Abstract:
Provided is an image display device, which includes: a display area including a plurality of pixels each of which includes a self light-emitting element; and a plurality of signal lines for inputting an image voltage to each of the plurality of pixels, in which: each of the plurality of pixels includes a field-effect transistor for driving the self light-emitting element based on the image voltage which is input through each of the plurality of signal lines to each of the plurality of pixels; the display area is divided into at least two regions including a first region and a second region; and a (channel width-to-channel length) ratio of the field-effect transistor in the first region is smaller than the ratio of any field-effect transistor in the second region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese application JP2009-025833 filed on Feb. 6, 2009, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image display device using, for example, an organic electroluminescence (EL) element, and more particularly, to an image display device capable of displaying a high-quality image with high definition at low voltage. 
     2. Description of the Related Art 
     In recent years, the demand has been increased for flat panel display devices instead of cathode ray tube (CRT) display devices, which are the mainstream of conventional display devices. In particular, an organic EL display device using an organic EL element such as an organic light-emitting diode (OLED) is excellent in terms of power consumption, weight, thickness, video image characteristic, view angle, and the like, and hence the development and practical use are advanced. 
     In the organic EL display device, each pixel has a driving transistor for driving the organic EL element. When a fluctuation in threshold voltage Vth of the driving transistor of each pixel is large, a fluctuation in light emission characteristic of each pixel occurs to reduce the uniformity of a screen, and hence high quality cannot be maintained. 
     The driving transistor for driving the organic EL element is normally a thin film transistor. The thin film transistor has a large fluctuation in threshold voltage Vth. 
     Therefore, the organic EL display device has the following problem. That is, the fluctuation in threshold voltage Vth of the driving transistor of each pixel becomes larger, and hence the fluctuation in light emission characteristic of each pixel occurs to reduce the uniformity of the screen. Thus, high quality cannot be maintained. 
     In view of this point, it is necessary for the organic EL display device to cancel the fluctuation in threshold voltage Vth of the driving transistor of each pixel. 
     An image display device in which the fluctuation in threshold voltage Vth of the driving transistor of each pixel is cancelled is disclosed in, for example, R. M. A. Dawson et al., “Design of an Improved Pixel for a Polysilicon Active-Matrix Organic LED Display,” SID Symposium Digest 29 11 (1998). 
       FIG. 13  is a circuit diagram illustrating an equivalent circuit of an example of a pixel of a conventional organic EL display device, which illustrates an equivalent circuit of a most typical pixel employing a voltage program system. 
     A signal line  12 , a selection switch line Y, and a power supply line  6  are led to a pixel  1  illustrated in  FIG. 13 . 
     The pixel  1  includes an organic electroluminescence element (hereinafter referred to as organic EL element)  2  serving as a light emitting element. 
     The organic EL element  2  has a cathode electrode connected to a common ground line, and an anode electrode connected to the power supply line  6  through a p-type thin film transistor (hereinafter referred to as driving TFT)  4 . 
     A holding capacitor element  3  is connected between a gate electrode and a source electrode of the driving TFT  4 . The gate electrode of the driving TFT  4  is connected to the signal line  12  through a selection switch element  32  consisting of an n-type thin film transistor. A gate electrode of the selection switch element  32  is connected to the selection switch line Y. 
       FIG. 14  is a time chart for illustrating an operation of the pixel  1  illustrated in  FIG. 13 . 
     In the organic EL display device having the pixel  1  illustrated in  FIG. 13 , one frame period (FRM) includes a write period and a light emission period. An image voltage is written into the pixel  1  during the write period. Light is emitted for display during the light emission period. The writing of the image voltage is performed for each display line, that is, for each selection switch line Y. 
     First, as illustrated in  FIG. 14 , during the write period, the selection switch element  32  is turned on. Then, an analog image voltage is supplied from the signal line  12 , and the image voltage is input to the holding capacitor element  3 . 
     During the light emission period, the organic EL element  2  emits light. During the light emission period, the selection switch element  32  is turned off, and hence a voltage corresponding to the image voltage which has been stored in the holding capacitor element  3  is applied to the gate electrode of the driving TFT  4 , with the result that a current corresponding to the applied voltage flows through the organic EL element  2 , to thereby adjust light emission luminance. 
