Abstract:
The present invention provides an image display device having about the same numbers of gate lines and data lines as before and capable of reducing the power consumption of a static memory during rewriting of a display image. In the configuration of the image display device, the drain electrode of a first transistor  15  included in a pixel circuit is connected to an input for setting a storing state of the static memory, the drain electrode of a second transistor  18  is connected to an input for resetting a storing state of the static memory, the source electrode of the first transistor is connected to a data line, the gate electrode of the first transistor included in a row of pixel circuits arranged parallel to gate lines is connected to one gate line of the plurality of gate lines, and the gate electrode of the second transistor included in another row of pixel circuits arranged adjacent to the row of pixel circuits is connected to the one gate line.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application JP 2006-018500 filed on Jan. 27, 2006, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an image display device and a driver circuit thereof, and more particularly to an image display device incorporating a static memory in each pixel circuit and having reduced power consumption. 
       BACKGROUND OF THE INVENTION 
       [0003]    In an active matrix type display, typically an active matrix type liquid crystal display, a thin film transistor (hereinafter abbreviated as TFT) is formed in each pixel, and display information is stored on a pixel-by-pixel basis to display images. A TFT formed by using a polysilicon film which is fabricated by polycrystallization of an amorphous silicon film by laser annealing, with its mobility being raised to about 100 cm 2 /VS is called a polysilicon TFT. Since a circuit configured of such polysilicon TFTs operates with signals of a few MHz to dozens of MHz at the maximum, not only pixels but also a data driver circuit generating image signals and a scanning circuit can be formed over the substrate of a liquid crystal display device or the like in the same process as the formation of the TFTs constituting pixel circuits. 
         [0004]    A transmissive liquid crystal display performs display by controlling the transmittance of transmitted light of a backlight. On the other hand, a reflective liquid crystal display which has a reflecting electrode for reflecting external light in a pixel performs display by controlling the reflectance of sunlight or room illumination light that comes in pixels, thereby negating the need for a backlight. 
         [0005]    Further, a liquid crystal display having both the functions of transmission and reflection is called a semi-transmissive liquid crystal display. In general, the reflective liquid crystal display and the semi-transmissive liquid crystal display in a state where the backlight is not lit feature much lower power consumption compared to the transmissive liquid crystal display which requires the backlight to light up. 
         [0006]    Liquid crystal displays enhancing such a low power consumption feature include a liquid crystal display with built-in pixel memory. Since an ordinary liquid crystal display without built-in pixel memory temporarily stores electric charge in a capacitor in a pixel to hold voltage that is applied to the liquid crystal, it is necessary to refresh the voltage at regular time intervals even in the case of displaying a static image. Thus, in either case of displaying a moving image or a static image, data lines for transferring data signals to pixels needs to be driven at about dozens of kHz; therefore, the data lines and the data driver circuit for driving the data lines consume much power. 
         [0007]    The liquid crystal display with built-in pixel memory which places emphasis on displaying static images incorporate a static memory in each pixel, thereby negating the need for refresh operation and therefore making it possible to completely cut power consumed by the data lines and the data driver circuit. 
         [0008]      FIG. 9  shows the configuration of a conventional display with built-in memory. Pixel circuits  82  are arranged in a matrix form over a glass substrate  81 . 
         [0009]    In  FIG. 9 , the pixel circuits  82  are arranged only in two columns by three rows, for simplicity of explanation. However, the actual numbers of columns and rows are both over several hundreds. A pixel circuit  82  is composed of a sampling TFT  83  for sampling data from a data line, a static memory  84  for storing 1 bit of data, and an AC circuit  85  for applying AC voltage corresponding to the storing state of the static memory  84  to a liquid crystal LC as a display section. 
         [0010]    Each pixel circuit  82  is connected to data lines s 1  to s 2  and gate lines g 1  to g 3  through the sampling TFT  83 . The data lines s 1  to s 2  are connected to a data driver circuit  86 , and the gate lines g 1  to g 3  are connected to a scanning circuit  87 . The data driver circuit  86  has the function of temporarily storing video signals serially inputted from the outside of the display and parallelly outputting to the data lines s 1  to s 2 . 
         [0011]    The scanning circuit  87  sequentially outputs pulses to the gate lines g 1  to g 3  in synchronization with the output operation of the data driver circuit  86 , thereby determining a horizontal row of pixel circuits  82  for writing a video signal generated on the data lines s 1  to s 2 . The sampling TFT  83  is turned on by a pulse supplied to the connected gate line, thereby writing the signal of the connected data line into the static memory  84 . 
