# Patent

## Title
Display device and electronic appliance

## Abstract

A display device includes: a plurality of arranged pixels, each of which includes an electro-optical component, a write-in transistor writing an image signal in a pixel, a maintenance capacity maintaining the image signal written by the write-in transistor, and a driving transistor driving the electro-optical component based on the image signal maintained by the maintenance capacity, wherein the write-in transistor has a plurality of gates, the gate of the driving transistor side among the plurality of gates has a structure in which a channel region is sandwiched between a first gate electrode and a second gate electrode, and the width of the channel region of the gate of the driving transistor side is narrower than the width of the channel region of other gates.

## Background
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to a display device and an electronic appliance, and more particularly to a display device in which pixels including electro-optical components are two-dimensionally (2D) arranged in the form of a matrix and an electronic appliance having the display device.
2. Description of the Related Art
Recently, in the field of display devices that perform image display, plane-type (flat panel type) display devices in which pixels (pixel circuits) are arranged in the form of a matrix have been rapidly spread. As a kind of plane type display device, there is a display device that uses a so-called current driving type electro-optical component, in which luminance is changed according to a current value that flows in the device, as a light-emitting device of a pixel. As a current driving type electro-optical component, an organic electroluminescence (EL) device is known, which has a phenomenon of emitting light when an electric field is applied to an organic thin film using EL that is an organic material.
An organic EL display device that uses organic EL devices as light-emitting devices of pixels has the following characteristics. That is, since the organic EL device can be driven by an applied voltage equal to or lower than 10V, it consumes little power. Since the organic EL device is a self-light emitting device, it has a high visual recognition of an image in comparison to a liquid crystal display, and since it does not require an illumination member such as a backlight or the like, it is easy to make it light-weight and ultra-thin. Also, since the response speed of the organic EL device is very high to the extent of several μs, no afterimage is generated when a moving image is displayed.
In the same manner as a liquid crystal display, an organic EL display device may adopt a simple (passive) matrix type and an active matrix type as its driving type. However, according to the simple matrix type display device, although it has a simple structure, the light-emitting term of the electro-optical components is decreased as the number of scanning lines (that is, the number of pixels) is increased, and thus it is difficult to realize a large-scale high-definition display device.
Because of this, the development of an active matrix type display device in which current flowing through electro-optical components is controlled by active elements installed in pixels such as the electro-optical components, for example, insulated gate field effect transistors, have been actively made. As the insulated gate field effect transistor, generally, a TFT (Thin Film Transistor) is used. According to the active matrix type display device, the electro-optical components continue light emission through a period of one display frame, and thus it is easy to realize a large-scale high-definition display device.
A pixel circuit that includes a current driving type electro-optical component, which is driven by the active matrix type, is provided with a driving circuit for driving the electro-optical component in addition to the electro-optical component. A pixel circuit is known, which is configured to have an organic EL device 21 that is a current driving type electro-optical component, a driving transistor 22 as a driving circuit, a write-in transistor 23, and a maintenance capacity 24 (for example, see JP-A-2008-310127).
JP-A-2008-310127 discloses that when a gate electrode of a driving transistor 22 is in a floating state, a gate potential Vg is changed in association with a source potential Vs of the driving transistor 22 to perform a so-called bootstrap operation (see Paragraph No. 0071 of JP-A-2008-310127). JP-A-2008-310127 also discloses that even if the I-V characteristic of the organic EL device 21 is time-dependently changed, the gate-source voltage Vgs of the driving transistor 22 is maintained constant, and thus light emitting luminance is maintained constant (see Paragraph No. 0093 of JP-A-2008-310127).
SUMMARY OF THE INVENTION
In the above-described bootstrap operation, the ratio (=ΔVg/ΔVs) of a variation ΔVg of the gate potential Vg to a variation ΔVs of the source potential Vs of the driving transistor 22 becomes a bootstrap gain Gb. This bootstrap gain Gb is determined by a capacitance value of the maintenance capacity 24 and a capacitance value of parasitic capacitance that is parasitic on the gate electrode of the driving transistor 22.
On the other hand, parasitic capacitance exists also in the write-in transistor 23. The parasitic capacitance of the write-in transistor 23 corresponds to one parasitic capacitance that is parasitic on the gate electrode of the driving transistor 22. Accordingly, under the influence of the parasitic capacitance that exists in the write-in transistor 23, the bootstrap gain Gb is changed from an ideal state (Gb=1). Specifically, the bootstrap gain Gb deteriorates.
If the bootstrap gain Gb deteriorates, the light emitting state is not maintained with respect to the gate-source voltage Vgs of the driving transistor 22 in a state where a difference ΔVth in threshold voltage Vth between pixels is maintained, dispersion in luminance occurs between the pixels (the details thereof will be described later). The dispersion in luminance between pixels is visually recognized as a vertical stripe, a horizontal stripe, or luminance non-uniformity. As a result, the uniformity of a screen is damaged.
Accordingly, it is desirable to provide a display device which can improve the bootstrap gain by reducing the capacitance value of the parasitic capacitance of the write-in transistor and obtain a good-quality display image without damaging the uniformity of the screen, and an electronic appliance having the display device.
According to an embodiment of the invention, there is provided a display device including: a plurality of arranged pixels, each of which includes an electro-optical component, a write-in transistor writing an image signal in a pixel, a maintenance capacity maintaining the image signal written by the write-in transistor, and a driving transistor driving the electro-optical component based on the image signal maintained by the maintenance capacity; wherein the write-in transistor has a plurality of gates, the gate of the driving transistor side among the plurality of gates has a structure in which a channel region is sandwiched between a first gate electrode and a second gate electrode, and the width of the channel region of the gate of the driving transistor side is narrower than the width of the channel region of other gates.
In the display device having the above-described configuration, the write-in transistor has a structure in which the plurality of gates are provided, for example, a double-gate structure. According to this double-gate structure, leak currents between a source region and a drain region can be reduced. Also, the write-in transistor has a sandwich structure, in which the second gate electrode is provided as a back gate electrode and the channel region is sandwiched between two gate electrodes (first and second gate electrodes), with respect to the gate of the driving transistor side. According to this sandwich structure, for example, the transistor characteristic can be improved in comparison to a bottom gate structure. In the write-in transistor, the width of the channel region of the gate of the driving transistor side is set to be narrower than the width of the channel region of other gates.
Here, between the second gate electrode that is the back gate electrode and the channel region, parasitic capacitance is formed, which has a capacitance value according to the opposite region between the second gate electrode and the channel region. In this case, in the gate of the driving transistor side, the width of the channel region is narrower than the width of the channel region of other gates, and thus the capacitance value of the parasitic capacitance becomes smaller than the capacitance value of the parasitic capacitance formed in other gates. The parasitic capacitance of the write-in transistor, particularly, the parasitic capacitance of the gate of the driving transistor side, becomes one parameter that determines the bootstrap gain. Accordingly, the capacitance value of the parasitic capacitance can be reduced, and thus the bootstrap gain can be improved.
According to the embodiment of the invention, since the bootstrap gain is improved by reducing the capacitance value of the parasitic capacitance of the write-in transistor, a good-quality display image can be obtained without damaging the uniformity of the screen.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system configuration diagram briefly illustrating the configuration of an organic EL display device to which the invention is applied;
FIG. 2 is a circuit diagram illustrating an example of a circuit configuration of a pixel of an organic EL display device to which the invention is applied;
FIG. 3 is a cross-sectional diagram illustrating an example of a cross-sectional structure of a pixel;
FIG. 4 is a timing waveform diagram illustrating a basic circuit operation of an organic EL display device to which the invention is applied;
FIGS. 5A to 5D are diagrams illustrating a (one of) basic circuit operation of an organic EL display device to which the invention is applied;
FIGS. 6A to 6D are diagrams illustrating a (another)
basic circuit operation of an organic EL display device to which the invention is applied;
FIG. 7 is a characteristic diagram illustrating the subject that is caused by dispersion of the threshold voltages Vth of a driving transistor;
FIG. 8 is a characteristic diagram illustrating the subject that is caused by dispersion of the mobility μ of a driving transistor;
FIGS. 9A to 9C are characteristic diagrams illustrating the relationship between the signal voltage Vsig of an image signal and the drain-source current Ids of the driving transistor according to the existence/nonexistence of threshold value correction and mobility correction;