       FIG. 15  is a circuit diagram illustrating an equivalent circuit of another example of a pixel of a conventional organic EL display device, and the pixel illustrated in  FIG. 15  is a most typical pixel employing a voltage program system. 
     A signal line  12 , a reset line  7 , a selection switch line Y, a lighting switch line  21 , and a power supply line  6  are led to the pixel  1  illustrated in  FIG. 15 . 
     The pixel  1  includes an organic EL element  2  serving as a light emitting element. 
     The organic EL element  2  has a cathode electrode connected to a common ground line, and an anode electrode connected to the power supply line  6  through a lighting switch element  20  consisting of an n-type thin film transistor and a driving TFT  4  consisting of an p-type thin film transistor. 
     A second holding capacitor element  30  is connected between a gate electrode and a source electrode of the driving TFT  4 . A reset switch element  5  consisting of an n-type thin film transistor is provided between a drain electrode and the gate electrode of the driving TFT  4 . Further, the gate electrode of the driving TFT  4  is connected to the signal line  12  through a first holding capacitor element  3  and a selection switch element  32  consisting of an n-type thin film transistor. 
     A gate electrode of the reset switch element  5  is connected to the reset line  7 . A gate electrode of the selection switch element  32  is connected to the selection switch line Y. A gate electrode of the lighting switch element  20  is connected to the lighting switch line  21 . 
     In the organic EL display device having the pixel  1  illustrated in  FIG. 15 , one frame period (FRM) includes a write period and a light emission period. An image voltage is written into the pixel  1  during the write period. Light is emitted for display during the light emission period. The writing of the image voltage is performed for each display line, that is, for each selection switch line Y. 
       FIG. 16  is a time chart for illustrating an operation of the pixel  1  illustrated in  FIG. 15 . 
     In the following, an operation during each of the write period and the light emission period is described. 
     First, as illustrated in  FIG. 16 , during a period between a time t 1  and a time t 2  of the write period, the reset switch element  5  and the lighting switch element  20  are turned on. As a result, the driving TFT  4  has a diode connection in which the gate electrode is connected to the drain electrode, and hence a voltage of the gate electrode of the driving TFT  4  which has been stored in each of the holding capacitor elements ( 3  and  30 ) in a preceding field is cleared. 
     Next, when the lighting switch element  20  is turned off at the time t 2 , the driving TFT  4  and the organic EL element  2  forcedly become a current off state. At this time, the gate electrode and the drain electrode of the driving TFT  4  are short-circuited through the reset switch element  5 , and hence the voltage of the gate electrode of the driving TFT  4 , the gate electrode also corresponding to one end of the first holding capacitor element  3 , is automatically reset to a voltage (VDD-Vth) which is lower than a voltage VDD of the power supply line  6  by a threshold voltage Vth. 
     During a period between the time t 1  and the time t 3 , a fixed voltage (reference voltage) is supplied to the signal line  12 . Further, the selection switch element  32  is turned on during the period between the time t 1  and the time t 3 . Therefore, the fixed voltage (reference voltage) is input from the signal line  12  to the other end of the first holding capacitor element  3 . 
     Next, at the time t 3 , the reset switch element  5  is turned off. After that, an analog image voltage is supplied to the signal line  12 , and the image voltage is input to the other end of the first holding capacitor element  3 . 
     During the light emission period starting from the time t 5 , the reset switch element  5  and the selection switch element  32  are turned off and the lighting switch element  20  is turned on, and hence the organic EL element  2  emits light. 
     During the light emission period, a voltage corresponding to the change from the reference voltage to the image voltage is applied to the gate electrode of the driving TFT  4 , and hence a current corresponding to the applied voltage flows through the organic EL element  2 , to thereby adjust light emission luminance. 
     As described above, in each pixel  1  of the organic EL display device illustrated in  FIG. 15 , the voltage of the gate electrode of the driving TFT  4  is automatically reset to the voltage (VDD-Vth) which is lower than the voltage VDD on the power supply line  6  by the threshold voltage Vth. Therefore, the fluctuation in threshold voltage of the driving TFT  4  is suppressed, and hence the light emission with high uniformity may be realized. 
     SUMMARY OF THE INVENTION 
       FIG. 12  is a diagram for illustrating a problem which arises in an organic EL display device which includes one of the pixels illustrated in  FIGS. 13 and 15 . 