         [0012]    The AC circuit  85  selects a square wave voltage VLCa or VLCb in accordance with the state of 1-bit data stored in the static memory. The voltage Vcom is a square wave voltage having a frequency of about 30 to 60 Hz, the voltage VLCa is a square wave voltage in phase with Vcom, and the voltage VLCb is a square wave voltage of opposite phase to Vcom. For example, assume that a normally white liquid crystal (in which bright display is performed when the applied AC voltage is small in amplitude) and an optical structure required therefor are employed, for example. When the voltage VLCa is selected, in-phase signals are applied to the liquid crystal LC; therefore, the applied AC voltage becomes low and the liquid crystal cell LC displays white. On the other hand, when the voltage VLCb is selected, opposite-phase signals are applied to the liquid crystal LC; therefore, the applied AC voltage becomes high and the liquid crystal cell LC displays black. The liquid crystal display device with built-in memory is described in more detail in JP-A-8-194205 (194205/1996) and JP-A-8-286170 (286170/1996). 
         [0013]    In accordance with the state of 1-bit data stored in the static memory  84 , the white display or black display of each pixel can be selected. Accordingly, in the case where video data is not rewritten, it is possible to display a static image even if the operation of the data driver circuit  86  and the scanning circuit  87  is stopped. Since this makes it possible to cut all the power for driving the data lines s 1  to s 2  and the gate lines g 1  to g 3 , the display with built-in memory can reduce power consumption during static image display, compared to an ordinary liquid crystal display. 
       SUMMARY OF THE INVENTION 
       [0014]    However, even the liquid crystal display with built-in pixel memory needs to drive the data driver circuit  86  and the scanning circuit  87  in the case of rewriting a static image; therefore, it is important to reduce power during rewriting. 
         [0015]    In  FIG. 9 , when the sampling TFT  83  rewrites the storing state of the static memory  84 , the current supply capacity of the sampling TFT  83  in writing a low level voltage of the data line differs from that in writing a high level voltage of the data line. In order to rewrite the storing state of the static memory  84 , it is necessary that the supply current of the sampling TFT  83  is sufficiently larger than the driving current of TFTs constituting the static memory  84 . 
         [0016]      FIG. 10A  is an illustration showing a sink current I sink  flowing through the sampling TFT in the case where the sampling TFT supplies the low level potential of the data line to the static memory to rewrite the storing state. Since  FIG. 10A  is an illustration for explaining a general principle, the sampling TFT is represented by symbol Ts and the static memory is represented by symbol Mem.  FIG. 11A  is a graph showing the operating point of the sink current I sink  and a voltage Va generated at the signal input portion of the static memory Mem in  FIG. 10A . In  FIGS. 11A and 11B , I Mem  denotes the supply current of the static memory Mem, and I TS  denotes the supply current of the sampling TFT Ts. Further, H denotes a high level, and L denotes a low level. 
         [0017]    As shown in  FIG. 11A , by way of example, the supply current of the sampling TFT Ts is twice as large as the driving current of TFTs constituting the static memory Mem. In this case, since the gate-source voltage which affects the current supply capacity of the sampling TFT Ts is the difference voltage between the data line and the gate line connected, the sampling TFT has relatively large current supply capacity so that the voltage Va at the operating point is low enough (a left-of-center position on the graph). Since the voltage Va at the operating point is recognized as the low level voltage, the static memory Mem can store the low level voltage of the data line. 
         [0018]    On the other hand, in the case where the sampling TFT supplies the high level potential of the data line to the static memory to rewrite the storing state, the sampling TFT flows a source current I source  as shown in  FIG. 10B . Since  FIG. 10B  is also an illustration for explaining a general principle, the sampling TFT is represented by symbol Ts and the static memory is represented by symbol Mem.  FIG. 11B  is a graph showing the operating point OP of the source current I source  and a voltage Va generated at the signal input portion of the static memory Mem in  FIG. 10B . As shown in  FIG. 11B  as well, by way of example, the supply current of the sampling TFT Ts is twice as large as the driving current of TFTs constituting the static memory. In this case, since the gate-source voltage which affects the current supply capacity of the sampling TFT Ts is the difference voltage between the voltage Va and the gate line voltage, the current supply capacity decreases sharply as the voltage Va increases, thus making it difficult to increase the voltage Va of the operating point OP (bring the operating point to a right-of-center position on the graph). If the voltage Va of the operating point OP does not become high enough, the static memory Mem may not recognize the voltage Va of the operating point as the high level voltage and therefore may fail to store the high level voltage of the data line. 
         [0019]    In order to avoid this problem, the high level voltage of the gate line needs to be higher than the power supply voltage VDD of the static memory Mem. Generating a voltage higher than the power supply voltage VDD requires an additional circuit such as a DC-DC converter, which leads to an increase in the power consumption of the entire image display device. 
         [0020]    In order to avoid this problem without increasing the power consumption, the pixel circuit is configured not to rewrite the static memory Mem under the condition of  FIG. 10B , but to rewrite the static memory Mem only under the condition of  FIG. 10A . 
         [0021]    For example, as shown in  FIG. 12 , it is known that the sampling TFT is configured as a CMOS analog switch having an n-channel TFT  95  and a p-channel TFT  96 . A sufficient current is supplied to the static memory Mem through the n-channel TFT  95  at the time of writing a low potential or through the p-channel TFT  96  at the time of writing a high potential. However, this method requires two kinds of different gate lines which are a gate line G for driving the n-channel TFT  95  and a gate line Gz for driving the p-channel TFT  96 , thus doubling the number of gate lines in the entire image display device. 