FIG. 10 is a timing waveform diagram illustrating the bootstrap operation;
FIG. 11 is a diagram illustrating the bootstrap gain Gb;
FIG. 12 is a timing waveform diagram illustrating the recurrence of the dispersion of the threshold voltage Vth;
FIG. 13 is a diagram illustrating a state where an operation point of an organic EL device is shifted when the organic EL device deteriorates;
FIG. 14 is a timing waveform diagram illustrating that the current of a driving transistor is decreased by the high-voltage of an organic EL device;
FIGS. 15A and 15B are diagrams illustrating the structure of a write-in transistor in the related art, in which FIG. 15A is a plane pattern diagram, and FIG. 15B is a cross-sectional diagram;
FIGS. 16A and 16B are diagrams illustrating the structure of a write-in transistor according to an embodiment of the invention, in which FIG. 16A is a plane pattern diagram, and FIG. 16B is a cross-sectional diagram;
FIG. 17 is a diagram illustrating the relationship between the gate voltage Vg of an N-channel transistor and the drain-source current Ids;
FIG. 18 is a perspective diagram illustrating an external appearance of a television set to which the invention is applied;
FIGS. 19A and 19B are perspective diagrams illustrating an external appearance of a digital camera to which the invention is applied, in which FIG. 19A is a perspective diagram as seen from the surface side, and FIG. 19B is a perspective diagram as seen from the rear surface side;
FIG. 20 is a perspective diagram illustrating an external appearance of a notebook type personal computer to which the invention is applied;
FIG. 21 is a perspective diagram illustrating an external appearance of a video camera to which the invention is applied; and
FIGS. 22A to 22G are diagrams illustrating external appearances of a portable phone to which the invention is applied, in which FIG. 22A is a front diagram of a portable phone in an open state, FIG. 22B is a side diagram thereof, FIG. 22C is a front diagram of a portable phone in a closed state, FIG. 22D is a left side diagram thereof, FIG. 22E is aright side diagram thereof, FIG. 22F is a plan diagram thereof, and FIG. 22G is a bottom diagram thereof.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, modes for carrying out the invention (hereinafter referred to “embodiments”) will be described with reference to the accompanying drawings. In this case, the explanation will be made in the following order.
1. Organic EL display device to which the invention is applied
1-1. System configuration
1-2. Basic circuit operation
1-3. Regarding bootstrap operation
2. Explanation of organic EL device according to embodiments
3. Modified examples
4. Electronic appliance
<1. Organic EL Display Device to which the Invention is Applied>
[1-1. System Configuration]
FIG. 1 is a system configuration diagram briefly illustrating the configuration of an active matrix type display device to which the invention is applied.
An active matrix type display device is a display device that controls the current flowing through electro-optical components by active elements installed in pixels such as the electro-optical components, for example, insulated gate field effect transistors. As the insulated gate field effect transistor, generally, a TFT (Thin Film Transistor) is used.
Here, as an example, a current drive type electro-optical component, in which luminance is changed according to a current value flowing through the device, for example, an active matrix type organic EL display device that uses organic EL devices as light-emitting devices of pixels (pixel circuits), will be described.
As illustrated in FIG. 1, an organic EL display device 10 according to this application includes a plurality of pixels 20 including organic EL devices, a pixel array unit 30 in which the pixels 20 are two-dimensionally (2D) arranged in the form of a matrix, and a driving unit arranged in the neighborhood of the pixel array unit 30. The driving unit includes a write-in scanning circuit 40, a power supply scanning circuit 50, and a signal output circuit 60, and drives the respective pixels 20 of the pixel array unit 30.
Here, in the case where the organic EL display device 10 corresponds to a color display, one pixel is composed of a plurality of sub-pixels, and the sub-pixels constitute a pixel 20. More specifically, in a color display device, one pixel is composed of three sub-pixels, that is, a sub-pixel that emits a red light (R), a sub-pixel that emits a green light (G), and a sub-pixel that emits a blue light (B).
However, one pixel is not limited to a combination of sub-pixels for the three primary colors of RGB, and it is also possible to configure one pixel through the addition of sub-pixel(s) for one color or a plurality of colors to the sub-pixels for three primary colors. More specifically, for example, one pixel may be configured by adding a sub-pixel that emits a white light (W) to improve the luminance to the sub-pixels for three primary colors or by adding at least one sub-pixel that emits a complementary color light to extend the color reproduction range to the sub-pixels for three primary colors.
In the pixel array unit 30, with respect to an arrangement of pixels 20 with m rows and n columns, scanning lines 31−1 to 31−m and power supply lines 32−1 to 32−m are wired for each pixel row along the row direction (pixel arrangement direction of a pixel row). Also, signal lines 33−1 to 33−n are wired for each pixel row along the column direction (pixel arrangement direction of a pixel column).
The scanning lines 31−1 to 31−m are respectively connected to output terminals of the rows that correspond to the write-in scanning circuit 40. The power supply lines 32−1 to 32−m are respectively connected to output terminals of the columns that correspond to the power supply scanning circuit 50. The signal lines 33−1 to 33−n are connected to output terminals of the columns that correspond to the signal output circuit 60.
The pixel array unit 30 is typically formed on a transparent insulating substrate such as a glass substrate or the like. Accordingly, the organic EL display device 10 has a plane type (flat type) panel structure. The driving circuit of the respective pixels 20 of the pixel array unit 30 may be formed using amorphous silicon TFTs or low-temperature polysilicon TFTs. In the case of using the low-temperature polysilicon TFTs, as illustrated in FIG. 1, the write-in 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-in scanning circuit 40 includes a shift register that shifts (transmits) a start pulse sp in order in synchronization with a clock pulse ck. In writing an image signal in the respective pixels 20 of the pixel array unit 30, the write-in scanning circuit 40 scans in order (progressively scans) the respective pixels 20 of the pixel array unit 30 in the unit of a row by progressively supplying the write scan signal WS (WS1 to WSm) with respect to the scanning lines 31−1 to 31−m.
The power supply scanning circuit 50 includes a shift register that shifts a start pulse sp in order in synchronization with a clock pulse ck. In synchronization with the progressive scanning by the write-in scanning circuit 40, the power supply scanning circuit 50 supplies the power supply potential DS (DS1 to DSm), which can be switched between a first power supply potential Vccp and a second power supply potential Vini that is lower than the first power supply potential Vccp, to the power supply lines 32−1 to 32−m. As described later, by switching Vccp/Vini of the power supply potential DS, the control of light emission/non-light emission of the pixels 20 is performed.
The signal output circuit 60 selectively outputs a signal voltage Vsig of an image signal according to luminance information that is supplied from a signal supply source (not illustrated) (hereinafter may be simply referred to as “signal voltage”) and a reference voltage Vofs. Here, the reference voltage Vofs is a voltage that becomes a reference against the signal voltage Vsig of the image signal (for example, a voltage that corresponds to the black level of the image signal), and is used to perform correction of the threshold value to be described later.
The signal voltage Vsig output from the signal output circuit 60/the reference voltage Vofs is written in the unit of a pixel row that is selected by scanning through the write-in scanning circuit 40, with respect to the respective pixels 20 of the pixel array unit 30 through the signal lines 33−1 to 33−n. That is, the signal output circuit 60 adopts a line-sequential writing driving type that writes the signal voltage Vsig in the unit of a row (line).
(Pixel Circuit)
FIG. 2 is a circuit diagram illustrating an example of a circuit configuration of a pixel (pixel circuit) 20.
As illustrated in FIG. 2, the pixel 20 is composed of an organic EL device 21 that is a current drive type electro-optical component, in which luminance is changed according to a current value flowing through the device, and a driving circuit driving the organic EL device 21 by flowing a current to the organic EL device 21. The cathode electrode of the organic EL device 21 is connected to a common power supply line 34 that is commonly wired (so-called solid-wired) with respect to all the pixels 20.
The driving circuit that drives the organic EL device 21 is composed of a driving transistor 22, a write-in transistor 23, and a maintenance capacity 24. As the driving transistor 22 and the write-in transistor 23, N-channel TFTs may be used. However, a conduction type combination of the driving transistor 22 and the write-in transistor 23 as described herein is merely exemplary, and the driving circuit is not limited to such a combination.
If the N-channel TFTs are used as the driving transistor 22 and the write-in transistor 23, they may be formed using an amorphous silicon (a-Si) process. By using the a-Si process, it becomes possible to provide a substrate for making the TFTs at a low cost, and further to provide the organic EL display device 10 at a low cost. Also, if the driving transistor 22 and the write-in transistor 23 are provided as a combination of the same conduction type, both the transistors 22 and 23 can be made in the same process, and thus this can contribute to the low-cost of the transistors.
One electrode (source/drain electrode) of the driving transistor 22 is connected to the anode electrode of the organic EL device 21, and the other electrode (drain/source electrode) thereof is connected to the power supply line 32 (32−1 to 32−m).
One electrode (source/drain electrode) of the write-in transistor 23 is connected to the signal line 33 (33−1 to 33−n), and the other electrode (drain/source electrode) thereof is connected to the gate electrode of the driving transistor 22. Also, the gate electrode of the write-in transistor 23 is connected to the scanning line 31 (31−1 to 31−m).