     In general, an organic EL display panel carries out display in high luminance in order to increase visibility. Further, in a case of displaying textual information or the like, a high-contrast display (in which, for example, characters are displayed at a highest tone while the rest is displayed at a lowest tone) is carried out. 
     As illustrated in  FIG. 12 , in a case where a display region  1  (AR 1 ) for displaying a still image such as textual information and a display region  2  (AR 2 ) for displaying a video image are provided by employing a conventional technology, a current amount at a highest tone in the display region  1  (AR 1 ) is fixed to a current amount defined by a maximum luminance in the display region  2  (AR 2 ). 
     Accordingly, in the display region  1  (AR 1 ), the rate of degradation of the organic EL element  2  largely varies between a pixel which is turned on at a high tone over a long time and a pixel which is not turned on. The pixels adjacent to each other bear a significant luminance difference along with the degradation over time, which appears as burn-in. Burn-in appears more significantly in the display region  1  (AR 1 ), which displays a fixed pattern for a long time, as compared with the display region  2  (AR 2 ). 
     Further, in the conventional technology, the display is driven by a hold drive that the light is emitted for one frame period after inputting of a video signal. Accordingly, when a video image is displayed, the image lag on the human eye causes a motion blur. 
     The present invention has been made to solve the problems of the conventional technologies described above, and therefore, it is an object of the present invention to provide an image display device having a pixel structure which may be suitably applied to a case that, for example, the display area is divided into a region for displaying a still image and a region for displaying a video image. 
     The above and other objects and novel features of the present invention become apparent from the description of this specification and the accompanying drawings. 
     Representative aspects of the invention disclosed in this application are generally and briefly described as follows. (1) There is provided an image display device including: a display area which includes a plurality of pixels each of which includes a self light-emitting element (such as an organic electroluminescence (EL) element); and a plurality of signal lines for inputting an image voltage to each of the plurality of pixels, in which: each of the plurality of pixels includes a field-effect transistor for driving the self light-emitting element based on the image voltage which is input through each of the plurality of signal lines to each of the plurality of pixels; and the display area is divided into at least two regions, based on a value on a (channel width-to-channel length) ratio of a channel width to a channel length of the field-effect transistor. (2) In the image display device according to item (1): the at least two regions are two regions including a first region and a second region; and the (channel width-to-channel length) ratio of the field-effect transistor in the first region is smaller than the (channel width-to-channel length) ratio of any field-effect transistor in the second region. (3) In the image display device according to item (2): the first region is a still image display region; and the second region is a video image display region. (4) There is provided an image display device including: a display area which includes a plurality of pixels each of which includes a self light-emitting element (such as an organic electroluminescence (EL) element); and a plurality of signal lines for inputting an image voltage to each of the plurality of pixels, each of the plurality of pixels includes a field-effect transistor for driving the self light-emitting element based on the image voltage which is input to each of the plurality of pixels, and the display area is divided into at least two regions, based on a pixel circuit configuration. (5) In the image display device according to item (4): the display area is divided into two regions including a first region and a second region; the first region is a still image display region; and the second region is a video image display region. 
     (6) In the image display device according to item (2) or (5): the first region of the display area includes a plurality of selection switch lines for inputting a selection voltage to each of the plurality of pixels; each of the plurality of pixels in the first region includes: a first holding capacitor element connected between a first electrode and a gate electrode of the field-effect transistor; and a selection switch element connected between a signal line of the plurality of signal lines and the gate electrode of the field-effect transistor; the selection switch element is controlled to be turned on and off based on the selection voltage which is input through each of the plurality of selection switch lines to each of the plurality of pixels; the second region of the display area includes a plurality of reset lines for inputting a reset voltage to each of the plurality of pixels; each of the plurality of pixels in the second region includes: a second holding capacitor element connected between a signal line of the plurality of signal lines and the gate electrode of the field-effect transistor; and a reset switch element connected between the gate electrode and a second electrode of the field-effect transistor; and the reset switch element is controlled to be turned on and off based on the reset voltage which is input through each of the plurality of reset lines to each of the plurality of pixels. 
