effect associated with the threshold correction processing can be sufficiently obtained.

  A parasitic capacitance Cgs is also interposed between the gate electrode and the source electrode of the control transistor 27. Therefore, at the timing when the control signal CT falls from the “H” level to the “L” level, the gate potential Vg of the drive transistor 22 decreases due to the coupling by the parasitic capacitance Cgs of the control transistor 27 and the parasitic capacitance Cgd of the drive transistor 22. . Then, as in the case of the first embodiment, the gate-source voltage Vgs of the drive transistor 22 is reduced.

  By coupling at the time of falling of the control signal CT, the gate potential Vg of the drive transistor 22 is lowered, and the gate-source voltage Vgs of the drive transistor 22 is reduced, whereby the current flowing through the drive transistor 22 can be suppressed. In particular, if the gate-source voltage Vgs is reduced below the threshold voltage Vth, no current flows through the drive transistor 22. If no current flows through the dynamic transistor 22, the source potential Vs of the drive transistor 22 does not rise, and therefore the gate potential Vg does not rise.

  As described above, the gate-source voltage Vgs of the drive transistor 22 is preferably reduced to the threshold voltage Vth or less by coupling when the power supply potential DS falls, so that the source potential Vs and the gate potential Vg of the drive transistor 22 are reduced. The rise can be prevented. As a result, the threshold value correction process, in particular, the divided Vth correction process can be stably realized, so that a display image with good image quality can be obtained.

  The connection position of the control transistor 27 is not limited between the power supply line 32 and the drain electrode of the drive transistor 22. For example, as shown in FIG. 15, the control transistor 27 may be connected between the source electrode of the drive transistor 22 and the anode electrode of the organic EL element 21. Also in this modification, the control signal CT becomes inactive and the control transistor 27 becomes non-conductive during the inactive period of the write scanning signal WS, so that the power supply line 32 to the drive transistor 22 is turned off. The supply of current can be cut off.

  In the case of this modification, the anode potential of the organic EL element 21 decreases due to coupling by the parasitic capacitance Cgs of the control transistor 27 when the control signal CT falls, and the gate-source voltage Vgs of the drive transistor 22 increases. There are concerns. However, the anode electrode of the organic EL element 21 has a storage capacitor 24 and an auxiliary capacitor 25 in addition to its own equivalent capacitance, and the combined capacitance value thereof is much greater than the capacitance value of the parasitic capacitance Cgs of the control transistor 27. large. Therefore, even if the coupling due to the parasitic capacitance Cgs of the control transistor 27 enters, the anode potential of the organic EL element 21 hardly changes.

[Modification]
In each of the above embodiments, the case where the driving circuit of the organic EL element 21 basically has a pixel configuration including two transistors, that is, the driving transistor 22 and the writing transistor 23 has been described as an example. The application is not limited to the configuration. 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.

  In each of the above embodiments, 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 pixels 20 and 20 'has been described as an example. However, the present invention is not limited to this application example. It is not something that can be done. 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. 16 to 20, for example, digital cameras, notebook personal computers, portable terminal devices such as mobile phones, and display devices such as video cameras.

  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 clear from the description of each embodiment described above, the display device according to the present invention can sufficiently obtain the display quality improvement effect associated with the threshold correction process, and thus obtain a high-quality display image. Can do.

  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. 16 is a perspective view showing an appearance of a television set to which the present invention is applied. The television 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. .

  17A and 17B are perspective views showing the appearance of a digital camera to which the present invention is applied. FIG. 17A is a perspective view seen from the front side, and FIG. 17B 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. 18 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. 19 is a perspective view showing the external appearance of a video camera to which the present invention is applied. The video camera according to this application example includes a main body 131, a lens 132 for shooting an object on a side facing forward, a start / stop switch 133 at the time of shooting, a display unit 134, and the like. It is manufactured by using a display device.