In the driving transistor 22 and the write-in transistor 23, one electrode means a metal wire that is electrically connected to the source/drain region, and the other electrode means a metal wire that is electrically connected to the drain/source region. Also, if one electrode becomes a source electrode by the potential relationship between one electrode and the other electrode, the other electrode becomes a drain electrode, while if one electrode becomes a drain electrode, the other electrode becomes a source electrode.
One electrode of the maintenance capacity 24 is connected to the gate electrode of the driving transistor 22, and the other electrode thereof is connected to the other electrode of the driving transistor 22 and the anode electrode of the organic EL device 21.
In this case, the driving circuit of the organic EL device 21 is not limited to the circuit configuration that is composed of two transistors, that is, the driving transistor 22 and the write-in transistor 23, and one capacitance device, that is, the maintenance capacity 24. For example, as one electrode is connected to the anode electrode of the organic EL device 21 and the other electrode is connected to a fixed potential, it becomes possible to adopt a circuit configuration in which a supplementary capacitance that supplements the capacitance shortfall of the organic EL device 21 is installed if necessary.
In the pixel 20 having the above-described configuration, the write-in transistor 23 is in a conductive state in response to a high (active) write-in scanning signal WS that is applied from the write-in scanning circuit 40 to the gate electrode through the scanning line 31. Accordingly, the write-in transistor 23 samples the signal voltage Vsig of the image signal according to the luminance information or the reference voltage Vofs, which is supplied from the signal output circuit 60 through the signal line 33, and writes the sampled voltage in the pixel 20. This written signal voltage Vsig or the reference voltage Vofs is applied to the gate electrode of the driving transistor 22 and is maintained in the maintenance capacity 24.
When the potential DS of the power supply line 32 (32−1 to 32−m) reaches the first power supply potential Vccp, one electrode of the driving transistor 22 becomes a drain electrode and the other electrode thereof becomes a source electrode, and thus the driving transistor 22 operates in a saturation region. Accordingly, the driving transistor 22 receives a current supply from the power supply line 32 and current-drives the organic EL device 21 to emit light. More specifically, the driving transistor 22, which operates in a saturation region, supplies a drive current having a current value according to the voltage value of the signal voltage Vsig that is maintained in the maintenance capacity 24 to the organic EL device 21, and current-drives the organic EL device 21 to emit light.
Also, when the power supply potential DS is changed from the first power supply potential Vccp to the second power supply potential Vini one electrode of the driving transistor 22 becomes the source electrode and the other electrode thereof becomes the drain electrode, and thus the driving transistor 22 operates as a switching transistor. Accordingly, the driving transistor 22 stops the supply of the drive current to the organic EL device 21 to make the organic EL device 21 in a non-light emission state. That is, the driving transistor 22 also has a function as a transistor that controls light emission/non-light emission of the organic EL device 21.
By the switching operation of the driving transistor 22, the ratio (duty) of a light emission period to a non-light emission period of the organic EL device 21 can be controlled by setting the period in which the organic EL device 21 is in a non-light emission state (non-light emission period). Since afterimage blurring according to the pixel emits light through one display frame period can be reduced by the duty control, the image quality of a moving image becomes more superior.
Of the first and second power supply potentials Vccp and Vini that are selectively supplied from the power supply scanning circuit 50 through the power supply line 32, the first power supply potential Vccp is a power supply potential for supplying the drive current for driving the organic EL device 21 to the driving transistor 22. Also, the second power supply potential Vini is a power supply potential for applying a reverse bias to the organic EL device 21. The second power supply potential Vini is set to a potential that is lower than the reference voltage Vofs, for example, on the assumption that the threshold voltage of the driving transistor 22 is Vth, a potential that is lower than Vofs−Vth, and preferably, a potential that is sufficiently lower than Vofs−Vth.
(Pixel Structure)
FIG. 3 is a cross-sectional diagram illustrating an example of a cross-sectional structure of a pixel 20. As illustrated in FIG. 3, a driving circuit that includes a driving transistor 22 and the like is formed on a glass substrate 201. Also, the pixel 20 has a configuration in which an insulating film 202, an insulating planarization film 203, and a window insulating film 204 are formed in order on the glass substrate 201, and an organic EL device 21 is installed on a concave portion 204A of the window insulating film 204. Here, among the respective configuration devices of the driving circuit, only the driving transistor 22 is illustrated, but illustration of other configuration devices is omitted.
The organic EL device 21 is composed of an anode electrode 205, an organic layer (electron transport layer, a luminous layer, and a hole transport layer/hole injection layer) 206, and a cathode layer 207. The anode electrode 205 is composed of a metal and the like, which is formed on the bottom portion of the concave portion 204A of the window insulating film 204. The organic layer 206 is formed on the anode electrode 205. The cathode electrode 207 is composed of a transparent conduction layer and the like, which is formed commonly to the whole pixel on the organic layer 206.
In the organic EL device 21, the organic layer 206 is formed on the anode electrode 205 by sequentially depositing a hole transport layer/hole injection layer 2061, a luminous layer 2062, an electron transport layer 2063, and an electron injection layer (not illustrated). Also, as current flows from the driving transistor 22 to the organic layer 206 through the anode electrode 205 under the current driving by the driving transistor 22 of FIG. 2, the luminous layer 2062 emits light when electrons and holes are recombined in the luminous layer 2062 in the organic layer 206.
The driving transistor 22 is composed of a gate electrode 221, source/drain regions 223 and 224 installed on both sides of a semiconductor layer 222, and a channel forming region 225 of a portion that is opposite to 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 device 21 through contact holes.
Also, as illustrated in FIG. 3, after the organic EL device 21 is formed on the glass substrate 201 in the unit of a pixel via the insulating film 202, the insulating planarization film 203, and the window insulating film 204, a sealing substrate 209 is bonded via a passivation film 208 by an adhesive 210. As the organic EL device 21 is sealed by the sealing substrate 209, the display panel 70 is formed.
[1-2. Basic Circuit Operation]
Now, the basic circuit operation of the organic EL display device 10 as configured above will be described using operation diagrams of FIGS. 5A to 5D and 6A to 6D based on the timing waveform diagram of FIG. 4. In the operation diagrams of FIGS. 5A to 5D and 6A to 6D, for the simplicity of the drawings, the write-in transistor 23 is illustrated as a switch symbol. Also, an equivalent capacitance 25 of the organic EL device 21 is also illustrated.
The timing waveform diagram of FIG. 4 illustrates the changes of the potential (write-in scanning signal) WS of the scanning line 31, the potential (power supply potential) DS of the power supply line 32, the potential Vsig/Vofs of the signal line 33, the gate potential Vg, and the source potential Vs of the driving transistor 22.
(Light Emission Period of Previously Displayed Frame)
In the timing waveform diagram of FIG. 4, before the time tll, there exists the light emission period of the organic EL device 21 in the previously displayed frame. In the light emission period of the previously displayed frame, the potential DS of the power supply line 32 reaches the first power supply potential (hereinafter referred to as “high potential”) Vccp, and the write-in transistor 23 is in a non-conductive state.
In this case, the driving transistor 22 is designed to operate in a saturation region. Accordingly, as illustrated in FIG. 5A, the driving current (drain-source current) Ids according to the gate-source voltage Vgs of the driving transistor 22 is supplied from the power supply line 32 to the organic EL device 21 through the driving transistor 22. Accordingly, the organic EL device 21 emits light with luminance according to the current value of the driving current Ids.
(Threshold Value Correction Preparation Period)
At the time tll, a new display frame (current display frame) of the progressive scan line comes in. Also, as illustrated in FIG. 5B, the potential DS of the power supply line 32 is changed from a high potential Vccp to the second power supply potential (hereinafter described as “low potential”) Vini that is sufficiently lower than Vofs-Vth for the reference voltage Vofs.
Here, it is assumed that the threshold voltage of the organic EL device 21 is Vthel and the potential (cathode potential) of the common power supply line 34 is Vcath. In this case, if it is assumed that the low potential Vini is Vini<Vthel+Vcath, the source potential Vs of the driving transistor 21 becomes almost the same as the low potential Vini, and thus the organic EL device 21 is in a reverse bias state to be extinct.
Next, at the time t12, the potential WS of the scanning line 31 is shifted from the low potential side to the high potential side, and as illustrated in FIG. 5C, the write-in transistor 23 is in a conductive state. At this time, since the reference voltage Vofs has been supplied from the signal output circuit 60 to the signal line 33, the gate potential Vg of the driving transistor 22 becomes the reference voltage Vofs. Also, the source potential Vs of the driving transistor 22 reaches the potential Vini that is sufficiently lower than the reference voltage Vofs.
At this time, the gate-source voltage Vgs of the driving transistor 22 becomes Vofs-Vini. Here, if Vofs-Vini is not larger than the threshold voltage Vth of the driving transistor 22, the threshold value correction process to be described later may not be performed, and thus it is necessary to set the potential relationship in that Vofs−Vini becomes Vofs-Vini>Vth.