     (7) In the image display device according to item (2) or (5): the first region of the display area includes: a plurality of selection switch lines for inputting a selection voltage to each of the plurality of pixels; and a plurality of first reset lines for inputting a reset voltage to each of the plurality of pixels; each of the plurality of pixels in the first region includes: a first holding capacitor element connected between a first electrode and a gate electrode of the field-effect transistor; a second holding capacitor element whose one electrode connected to the gate electrode of the field-effect transistor; a selection switch element connected between a signal line of the plurality of signal lines and another electrode of the second holding capacitor element; and a first reset switch element connected between the gate electrode and a second electrode of the field-effect transistor; the selection switch element is controlled to be turned on and off based on the selection voltage which is input through each of the plurality of selection switch lines to each of the plurality of pixels; the first reset switch element is controlled to be turned on and off based on the reset voltage which is input through each of the plurality of first reset lines to each of the plurality of pixels; the second region of the display area includes a plurality of second reset lines for inputting a reset voltage to each of the plurality of pixels; each of the plurality of pixels in the second region includes: a third holding capacitor element connected between a signal line of the plurality of signal lines and the gate electrode of the field-effect transistor; and a second reset switch element connected between the gate electrode and the second electrode of the field-effect transistor; and the second reset switch element is controlled to be turned on and off based on the reset voltage which is input through each of the plurality of second reset lines to each of the plurality of pixels. (8) In the image display device according to item (6) or (7): the first region and the second region of the display area each include a plurality of lighting switch lines for inputting a lighting voltage to each of the plurality of pixels; each of the plurality of pixels in the first region and the second region includes a lighting switch element connected between the second electrode of the field-effect transistor and the self light-emitting element; and the lighting switch element is controlled to be turned on and off based on the lighting voltage input to each of the plurality of pixels through each of the plurality of lighting switch lines. 
     An effect obtained by the representative aspects of the invention disclosed in this application is briefly described as follows. 
     According to the present invention, there may be implemented an image display device having a pixel structure suitably applied to a case that a display area is divided into a region for displaying a still image and a region for displaying a video image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  each illustrate pixels corresponding to basic components according to the present invention; 
         FIG. 2  is a block diagram illustrating an entire configuration of an image display device according to a first embodiment of the present invention; 
         FIG. 3A  illustrates a pixel layout of the image display device according to the first embodiment of the present invention; 
         FIG. 3B  illustrates another pixel layout of the image display device according to the first embodiment of the present invention; 
         FIG. 4  is a circuit diagram illustrating an equivalent circuit of a pixel applied to an image display device according to a second embodiment of the present invention; 
         FIG. 5  is a time chart for illustrating an operation of the pixel illustrated in  FIG. 4 ; 
         FIG. 6  is a block diagram illustrating an entire configuration of the image display device according to the second embodiment of the present invention; 
         FIGS. 7A and 7B  each are a circuit diagram illustrating an equivalent circuit of a pixel circuit of the image display device according to the second embodiment of the present invention; 
         FIG. 8  is a time chart for illustrating operations of the pixels illustrated in  FIGS. 7A and 7B ; 
         FIG. 9  is a block diagram illustrating an entire configuration of an image display device according to a third embodiment of the present invention; 
         FIGS. 10A and 10B  each are a circuit diagram illustrating an equivalent circuit of a pixel circuit of the image display device according to the third embodiment of the present invention; 
         FIG. 11  is a time chart for illustrating operations of the pixels illustrated in  FIGS. 10A and 10B ; 
         FIG. 12  is a diagram for illustrating a problem which arises in an organic EL display device including one of pixels illustrated in  FIGS. 13 and 15 ; 
         FIG. 13  is a circuit diagram illustrating an equivalent circuit of an example of one pixel of a conventional organic EL display device; 
         FIG. 14  is a time chart for illustrating an operation of the one pixel illustrated in  FIG. 13 ; 
         FIG. 15  is a circuit diagram illustrating an equivalent circuit of another example of the one pixel of a conventional organic EL display device; and 
         FIG. 16  is a time chart for illustrating an operation of the one pixel illustrated in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention are described in detail with reference to the attached drawings. 
     Throughout the explanatory drawings for the embodiments, elements having the same functions are expressed by the same reference symbols and the duplicated description thereof is omitted. 
     (An Overview of an Image Display Device According to the Present Invention) 
       FIGS. 1A and 1B  schematically illustrate an image display device according to the present invention. 