  20A and 20B are external views showing a mobile terminal device to which the present invention is applied, for example, a mobile phone. FIG. 20A is a front view in an opened state, FIG. 20B is a side view thereof, and FIG. (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 illustrating an outline of a configuration of an organic EL display device according to a first embodiment of the present invention. FIG. 3 is a circuit diagram illustrating a circuit configuration of a pixel according to the first embodiment. 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 operation | movement description in the ideal operation state of an organic electroluminescence display. It is explanatory drawing (the 1) of the circuit operation | movement in an ideal state. It is explanatory drawing (the 2) of the circuit operation | movement in an ideal state. 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. 6 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 or not threshold correction and mobility correction are performed. It is a timing waveform diagram with which it uses for operation | movement description in the actual operation state of the organic electroluminescence display which concerns on 1st Embodiment. It is a circuit diagram which shows the circuit structure of the pixel which concerns on the modification of 1st Embodiment. It is a system block diagram which shows the outline of a structure of the organic electroluminescence display which concerns on 2nd Embodiment of this invention. It is a circuit diagram which shows the circuit structure of the pixel which concerns on 2nd Embodiment. It is a timing waveform diagram with which it uses for operation | movement description in the actual operation state of the organic electroluminescence display which concerns on 2nd Embodiment. It is a circuit diagram which shows the circuit structure of the pixel which concerns on the modification of 2nd Embodiment. 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 a timing waveform diagram for demonstrating the subject of a prior art.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10A, 10B ... Organic EL display device, 20, 20 '... Pixel (pixel circuit), 21 ... Organic EL element, 22 ... Drive transistor, 23 ... Write transistor, 24 ... Retention capacity, 25 ... Auxiliary capacity, 26 ... Cup link Capacitor 27... Control 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-) n) Signal line 34 Common power supply line 40 Write scanning circuit 50 Power supply scanning circuit 60 Signal output circuit 70 Display panel 80 Auxiliary scanning circuit

Claims (8)

An electro-optic element;
A write transistor having a gate electrode connected to the scan line and one electrode connected to the signal line;
A drive transistor connected in series to the electro-optic element and having a gate electrode connected to the other electrode of the writing transistor;
A pixel array section in which pixels having one storage electrode connected to the gate electrode of the driving transistor and the other electrode connected to the other electrode of the driving transistor are arranged in a matrix;
A writing scanning unit that provides a writing scanning signal to the writing transistor while scanning each pixel of the pixel array unit in units of rows;
A power supply scanning section capable of selectively supplying a drive current to the drive transistor through a power supply line and switching a power supply potential applied to the power supply line;
A display device comprising: a current supply cut-off unit that cuts off supply of current from the power supply line to the drive transistor during an inactive period of the write scan signal. The current supply cut-off unit includes the power supply scanning unit,
The power supply scanning unit can switch the power supply potential between a first power supply potential that supplies a drive current to the drive transistor and a second power supply potential that does not supply the drive current, and the write scan signal is inactive The display device according to claim 1, wherein the power supply potential is switched to the second power supply potential in a period. The display device according to claim 2, wherein the pixel has a coupling capacitor connected between a gate electrode of the driving transistor and an electrode on the power supply line side. The display device according to claim 1, wherein the current supply cut-off unit is a switch element that is connected in series to the drive transistor and is in a non-conductive state during an inactive period of the write scan signal. Threshold correction processing for changing the source potential toward the potential obtained by subtracting the threshold voltage of the drive transistor from the initialization potential with reference to the initialization potential of the gate potential of the drive transistor initialized during the active period of the write scan signal The display device according to claim 1. The display device according to claim 5, wherein the threshold correction process is performed a plurality of times by dividing into a plurality of horizontal periods of a horizontal period in which a video signal is written by the writing transistor and a horizontal period preceding the horizontal period. An electro-optic element;
A write transistor having a gate electrode connected to the scan line and one electrode connected to the signal line;
A drive transistor connected in series to the electro-optic element and having a gate electrode connected to the other electrode of the writing transistor;
Driving each pixel of a pixel array unit in which pixels having one electrode connected to the gate electrode of the driving transistor and the other electrode connected to the other electrode of the driving transistor are arranged in a matrix On the occasion
While providing each pixel of the pixel array unit in a row unit while giving a write scan signal to the write transistor,
While selectively supplying a drive current through a power supply line capable of switching a power supply potential to the drive transistor,
A method for driving a display device, wherein supply of current from the power supply line to the drive transistor is cut off during an inactive period of the write scan signal. An electro-optic element;
A write transistor having a gate electrode connected to the scan line and one electrode connected to the signal line;
A drive transistor connected in series to the electro-optic element and having a gate electrode connected to the other electrode of the writing transistor;
A pixel array section in which pixels having one storage electrode connected to the gate electrode of the driving transistor and the other electrode connected to the other electrode of the driving transistor are arranged in a matrix;
A writing scanning unit that provides a writing scanning signal to the writing transistor while scanning each pixel of the pixel array unit in units of rows;
A power supply scanning section capable of selectively supplying a drive current to the drive transistor through a power supply line and switching a power supply potential applied to the power supply line;
An electronic apparatus comprising: a display device comprising: a current supply cut-off unit that cuts off supply of current from the power supply line to the drive transistor during an inactive period of the write scan signal.

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