As described above, the initialization process of fixing the gate potential Vg of the driving transistor 22 to the reference voltage Vofs and fixing (deciding) the source potential Vs to the low potential Vini is a preparation (threshold value correction preparation) process before the threshold value correction process (threshold value correction operation) to be described later is performed. Accordingly, the reference voltage Vofs and the low potential Vini become the initialization potentials of the gate potential Vg and the source potential Vs of the driving transistor 22.
(Threshold Value Correction Period)
Next, at the time t13, as illustrated in FIG. 5D, if the potential DS of the power supply line 32 is changed from the low potential Vini to the high potential Vccp, the threshold value correction process starts in a state where the gate potential Vg of the driving transistor 22 is maintained. That is, the source potential Vs of the driving transistor 22 starts increasing toward the potential that is obtained by subtracting the threshold voltage Vth of the driving transistor 22 from the gate potential Vg.
Here, for convenience, the process of changing the source potential Vs toward the potential that is obtained by subtracting the threshold voltage Vth of the driving transistor from the initialization potential Vofs based on the initialization potential Vofs of the gate electrode of the driving transistor is called a threshold value correction process. If this threshold value correction process is performed, the gate-source voltage Vgs of the driving transistor 22 converges to the threshold voltage Vth of the driving transistor 22. The voltage that corresponds to the threshold voltage Vth is maintained in the maintenance capacity 24.
In a period (threshold value correction period) in which the threshold value correction process is performed, in order to make the current flow only to the side of the maintenance capacity 24 but not flow to the side of the organic EL device 21, the potential Vcath of the common power supply line 34 is set so that the organic EL device 21 is in a cutoff state.
Next, at the time t14, the potential WS of the scanning line 31 is shifted to the low potential side, and as illustrated in FIG. 6A, the write-in transistor 23 becomes a non-conductive state. At this time, the gate electrode of the driving transistor 22 is electrically cut off from the signal line 33, and thus becomes a floating state. However, since the gate-source voltage Vgs becomes equal to the threshold voltage Vth of the driving transistor 22, the driving transistor 22 is in a cutoff state. Accordingly, the drain-source current Ids does not flow through the driving transistor 22.
(Signal Write and Mobility Correction Period)
Next, at the time t15, as illustrated in FIG. 6B, the potential of the signal line 33 is changed from the reference voltage Vofs to the signal voltage Vsig of the image signal. Then, at the time t16, the potential WS of the scanning line 31 is shifted to the high potential side, and as illustrated in FIG. 6C, the write-in transistor 23 becomes a conductive state, and samples and stores the signal voltage Vsig of the image signal in the pixel 20.
As the write-in transistor 23 writes the signal voltage Vsig, the gate potential Vg of the driving transistor 22 becomes the signal voltage Vsig. Also, when the driving transistor 22 is driven by the signal voltage Vsig of the image signal, the threshold voltage Vth of the driving transistor 22 and the voltage that corresponds to the threshold voltage Vth maintained in the maintenance capacity 24 cancel each other. The principle of threshold value cancellation will be described in detail later.
At this time, the organic EL device 21 is in a cutoff state (in high impedance state). Accordingly, the current (drain-source current Ids) flowing from the power supply line 32 to the driving transistor 22 in accordance with the signal voltage Vsig of the image signal flows into the equivalent capacitance 25 of the organic EL device 21, and the charging of the equivalent capacitance 25 starts.
As the equivalent capacitance 25 of the organic EL device 21 is charged, the source potential Vs of the driving transistor 22 is increased as time lapses. In this case, the dispersion of the threshold voltage Vth of the driving transistor 22 for each pixel has already been cancelled, and the drain-source current Ids of the driving transistor 22 depends on the mobility μ of the driving transistor 22. The mobility μ of the driving transistor 22 is the mobility of a semiconductor thin film that forms the channel of the driving transistor 22.
Here, it is assumed that the ratio of the maintenance voltage Vgs of the maintenance capacity 24 to the signal voltage Vsig of the image signal, that is, the write gain G is 1 (ideal value). As the source potential Vs of the driving transistor is increased up to the potential of Vofs−Vth+ΔV, the gate-source voltage Vgs of the driving transistor 22 becomes Vsig−Vofs+Vth−ΔV.
That is, the increment ΔV of the source potential Vs of the driving transistor 22 acts to be subtracted from the voltage (Vsig−Vofs+Vth) maintained in the maintenance capacity 24, in other words, acts to perform discharge of the maintenance capacitance 24 to put a negative feedback. Accordingly, the increment ΔV of the source potential Vs becomes the feedback amount of the negative feedback.
As described above, by putting a negative feedback on the gate-source voltage Vgs with the feedback amount ΔV according to the drain-source current Ids flowing through the driving transistor 22, the dependence on the mobility μ of the drain-source current Ids of the driving transistor 22 can be cancelled. This process of cancelling the dependence is the mobility correction process that corrects the dispersion of the mobility μ of the driving transistor 22 for each pixel.
More specifically, since the drain-source current Ids is increased as the signal amplitude Vin (=Vsig−Vofs) of the image signal that is written on the gate electrode of the driving transistor 22 becomes high, an absolute value of the feedback amount ΔV of the negative feedback is also increased. Accordingly, the mobility correction process according to the luminance level is performed.
Also, in the case where the signal amplitude Vin of the image signal is constant, the absolute value of the feedback amount ΔV of the negative feedback becomes large as the mobility μ of the driving transistor 22 is increased, and thus the dispersion of the mobility μ for each pixel can be removed. Accordingly, the feedback amount ΔV of the negative feedback may be the correction amount of mobility correction. The details of the principle of the mobility correction will be described later.
(Light Emission Period)
Next, at time t17, the potential WS of the scanning line 31 is shifted to the low potential side, as illustrated in FIG. 6D, and thus the write-in transistor 23 becomes in a non-conductive state. Accordingly, the gate electrode of the driving transistor 22 is electrically cut off from the signal line 33, and thus is in a floating state.
Here, when the gate electrode of the driving transistor 22 is in a floating state, the gate potential Vg is also changed in association with the change of the source potential Vs of the driving transistor 22 since the maintenance capacity 24 is connected between the gate and source of the driving transistor 22. As described above, the change operation of the gate potential Vg of the driving transistor 22 in association with the change of the source potential Vs is a bootstrap operation by the maintenance capacity 24.
As the gate electrode of the driving transistor 22 is in a floating state and the drain-source current Ids of the driving transistor 22 flows to the organic EL device 21, the anode potential of the organic EL device 21 is increased according to the corresponding current Ids.
Also, if the anode potential of the organic EL device 21 exceeds Vthel+Vcath, a driving current flows to the organic EL device 21, and thus the light emission of the organic EL device 21 starts. Also, the increase of the anode potential of the organic EL device 21 corresponds to the increase of the source potential Vs of the driving transistor 22. If the source voltage of the driving transistor 22 is increased, the gate potential Vg of the driving transistor 22 is also increased in association by the bootstrap operation of the maintenance capacity 24.
In this case, if it is assumed that the bootstrap gain is 1 (ideal value), the increase amount of the gate potential Vg becomes equal to the increase amount of the source potential Vs. Accordingly, during the light emission period, the gate-source voltage Vgs of the driving transistor 22 is constantly maintained as Vsig−Vofs+Vth−ΔV. Also, at time t18, the potential of the signal line 33 is changed from the signal voltage Vsig of the image signal to the reference voltage Vofs.
In a series of circuit operation as described above, respective processing operations of threshold value correction preparation, threshold value correction, write (signal write) of the signal voltage Vsig, and mobility correction are performed in one horizontal scanning period (1H). Also, respective processing operations of signal write and mobility correction are executed in parallel in a time period of t6 to t7.
(Divided Threshold Value Correction)
Here, it is exemplified that the threshold value correction process is executed only once. However, this driving method is merely exemplary, and the invention is not limited to this driving method. For example, it is also possible to adopt a driving method that performs the threshold value correction process plural times in a divided manner through a plurality of horizontal scanning periods that precede the 1H period, that is, a driving method that performs a so-called divided threshold value correction in addition to the 1H period in which the threshold value correction process is performed together with the mobility correction and the signal write process.
According to the driving method for divided threshold value correction, even if the time that is allocated in one horizontal scanning period is shortened by the multi-pixels according to the high definition, a sufficient time can be secured through a plurality of horizontal scanning period as the threshold value correction period, and thus the threshold value correction process can be accurately performed.
[Principle of Threshold Value Cancellation]
Here, the principle of threshold value cancellation (that is, threshold value correction) of the driving transistor 22 will now be described. Since the driving transistor 22 is designed to operate in a saturation region, it operates as a constant current source. Accordingly, a constant drain-source current (driving current) Ids that is given by the following equation (1) is supplied from the driving transistor 22 to the organic EL device 21.