     According to the present invention, as illustrated in  FIGS. 1A and 1B , an image display area AR is divided into two regions, namely, a display region  1  (AR 1 ) and a display region  2  (AR 2 ), according to display information. In each of the display regions, pixels are driven in a manner suited for the display information to be displayed in the region. A pixel  1  ( 1 - 1 ) in the display region  1  (AR 1 ) is different in pixel configuration from a pixel  2  ( 1 - 2 ) in the display region  2  (AR 2 ). A description is given of how to drive each pixel in a manner suited for the display, with reference to the embodiments. 
     The display area may be divided into an upper part and a lower part as illustrated in  FIG. 1A , or may be divided into a right part and a left part as illustrated in  FIG. 1B . 
     Further, the display area may be divided into three or more regions. It goes without saying that the display area may be divided in any pattern other than the two patterns illustrated in  FIGS. 1A and 1B . 
     First Embodiment 
       FIG. 2  is a block diagram illustrating an entire configuration of an image display device according to a first embodiment of the present invention. In  FIG. 2 , reference numeral  1 - 1  denotes the pixel  1 ,  1 - 2  denotes the pixel  2 ,  6  denotes a power supply line,  8  denotes a scan line drive circuit,  9  denotes a signal line drive circuit,  12  denotes a signal line, and  51  denotes a signal input line, and reference symbol AR 1  denotes the display region  1 , AR 2  denotes the display region  2 , and Y denotes a selection switch line. 
       FIGS. 3A and 3B  each illustrate a pixel layout of the image display device according to the first embodiment of the present invention. An equivalent circuit of each of the pixels illustrated in  FIGS. 3A and 3B  is similar to an equivalent circuit illustrated in  FIG. 13 . 
     In  FIGS. 3A and 3B , reference numeral  101  denotes a semiconductor film of an n-type thin film transistor for forming a selection switch element  32 ,  102  denotes wiring for forming the signal line  12 ,  103  denotes wiring for forming the power supply line  6 ,  104  denotes gate electrode wiring for forming the selection switch line Y,  105  and  109  denote wiring,  106  denotes a semiconductor film for forming a driving TFT  4 ,  107  denotes gate electrode wiring for forming the driving TFT  4 ,  108  denotes wiring for forming one of electrodes of a holding capacitor element  3 ,  110  denotes an organic EL film, and  111  denotes an anode electrode of an organic EL element, and reference symbol CH denotes a contact hole. 
     The pixel  1  ( 1 - 1 ) in the display region  1  (AR 1 ) of  FIG. 2  corresponds to the pixel layout of  FIG. 3A , while the pixel  2  ( 1 - 2 ) in the display region  2  (AR 2 ) of  FIG. 2  corresponds to the pixel layout of  FIG. 3B . 
     As illustrated in  FIGS. 3A and 3B , the driving TFT  4  of the display region  1  (AR 1 ), which mainly displays a still image, is adapted to have a “channel width-to-channel length” value smaller than a “channel width-to-channel length” value of the driving TFT  4  of the display region  2  (AR 2 ), which mainly displays a video image. 
     With this configuration, even in a case where the pixel  1  ( 1 - 1 ) and the pixel  2  ( 1 - 2 ) are applied with the same image voltage, the pixel  1  ( 1 - 1 ) becomes lower in luminance than the pixel  2  ( 1 - 2 ), which reduces the rate of deterioration of the organic EL element  2 , to thereby suppress burn-in. 
     Second Embodiment 
       FIG. 4  is a circuit diagram illustrating an equivalent circuit of one pixel applied to an image display device according to a second embodiment of the present invention, and  FIG. 5  is a time chart for illustrating an operation of the pixel illustrated in  FIG. 4 . 
     In the case of the pixel illustrated in  FIG. 4 , the number of elements included in the pixel circuit is reduced as compared with the pixel illustrated in  FIG. 15 . 
     As illustrated in  FIG. 4 , the organic EL element  2  has a cathode electrode connected to a common ground line, and an anode electrode connected to the power supply line  6  through a lighting switch element  20  consisting of an n-type thin film transistor and the driving TFT (p-type thin film transistor)  4 . 
     A reset switch element  5  consisting of an n-type thin film transistor is provided between a drain electrode and a gate electrode of the driving TFT  4 . The gate electrode of the driving TFT  4  is connected to the signal line  12  through the holding capacitor element  3 . 
     A gate electrode of the reset switch element  5  is connected to a reset line  7 . A gate electrode of the lighting switch element  20  is connected to a lighting switch line  21 . 
     In the case of the pixel illustrated in  FIG. 4 , the number of elements included in the pixel is reduced. Accordingly, it is necessary to divide one frame period (FM) into a write period and a light emission period. 
     In the following, an operation during each of the write period and the light emission period is described with reference to  FIG. 5 . 
     First, at a time t 1  of the write period, the lighting switch element  20  and the reset switch element  5  are turned on. As a result, the driving TFT  4  has a diode connection in which the gate electrode is connected to the drain electrode, and hence a voltage of the gate electrode of the driving TFT  4  which has been stored in the holding capacitor element  3  in a preceding field is cleared. 
     Next, when the lighting switch element  20  is turned off at a time t 2 , the driving TFT  4  and the organic EL element  2  forcedly become a current off state. At this time, the gate electrode and the drain electrode of the driving TFT  4  are short-circuited through the reset switch element  5 , and hence the voltage of the gate electrode of the driving TFT  4 , the gate electrode also corresponding to one end of the holding capacitor element  3 , is automatically reset to a voltage (VDD-Vth) which is lower than a voltage VDD on the power supply line  6  by a threshold voltage Vth. At this time, an analog image voltage Vs (k) is input from the signal line  12  to the other end of the holding capacitor element  3 . 
     Next, at a time t 3 , the reset switch element  5  is turned off, and hence the writing of an analog image voltage into the pixel is stopped. In this manner, the writing of an analog image voltage into a pixel is sequentially performed for each display line. After the writing into all the pixels is completed, the “write period” ends. 
     During the “light emission period”, the reset switch element  5  is turned off while the lighting switch element  20  of each of all the pixels is simultaneously turned on. 
     At this time, a triangular wave voltage illustrated in  FIG. 5  is input to the signal line  12 . The organic EL element  2  of each pixel is driven by the driving TFT  4  based on a voltage relationship between the analog image voltage Vs (k) which is written in advance and the triangular wave voltage applied to the signal line  12 . More specifically, the driving TFT  4  causes a current corresponding to a differential voltage between the analog image voltage Vs (k) which is written in advance and the triangular wave voltage applied to the signal line  12  to flow through the organic EL element  2  of each pixel. 
     In this case, when a mutual conductance (gm) of the driving TFT  4  is sufficiently large, the organic EL element  2  may be assumed to be driven in such a digital manner as to be turned on and off. More specifically, the organic EL element  2  continues to emit light at a substantially constant luminance during only a period depending on the analog image voltage value Vs (k) which is written in advance. The modulation of the light emission time period is visually recognized as multi-gradation light emission. 
     Further, in the pixel illustrated in  FIG. 4 , the organic EL element  2  is forcedly turned off during the write period, to thereby eliminate an image lagon the human eye. As a result, a motion blur may be suppressed. 
       FIG. 6  is a block diagram illustrating an entire configuration of the image display device according to the second embodiment of the present invention. In  FIG. 6 , reference numeral  1 - 1  denotes the pixel  1 ,  1 - 2  denotes the pixel  2 ,  6  denotes the power supply line,  7  denotes the reset line,  8  denotes the scan line drive circuit,  9  denotes the signal line drive circuit,  12  denotes the signal line,  21  denotes the lighting switch line,  22  denotes a lighting control line,  50  denotes a triangular wave generation circuit,  51  denotes the signal input line,  52  denotes a triangular wave generator,  53  denotes a triangular wave selection switch element,  54  denotes a signal line selection switch element,  55  denotes a triangular wave selection switch control line, and  56  denotes a signal line selection switch control line, and reference symbol AR 1  denotes the display region  1 , AR 2  denotes the display region  2 , Y denotes the selection switch line, and OR denotes an OR circuit. 
       FIGS. 7A and 7B  each are a circuit diagram illustrating an equivalent circuit of a pixel circuit of the image display device according to the second embodiment of the present invention. 
     In this embodiment, the display region  1  (AR 1 ) of  FIG. 6  is a still image display area for mainly displaying a still image, in which a pixel illustrated by an equivalent circuit of  FIG. 7A  is employed as the pixel  1  of the display region  1  (AR 1 ). The display region  2  (AR 2 ) of  FIG. 6  is a video image display area for mainly displaying a video image, in which a pixel illustrated by an equivalent circuit of  FIG. 7B  is employed as the pixel  2  of the display region  2  (AR 2 ). 