Ids=(1⁄2)·μ(W/L)Cox(Vgs−Vth)2  (1)

Here, W denotes a channel width of the driving transistor 22, L denotes a channel length, and Cox denotes a gate capacitance per unit area.
FIG. 7 illustrates the characteristics of the drain-source current Ids versus the gate-source voltage Vgs of the driving transistor 22.
As illustrated in this characteristic diagram, if a cancellation process is not performed with respect to the dispersion for each pixel of the threshold voltage Vth of the driving transistor 22, the drain-source current Ids that corresponds to the gate-source voltage Vgs becomes Ids1 when the threshold voltage Vth is Vth1.
By contrast, if the threshold voltage Vth is Vth2 (Vth2>Vth1) in the same manner, the drain-source current Ids that corresponds to the gate-source voltage Vgs becomes Ids2 (Ids2<Ids1). That is, if the threshold voltage Vth of the driving transistor 22 is changed, the drain-source current Ids is changed even though the gate-source voltage Vgs is constant.
On the other hand, in the pixel (pixel circuit) 20 having the above-described configuration, as described above, the gate-source voltage Vgs of the driving transistor 22 during the light emission is Vsig−Vofs+VthΔV. Accordingly, by substituting this in equation (1), the drain-source current Ids is expressed as in the following equation (2).

Ids=(1⁄2)·μ(W/L)Cox(Vsig−VofsΔV)2  (2)

That is, the term of the threshold voltage Vth of the driving transistor 22 is cancelled, and the drain-source current Ids that is supplied from the driving transistor 22 to the organic EL device 21 is not dependent upon the threshold voltage Vth of the driving transistor 22. As a result, even if the threshold voltage Vth of the driving transistor 22 is changed for each pixel due to the dispersion or time-dependent change of the manufacturing process of the driving transistor 22, the drain-source current Ids is not changed, and thus the luminance of the organic EL device 21 can be maintained constant.
(Principle of Mobility Correction)
Next, the principle of mobility correction of the driving transistor 22 will be described. FIG. 8 illustrates characteristic curves in a state where a pixel A in which the mobility μ of the driving transistor 22 is relatively large and a pixel B in which the mobility μ of the driving transistor 22 is relatively small are compared with each other. In the case where the driving transistor 22 is formed of a polysilicon thin film transistor or the like, it is unavoidable that the mobility μ is changed between pixels such as pixel A and pixel B.
A case is considered, in which the signal amplitude Vin (=Vsig−Vofs) of the same level is written on the gate electrode of the driving transistor 22, for example, in both pixels A and B. In this case, if the correction of the mobility μ is not performed, there is a large difference between the drain-source current Ids1 that flows to the pixel A having a high mobility μ and the drain-source current Ids2′ that flows to the pixel B having a low mobilityμ. As described above, if there is a large difference in drain-source current Ids between the pixels due to the dispersion of the mobility μ for each pixel, the uniformity of the screen is damaged.
Here, as can be known from the transistor characteristic equation (1) as described above, if the mobility μ is high, the drain-source current Ids becomes large. Accordingly, the feedback amount ΔV of the negative feedback becomes large as the mobility μ becomes large. As illustrated in FIG. 8, the feedback amount ΔV1 of the pixel A having a high mobility is larger than the feedback amount ΔV2 of the pixel B having a low mobility.
Accordingly, by putting a negative feedback on the gate-source voltage Vgs with the feedback amount ΔV according to the drain-source current Ids of the driving transistor 22 by the mobility correction process, the negative feedback becomes larger as the mobility μ becomes higher. As a result, the dispersion of the mobility μ for each pixel can be suppressed.
Specifically, if the feedback amount ΔV1 is corrected in a pixel A having a high mobility μ, the drain-source current Ids greatly descends from Ids1′ to Ids1. On the other hand, since the feedback amount ΔV2 of the pixel B having a low mobility is small, the drain-source current Ids descends from Ids2′ to Ids2, and does not descend any further. As a result, since the drain-source current Ids1 of the pixel A becomes almost equal to the drain-source current Ids2, the dispersion of the mobility μ for each pixel is corrected.
In summary, if pixels A and B have different mobility μ, the feedback amount ΔV1 of the pixel A having a high mobility μ becomes larger than the feedback amount ΔV2 of the pixel B having a low mobility μ. That is, as the mobility μ becomes higher, the feedback amount ΔV of the pixel becomes larger and the reduction amount of the drain-source current Ids becomes larger.
Accordingly, by putting a negative feedback on the gate-source voltage Vgs with the feedback amount ΔV according to the drain-source current Ids of the driving transistor 22, the current values of the drain-source currents Ids of the pixels having different mobility μ become uniform. As a result, the dispersion of the mobilityμ for each pixel can be corrected. That is, the process of putting a negative feedback on the gate-source voltage Vgs of the driving transistor 22 with the feedback amount ΔV according to the current (the drain-source current Ids) that flows to the driving transistor 22 becomes the mobility correction process.
Here, in the pixel (pixel circuit) 20 as illustrated in FIG. 2, the relationship between the signal voltage Vsig of an image signal and the drain-source current Ids of the driving transistor 22 according to existence/nonexistence of the threshold value correction and mobility correction will be described using FIGS. 9A to 9C.
FIG. 9A shows a case where neither the threshold value correction nor the mobility correction is performed, FIG. 9B shows a case where the mobility correction is not performed, but the threshold value correction is performed, and FIG. 9C shows a case where both the threshold value correction and the mobility correction are performed. In the case where neither the threshold value correction nor the mobility correction is performed as shown in FIG. 9A, a great difference in drain-source current Ids occurs between the pixels A and B due to the dispersion of the threshold voltage Vth and the mobility μ between the pixels A and B.
In the case where only the threshold value correction is performed as shown in FIG. 9B, the dispersion of the drain-source current Ids can be somewhat reduced, but there remains a difference in drain-source current Ids between the pixels A and B due to the dispersion of the mobility μ between the pixels A and B. Also, in the case where both the threshold value correction and the mobility correction are performed as shown in FIG. 9C, the difference in drain-source current Ids between the pixels A and B due to the dispersion of the threshold voltage Vth and the mobility μ between the pixels A and B can be almost eliminated. Accordingly, the luminance dispersion of the organic EL device 21 does not occur in any grayscale, and thus a good quality display image can be obtained.
Also, since the pixel 20 illustrated in FIG. 2 has a function of a bootstrap operation by the above-described maintenance capacity 24 in addition to the function of the threshold value correction and the mobility correction, the following effects can be obtained.
That is, even if the source potential Vs of the driving transistor 22 is changed according to the time-dependent change of the I-V characteristics of the organic EL device 21, the gate-source potential Vgs of the driving transistor 22 can be maintained constant by the bootstrap operation through the maintenance capacity 24. Accordingly, the current that flows to the organic EL device 21 is not changed but is maintained constant. As a result, the luminance of the organic EL device is maintained constant, and thus even if the I-V characteristic of the organic EL device 21 is time-dependently changed, an image display accompanying no luminance deterioration can be realized.
[1-3. Regarding Bootstrap Operation]
Here, the above-described bootstrap operation will be described in detail using the timing waveform diagram of FIG. 10.
As can be known from the circuit operation as described above, at a time when the signal write and mobility correction period is ended, the signal voltage Vsig of the image signal is written on the gate electrode of the driving transistor 22. In this case, the source potential Vs of the driving transistor 22 reaches the potential Vs1 (=Vofs−Vth+ΔVs) that has ascended as high as the increment ΔVs of potential according to the mobility μ from the time when the threshold value correction process is completed.
Here, if the write-in transistor 23 is in a non-conductive state, the gate-source voltage Vgs of the driving transistor 22 is maintained by the maintenance capacity 24, and thus the source potential Vs ascends up to the potential Voled according to the current Ids that flows to the driving transistor 22. The increment amount at this time is ideally equal to the increment amount Voled−Vs1 of the source potential Vs. However, in the case where parasitic capacitance exists in the driving transistor 22 and the write-in transistor 23, the increment amount becomes smaller than the increment amount of the source potential Vs.
(Regarding Bootstrap Gain Gb)
As illustrated in FIG. 11, parasitic capacitances Cgs, Cgd, and Cws exist in the driving transistor 22 and the write-in transistor 23. The parasitic capacitance Cgs is a parasitic capacitance between the gate and source of the driving transistor 22, and the parasitic capacitance Cgd is a parasitic capacitance between the gate and drain of the driving transistor 22. The parasitic capacitance Cws is a parasitic capacitance between the gate and drain of the write-in transistor 23.
Here, it is assumed that the gate potential Vg and the source potential Vs before the bootstrap operation of the driving transistor 22 are Vg1 and Vs1, respectively, and the gate potential Vg and the source potential Vs after the bootstrap operation are Vg2 and Vs2, respectively.
Now, if it is assumed that the source potential Vs of the driving transistor 22 has ascended from the potential Vs1 to the potential Vs2, the gate potential Vg ascends only up to (Cs+Cgs)/(Cs+Cgs+Cgd+Cws)×(Vs2Vs1). The coefficient at this time, that is, (Cs+Cgs)/(Cs+Cgs+Cgd+Cws), becomes the bootstrap gain Gb, and this bootstrap gain Gb should be equal to or less than 1. Accordingly, the increment amount ΔVs of the gate potential Vg becomes smaller than the increment amount ΔVg of the source potential Vs.
As described above, in the case where the parasitic capacitance exists in the driving transistor 22 and the write-in transistor 23, the increment amount ΔVg of the gate potential Vg becomes smaller than the increment amount ΔVs of the source potential Vs. As a result, by the bootstrap operation, the gate-source voltage Vgs of the driving transistor 22 becomes lower than the gate-source voltage Vgs at a time when the mobility correction process is completed. Accordingly, in the case where the parasitic capacitance that is parasitic on the gate electrode of the driving transistor 22 is high and the bootstrap gain Gb is low, a desired luminance may not be obtained.
(Regarding Reoccurrence of Dispersion of Threshold Voltage Vth)
Also, as illustrated in FIG. 12, it is considered that the driving transistor 22 has different threshold voltages Vtha and Vthb. After completion of the threshold value correction operation, the difference in gate-source voltage Vgs between a transistor having the threshold voltage Vtha and a transistor having the threshold voltage Vthb becomes Vthb−Vtha. Even in the mobility correction operation, the increment amount ΔVs of the source potential Vs is not dependent upon the threshold voltage Vth, and thus the different in the gate-source voltage Vgs is maintained as Vthb−Vtha.
In the case of the bootstrap operation, the source voltage Vs ascends up to the voltage Voled that is determined by the current Ids of the driving transistor 22, and thus the increment amounts ΔVsa and ΔVsb of the source potential Vs differ from each other to the extent of the difference Vthb-Vtha of the threshold voltage Vth. In this case, the increment amount ΔVg of the gate potential Vg is determined by the increment amount ΔVs of the source potential Vs.
Accordingly, as illustrated in FIG. 12, the difference in gate-source voltage Vgs after the bootstrap operation becomes (Cs+Cgs)/(Cs+Cgs+Cgd+Cws)×(Vthb−Vtha), which is decreased even after the threshold value correction. Accordingly, although the threshold value correction process has been performed, the dispersion of the threshold voltage Vth occurs. If the parasitic capacitance is high, the change amount becomes large, and this causes the luminance non-uniformity.
(Regarding High Voltage of Voltage Voled of Organic EL Device 21)
In the case where the organic EL device 21 deteriorates, as illustrated in FIG. 13, the operation point of the organic El device 21 is shifted from the voltage Voled1 to the voltage Voled. That is, the operation point becomes high voltage. Here, it is considered that the voltage Voled of the organic El device 21 becomes high.
In a pixel where the organic EL device 21 does not deteriorate, the increment amount of the source potential Vs during the bootstrap operation is ΔVsa. By contrast, in a pixel where the organic EL device 21 deteriorates, the increment amount ΔVsb of the source potential Vs becomes ΔVsa+Voled2−Voled1. Accordingly, the increment amount ΔVg of the gate potential Vg is as illustrated in FIG. 14, and the gate-source voltage Vgs of the driving transistor 22 is lowered to the extent of (Cs+Cgs)/(CsCgs+Cgd+Cws)×(Voled2−Voled1). As a result, if the parasitic capacitance is high, the decrement amount of the gate-source voltage Vgs becomes large. That is, the current Ids of the driving transistor 22 deteriorates to cause burn-in.
(Structure of Write-in Transistor in the Related Art)
FIGS. 15A and 15B illustrate a general structure of a write-in transistor. FIG. 15A is a plan pattern diagram, and FIG. 15B is a cross-sectional diagram.
The write-in transistor 23A in the related art has a double gate structure having a plurality of gates, for example, two gates GA and GB as a leak prevention measure. The write-in transistor 23A in the related art also has a shield structure as shield and leak prevention measures for channel regions 231A and 231B. Specifically, the write-in transistor has a shield structure in which the opposite sides of the gate electrodes 232A and 232B of the channel regions 231A and 231B are covered by metal wiring layers 233A and 233B. The write-in transistor 23A also adopts an LDD (Lightly Doped Drain) structure having a low-density impurity region, that is, an LDD region 235, between the channel regions 231A and 231B and the source/drain region 234.
In the write-in transistor 23A having the above-described configuration in the related art, parasitic capacitances having capacitance values according to the gate width of the gate electrodes 232A and 232B are formed between the LDD region 235 and the gate electrodes 232A and 232B. Also, parasitic capacitance is formed between the metal wiring layers 233A and 233B and the channel regions 231A and 232B. These parasitic capacitances form the parasitic capacitance Cws between the gate and drain of the write-in transistor 23. If the capacitance value of the parasitic capacitance Cws is large, the bootstrap gain Gb deteriorates.
<2. Explanation of Organic El Device According to Embodiments>
The organic EL device according to the embodiment is based on the system configuration as illustrated in FIG. 1, and in the corresponding system configuration, the structure of the write-in transistor constituting a pixel is characterized. Hereinafter, the detailed structure of the write-in transistor 23B will be described.