     Here, the pixel of  FIG. 7A  is similar to the pixel illustrated in  FIG. 13 , and the pixel of  FIG. 7B  is similar to the pixel illustrated in  FIG. 4 . Further, similarly to the first embodiment described above, the driving TFT  4  of the display region  1  (AR 1 ) is adapted to have a “channel width-to-channel length” value smaller than a “channel width-to-channel length” value of the driving TFT  4  of the display region  2  (AR 2 ). 
     With this configuration, even in a case where the pixel  1  ( 1 - 1 ) and the pixel  2  ( 1 - 2 ) are applied with the same image voltage, the pixel  1  ( 1 - 1 ) becomes lower in luminance than the pixel  2  ( 1 - 2 ), which reduces the rate of deterioration of the organic EL element  2 , to thereby suppress burn-in. 
       FIG. 8  is a time chart for illustrating operations of the pixels illustrated in  FIGS. 7A and 7B . 
     As illustrated in  FIG. 8 , in principle, the pixel  1  ( 1 - 1 ) is driven in a manner similar to that illustrated in  FIG. 14 , and the pixel  2  ( 1 - 2 ) is driven in a manner similar to that illustrated in  FIG. 5 . 
     The pixel  1  ( 1 - 1 ) of the display region  1  (AR 1 ) emits light for one frame period (FRM) after receiving a signal voltage input during the “display region  1  write/light emission period” in the one frame period (FRM). 
     In the display region  2  (AR 2 ), during the “write period”, the scan line drive circuit  8  outputs a control voltage at High level for selecting each display line sequentially, and the control voltage is input to each OR circuit OR. Accordingly, each lighting switch line  21  connected to an output terminal of each OR circuit OR is sequentially raised to High level. 
     Further, during the “light emission period”, the lighting control line  22  is raised to High level, and all the lighting switch lines  21  respectively connected to the output terminals of all the OR circuits OR are raised to High level. As a result, all the lighting switch elements  20  of all the pixels of the display region  2  (AR 2 ) are turned on and the organic EL elements  2  of all the pixels emit light. 
     In this case, during the “display region  1  write period” and the “display region  2  write period”, the triangular wave selection switch element  53  is turned off while the signal line selection switch element  54  is turned on, and the signal line  12  is connected to the signal line drive circuit  9 . On the other hand, during the “light emission period”, the triangular wave selection switch element  53  is turned on while the signal line selection switch element  54  is turned off, and the signal line  12  is connected to the triangular wave generation circuit  50 . 
     As described above, the pixel  2  ( 1 - 2 ) of the display region  2  (AR 2 ) stores a signal voltage in the holding capacitor element  3  during the “display region  2  write period” and emits light during only the “light emission period”. With this configuration, the quality of a video image displayed in the display region  2  (AR 2 ) may be enhanced. 
     Third Embodiment 
       FIG. 9  is a block diagram illustrating an entire configuration of an image display device according to a third embodiment of the present invention. In  FIG. 9 , reference numeral  1 - 1  denotes the pixel  1 ,  1 - 2  denotes the pixel  2 ,  6  denotes the power supply line,  7  denotes the reset line,  8  denotes the scan line drive circuit,  9  denotes the signal line drive circuit,  12  denotes the signal line,  21  denotes the lighting switch line,  22  denotes the lighting control line,  50  denotes the triangular wave generation circuit,  51  denotes the signal input line,  52  denotes the triangular wave generator,  53  denotes the triangular wave selection switch element,  54  denotes the signal line selection switch element,  55  denotes the triangular wave selection switch control line,  56  denotes the signal line selection switch control line,  57  denotes a reference signal selection switch element,  58  denotes a reference voltage line, and  59  denotes a reference signal selection switch control line, and reference symbol AR 1  denotes the display region  1 , AR 2  denotes the display region  2 , Y denotes the selection switch line, and OR denotes the OR circuit. 
       FIGS. 10A and 10B  each are a circuit diagram illustrating an equivalent circuit of a pixel circuit of the image display device according to the third embodiment of the present invention. In this embodiment, the display region  1  (AR 1 ) of  FIG. 9  is a still image display area for mainly displaying a still image, in which a pixel illustrated by an equivalent circuit of  FIG. 10A  is employed as the pixel  1  of the display region  1  (AR 1 ). The display region  2  (AR 2 ) of  FIG. 9  is a video image display area for mainly displaying a video image, in which a pixel illustrated by an equivalent circuit of  FIG. 10B  is employed as the pixel  2  of the display region  2  (AR 2 ). 