The write-in transistor 23B according to the embodiment has a structure having a plurality of gates, for example, has a double gate structure having two gates. This double gate structure has an advantage that it can reduce leak current between the source region and the drain region.
Also, the write-in transistor 23B adopts a sandwich structure with respect to the gate on the side of the driving transistor 22 among a plurality of gates. Specifically, the write-in transistor has a sandwich structure in which a second gate electrode that is positioned on the opposite side of a first gate electrode is provided as a back gate electrode with respect to the channel region, and the channel region is sandwiched between the two gate electrodes (first and second gate electrodes). According to this sandwich structure, for example, the transistor characteristic can be improved in comparison to a bottom gate structure.
In the write-in transistor 23B that adopts a double gate structure and the sandwich structure, the width of the channel region of the gate of the driving transistor side 22 is set to be narrower than the width of the channel region of other gates.
Here, between the second gate electrode that is the back gate electrode and the channel region, parasitic capacitance is formed, which has a capacitance value according to the opposite region between the second gate electrode and the channel region. In this case, in the gate of the driving transistor side 22, the width of the channel region is narrower than the width of the channel region of other gates, and thus the capacitance value of the parasitic capacitance becomes smaller than the capacitance value of the parasitic capacitance formed in other gates.
As described above, the parasitic capacitance that is parasitic on the write-in transistor 23B, particularly, the parasitic capacitance of the gate of the driving transistor side 22, becomes one parameter that determines the bootstrap gain Gb. Accordingly, since the capacitance value of the parasitic capacitance can be reduced, the bootstrap gain Gb can be improved and a good quality display image can be obtained without damaging the uniformity of the screen.
In the gate on the side of the driving transistor 22, it is preferable that the gate electrodes are formed so that the width of the second gate electrode is narrower than the width of the first gate electrode on the point of reducing the capacitance value of the parasitic capacitance. Also, on the point of simplifying the manufacturing process, it is preferable to form the second gate electrode with the same wire material as the signal line 33 (33−1 to 33−n) for transmitting the image signal. On the point of shielding and leak measure, it is preferable to adopt a shield structure in which the channel region is covered by the metal wiring layer even with respect to other gates.
EXAMPLES
The detailed examples of the write-in transistor 23B will be described using FIGS. 16A and 16B. FIGS. 16A and 16B are diagrams illustrating the structure of a write-in transistor 23B according to an example of the invention. FIG. 16A is a plane pattern diagram, and FIG. 16B is a cross-sectional diagram. The same reference numerals are used for the same portions as in FIGS. 15A and 15B.
The write-in transistor 23B according to the example of the invention, for example, adopts a double gate structure having two gates GA and GB. By adopting the double gate structure, leak current between the source region (source/drain region 234 on one side) and the drain region (source/drain region 234 on the other side) can be reduced.
Of the two gates GA and GB, the gate GA on the side of the signal line 33 adopts a shield structure as a shield and leak prevention measures for the channel region 231A. Specifically, in the gate GA on the side of the signal line 33, the gate electrode (first gate electrode) 232A and the metal wiring layer 233A on the opposite side are formed with respect to the channel region 231A, and the channel region 231A is shielded by the metal interconnection layer 233A.
Of the two gates GA and GB, the gate GB on the side of the driving transistor 22 adopts a shield structure in the same manner as the side of the gate GA as a shield and leak prevention measures for the channel region 231B. However, on the gate GB on the side of the driving transistor 22, the second gate electrode 236 is arranged on an opposite side to the first gate electrode 232B with respect to the channel region 231B, as a back gate electrode.
That is, with respect to the gate GB on the side of the driving transistor 22, a sandwich gate structure is formed, in which the channel region 231B is sandwiched between the gate electrode 232B and two gate electrodes 232B and 236 of the back gate electrode 236. In the gate GB having the sandwich gate structure, the back gate electrode 236 functions as a shield member for shielding measures. In forming the back gate electrode 236, on the point of seeking the simplicity of the manufacturing process, it is preferable to form the back gate electrode 236 with the same wiring material as the metal wiring layer such as the signal line 33 (33−1 to 33−n).
FIG. 17 is a diagram illustrating the relationship between the gate voltage Vg of an N-channel transistor and the drain-source current Ids. In FIG. 17, a solid line represents the characteristic in the case of the sandwich gate structure, and a dashed line represents the characteristic in the case of a bottom gate structure. As can be known from the drawing, the sandwich gate structure side has a superior characteristic than that of the bottom gate structure. Also, by using the N-channel transistor having the sandwich gate structure as the write-in transistor 23, the improvement of the characteristic of the write-in transistor 23 can be sought.
In the gate GB of the write-in transistor 23B adopting the double gate structure and the sandwich gate structure, the width WB of the channel region 231B is set to be narrower than the width WA of the channel region 231A of other gates GA. Here, between the back gate electrode (second gate electrode) 236 and the channel region 231B, a parasitic capacitance having a capacitance value according to the opposite area between the back gate electrode 236 and the channel region 231B.
In the gate GB on the side of the driving transistor 22, since the width WB of the channel region 231B is narrower than the width WA of the channel region 231A of the gate GA, the opposite area can be reduced. Accordingly, the capacitance value of the parasitic capacitance that is formed on the gate GB can be set to be smaller than the capacitance value of the parasitic capacitance that is formed on the gate GA. Here, on the point of reducing the capacitance value of the parasitic capacitance, it is preferable that the length L2 of the back gate electrode 236 in the channel length direction is shorter than the length L1 of the first gate electrode 232B.
The parasitic capacitance of the gate GB on the side of the driving transistor 22 becomes one parameter that determines the bootstrap gain Gb. Accordingly, since the bootstrap gain Gb can be improved by reducing the capacitance value of the parasitic capacitance of the gate GB, a good quality display image can be obtained without damaging the uniformity of the screen. At this time, although the characteristic of the write-in transistor 23 deteriorates through narrowing of the width WB of the channel region 231B, the deterioration amount can be covered by adopting the sandwich gate structure, and thus the equivalent transistor characteristic to that of the structure in the related art can be maintained.
Also, since the sandwich gate structure is adopted with respect to the gate GB on the side of the driving transistor 22, the shielding measures for the channel region 231B can be devised without adopting a dedicated shielding structure. Since the dedicated shielding structure is not adopted, the parasitic capacitance between the gate electrode and the shield (back gate electrode 236) can be eliminated.
Also, the write-in transistor 23 adopts an LDD structure in which an impurity region having a density that is lower than that of the corresponding region 234, that is, an LDD region 235, is installed between the channel regions 231A and 231B and the source/drain region 234 so that high electric field is not concentrated onto the region. Especially, in the gate GB on the side of the driving transistor 22, which adopts the LDD structure, the back gate electrode 236 is formed not to overlap the LDD region 235, and thus the capacitance value of the parasitic capacitance is not increased by the back gate electrode 236. Accordingly, the capacitance value of the parasitic capacitance of the write-in transistor 23 can be further reduced.
<3. Modified Examples>
In the above-described embodiment, it is exemplified that the pixel is configured so that the driving circuit of the organic EL device 21 is basically composed of two transistors including the driving transistor 22 and the write-in transistor 23. However the invention is not limited thereto. That is, the invention can be applied to the whole display devices configured to have a write-in transistor 23 of a structure in which the pixel has a plurality of gates.
Also, in the above-described embodiment, it is exemplified that an organic EL display in which an organic EL device is used as an electro-optical component of the pixel 20 is adopted. However, the invention is not limited to such an application. Specifically, the invention can be applied to the entire display devices that use current driving type electro-optical component (light emitting device) of which the luminance is changed according to the current value flowing through the device, such as an inorganic EL device, an LED device, a semiconductor layer device, and the like.
<4. Applications>
As described above, the display device according to the embodiment of the invention can be applied to display devices of electronic appliances in all fields where an image signal input to the electronic appliance or an image signal generated in the electronic appliance is displayed as an image or a video. As an example, it is possible to apply the invention to display devices of diverse electronic appliances, such as a digital camera, a notebook type personal computer, a portable terminal such as a portable phone, and a video camera.
As described above, by using the display device according to the embodiment of the invention as the display device of electronic appliances in all fields, the picture quality of the display image can be improved in various kinds of electronic appliances. That is, as can be understood from the explanation of the embodiment as described above, since the display device according to the embodiment of the invention can obtain a good-quality display image without damaging the uniformity of the screen by improving the bootstrap gain Gb, the picture quality of the display image can be improved in various kinds of electronic appliances.
The display device according to the embodiment of the invention includes a module-shaped device of a sealed configuration. For example, it corresponds to a display module that is formed by attaching an opposite unit such as transparent glass to a pixel array unit 30. In this transparent opposite unit, a color filter, a protection film, and the above-described shielding film may be installed. In this case, a circuit unit for inputting/outputting signals from the outside to the pixel array unit or FPC (Flexible Printed Circuit) may be installed in the display module.
Hereinafter, detailed example of electronic appliances to which the invention is applied will be described.
FIG. 18 is a perspective diagram illustrating an external appearance of a television set to which the invention is applied. The television set in this application includes an image display screen unit 101 that is composed of a front panel 102 or a filter glass 103, and is manufactured using the display device according to the embodiment of the invention as the image display screen unit 101.
FIGS. 19A and 19B are perspective diagrams illustrating an external appearance of a digital camera to which the invention is applied. FIG. 19A is a perspective diagram as seen from the surface side, and FIG. 19B is a perspective diagram as seen from the rear surface side. The digital camera according to this application includes a light-emitting unit 111 for a flash, a display unit 112, a menu switch 113, a shutter button 114, and the like, and is manufactured using the display device according to the embodiment of the invention as the display unit 112.
FIG. 20 is a perspective diagram illustrating an external appearance of a notebook type personal computer to which the invention is applied. The personal computer according to this application includes a main body 121, a keyboard 122 that is operated when inputting characters and the like, and a display unit 123 for displaying an image, and is manufactured using a display device according to the embodiment of the invention as the display unit 123.
FIG. 21 is a perspective diagram illustrating an external appearance of a video camera to which the invention is applied. The video camera according to this application includes a main body unit 131, a lens 132 provided on a side surface toward the front to capture an image of an object, a start/stop switch 133 used during capturing an image, and a display unit 134, and is manufactured using the display device according to the embodiment of the invention as the display unit 134.
FIGS. 22A to 22G are diagrams illustrating external appearances of a portable terminal, for example, a portable phone, to which the invention is applied. FIG. 22A is a front diagram of a portable phone in an open state, FIG. 22B is a side diagram thereof, FIG. 22C is a front diagram of a portable phone in a closed state, FIG. 22D is a left side diagram thereof, FIG. 22E is a right side diagram thereof, FIG. 22F is a plan diagram thereof, and FIG. 22G is a bottom diagram thereof. The portable phone according to this application includes an upper housing 141, a lower housing 142, a connection unit (here, hinge unit) 143, a display 144, a sub-display 145, a picture light 146, and a camera 147, and is manufacture using the display device according to the embodiment of the invention as the display 144 or the sub-display 145.
The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-075059 filed in the Japan Patent Office on Mar. 29, 2010, the entire contents of which is hereby incorporated by reference.
It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.

## Claims

1. A display device comprising a plurality of arranged pixels, each of which includes an electro-optical component, a write-in transistor for writing an image signal into the pixel, a maintenance capacity element for maintaining the image signal written by the write-in transistor, and a driving transistor for driving the electro-optical component based on the image signal maintained by the maintenance capacity element,
wherein,
the write-in transistor has a plurality of gates,
a driving transistor side gate from among the plurality of gates has a structure in which a channel region is sandwiched between a first gate electrode and a second gate electrode,
the width of the channel region of the driving transistor side gate is narrower than the width of the channel region of each of the other gates, and
except for the driving transistor side gate, the gates are shielded by a metal wiring layer.

2. The display device according to claim 1, wherein the second gate electrode has a width that is narrower than a width of the first gate electrode.

3. The display device according to claim 1, wherein the second gate electrode is formed of the same wiring material as a signal line for transmitting the image signal.

4. The display device according to claim 1, wherein the write-in transistor has an LDD structure in which an impurity region having a density that is lower than that of a source/drain region is provided between the source/drain region and a channel region.

5. The display device according to claim 4, wherein the second gate electrode does not overlap the impurity region.

6. The display device according to claim 1, wherein, in the write-in transistor, a parasitic capacitance exists between the channel region and the second gate electrode, and the capacitance value of the parasitic capacitance is one parameter that determines a gain during a bootstrap operation in which a gate potential of the driving transistor is changed to follow a source potential of the driving transistor when the write-in transistor is in a non-conductive state.

7. The display device according to claim 6, wherein the source potential of the driving transistor is changed according to a current flowing through the driving transistor.

8. An electronic appliance comprising: a display device including a plurality of arranged pixels, each of which includes an electro-optical component, a write-in transistor for writing an image signal into the pixel, a maintenance capacity element for maintaining the image signal written by the write-in transistor, and a driving transistor for driving the electro-optical component based on the image signal maintained by the maintenance capacity element,
wherein,
the write-in transistor has a plurality of gates, the driving transistor-side gate from among the plurality of gates has a structure in which a channel region is sandwiched between a first gate electrode and a second gate electrode, and a width of the channel region of the driving transistor-side gate is narrower than a width of the channel region of each of the other gates, and
except for the driving transistor-side gate, the plurality of gates are shielded by a metal wiring layer.