     Here, the pixel of  FIG. 10A  is similar to the pixel illustrated in  FIG. 15 , and the pixel of  FIG. 10B  is similar to the pixel illustrated in  FIG. 4 . Further, similarly to the first embodiment described above, the driving TFT  4  of the display region  1  (AR 1 ) is adapted to have a “channel width-to-channel length” value smaller than a “channel width-to-channel length” value of the driving TFT  4  of the display region  2  (AR 2 ). 
     With this configuration, even in a case where the pixel  1  ( 1 - 1 ) and the pixel  2  ( 1 - 2 ) are applied with the same image voltage, the pixel  1  ( 1 - 1 ) becomes lower in luminance than the pixel  2  ( 1 - 2 ), which reduces the rate of deterioration of the organic EL element  2 , to thereby suppress burn-in. 
       FIG. 11  is a time chart for illustrating operations of the pixels illustrated in  FIGS. 10A and 10B . 
     As illustrated in  FIG. 11 , in principle, the pixel  1  ( 1 - 1 ) is driven in a manner similar to that illustrated in  FIG. 16 , and the pixel  2  ( 1 - 2 ) is driven in a manner similar to that illustrated in  FIG. 5 . 
     The pixel  1  ( 1 - 1 ) of the display region  1  (AR 1 ) emits light for one frame period (FRM) after receiving a signal voltage input during the “display region  1  write/light emission period” in the one frame period (FRM). 
     In the display region  2  (AR 2 ), during the “write period”, the scan line drive circuit  8  sequentially outputs a control voltage at High level for selecting each display line, and the control voltage is input to each OR circuit OR. Accordingly, each lighting switch line  21  connected to the output terminal of each OR circuit OR is sequentially raised to High level. 
     Further, during the “light emission period”, the lighting control line  22  is raised to High level, and all the lighting switch lines  21  respectively connected to the output terminals of all the OR circuits OR are raised to High level. As a result, all the lighting switch elements  20  of all the pixels of the display region  2  (AR 2 ) are turned on and the organic EL elements  2  of all the pixels emit light. 
     In this case, during the “display region  1  write period”, the triangular wave selection switch element  53  is turned off. However, when the signal line selection switch element  54  is turned on, the reference signal selection switch element  57  is turned off, and hence the signal line  12  is connected to the signal line drive circuit  9 . On the other hand, when the signal line selection switch element  54  is turned off, the reference signal selection switch element  57  is turned on, and hence the signal line  12  is connected to the reference voltage line  58 . 
     Meanwhile, during the “display region  2  write period”, the triangular wave selection switch element  53  is turned off while the signal line selection switch element  54  is turned on and the reference signal selection switch element  57  is turned off. Accordingly, the signal line  12  is connected to the signal line drive circuit  9 . 
     Further, during the “light emission period”, the triangular wave selection switch element  53  is turned on while the signal line selection switch element  54  is turned off and the reference signal selection switch element  57  is turned off. Accordingly, the signal line  12  is connected to the triangular wave generation circuit  50 . 
     As described above, the pixel  2  ( 1 - 2 ) of the display region  2  (AR 2 ) stores a signal voltage in the holding capacitor element  3  during the “display region  2  write period” and emits light during only the “light emission period”. With this configuration, the quality of a video image displayed in the display region  2  (AR 2 ) may be enhanced. 
     It should be noted that in the second embodiment and the third embodiment described above, the driving TFT  4  of the display region  1  (AR 1 ) is adapted to have a “channel width-to-channel length” value smaller than a “channel width-to-channel length” value of the driving TFT  4  of the display region  2  (AR 2 ). Alternatively, in the second embodiment and the third embodiment described above, the driving TFT  4  of the display region  1  (AR 1 ) may be adapted to have a “channel width-to-channel length” value which is substantially equal to a “channel width-to-channel length” value of the driving TFT  4  of the display region  2  (AR 2 ). In this case, in the display region  2  (AR 2 ), the organic EL element  2  is forcedly turned off during the write period in the one frame period (FRM), to thereby eliminate an image lag on the human eye. As a result, a motion blur may be suppressed. 
     While there have been described what are at present considered to be certain embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claim cover all such modifications as fall within the true spirit and scope of the invention.