         [0022]    Further, as shown in  FIG. 13 , there is a method for writing signal voltages of complementary logic (in which a high level voltage is provided at one end while a low level voltage is provided at the other end) to the two complementary signal input portions of the static memory through sampling TFTs  97  and  98  of two n-channel TFTs. However, this method requires two kinds of different data lines S and Sz for supplying complementary logic signals, thus doubling the number of data lines in the entire image display device. 
         [0023]    Such a significant increase in the number of gate lines or data lines causes the adverse effect of reducing manufacturing yield and lowering the upper limit of the definition of the image display device. Further, as the number of lines is increased, the parasitic capacitance of the lines increases proportionally, so that the power consumption of the data driver circuit or the scanning circuit for driving the lines increases unpreferably. 
         [0024]    Accordingly, it is an object of the present invention to provide an image display device for rewriting the static memory Mem only under the condition of  FIG. 10A  with a simple wiring structure requiring little increase in the number of gate lines or data lines compared to a conventional liquid crystal display device. 
         [0025]    A representative aspect of the invention disclosed in this specification will be briefly described as follows. The invention provides an image display device comprising a plurality of pixel circuits arranged in a matrix form over a substrate and each including at least one static memory; a plurality of data lines for conveying an image signal to the plurality of pixel circuits; a plurality of gate lines, intersecting the data lines, for conveying a scanning pulse to the plurality of pixel circuits; and a scanning circuit for sequentially supplying a scanning pulse to the plurality of gate lines, wherein the pixel circuits includes a first transistor for setting a storing state of the static memory and a second transistor for resetting a storing state of the static memory, a drain electrode of the first transistor is connected to an input for setting a storing state of the static memory, a drain electrode of the second transistor is connected to an input for resetting a storing state of the static memory, a source electrode of the first transistor is connected to one of the data lines, a gate electrode of the first transistor included in a row of pixel circuits arranged parallel to the gate lines is connected to one gate line of the plurality of the gate lines, and a gate electrode of the second transistor included in another row of pixel circuits arranged adjacent to the row of pixel circuits is connected to the one gate line. 
         [0026]    According to the aspect of the invention, it is possible to reduce power consumption required to rewrite pixel circuits and therefore lower the power consumption of an image display device. Particularly in an image display device, such as a reflective liquid crystal display device or a semi-transmissive liquid crystal display device, in which most of the operating power is consumed for circuit operation, it is easy to obtain the effect of reducing power consumption. Further, it is possible to reduce the power consumption of an electronic device equipped with an image display device according to the invention and thereby obtain the effect of prolonging the operating time of an attached battery. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    In the accompanying drawings: 
           [0028]      FIG. 1  is an illustration showing the circuit configuration of an image display device according to the present invention; 
           [0029]      FIG. 2  is a timing chart of voltage waveforms supplied to pixel circuits PX and generated at the pixel circuits PX; 
           [0030]      FIG. 3  is a graph showing a general relationship between AC voltage amplitude applied to a liquid crystal cell LC and light reflectance (or transmittance); 
           [0031]      FIG. 4  is an illustration showing another configuration of a pixel circuit PX; 
           [0032]      FIG. 5  is an illustration showing the structure of the image display device according to the invention; 
           [0033]      FIG. 6  is a front layout view of pixel circuits PX; 
           [0034]      FIG. 7  is an illustration showing a cross section structure along line A-A′ shown in  FIG. 6 ; 
           [0035]      FIG. 8  is an illustration showing a mobile electronic device to which the image display device according to the invention applied; 
           [0036]      FIG. 9  is an illustration showing the configuration of a conventional display with built-in memory; 
           [0037]      FIG. 10A  is an illustration showing a sink current I sink  flowing through a sampling TFT; 
           [0038]      FIG. 10B  is an illustration showing a source current I source  flowing through a sampling TFT; 
           [0039]      FIG. 11A  is a graph showing the operating point of the sink current I sink  and a voltage Va in  FIG. 10A ; 
           [0040]      FIG. 11B  is a graph showing the operating point of the source current I source  and a voltage Va in  FIG. 10B ; 
           [0041]      FIG. 12  is an illustration showing the configuration of a conventional pixel circuit; and 
           [0042]      FIG. 13  is an illustration showing the configuration of another conventional pixel circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    Preferred embodiments of an image display device according to the present invention will be described with reference to the accompanying drawings. 
         [0044]      FIG. 1  shows the circuit configuration of the image display device according to the invention. There are formed over a glass substrate  1  a data driver circuit HCIR, a scanning circuit VCIR, and a display area  2 . The glass substrate  1  is a substrate that is generally used in a low-temperature polysilicon manufacturing process. However, the material of the substrate is not limited to glass as long as insulation on the surface is secured. On the display area  2 , a plurality of data lines S 1  to S 2  are wired in vertical directions and a plurality of gate lines G 0  to G 3  are wired in horizontal directions. Pixel circuits PX and PX 1  to PX 3  are disposed at intersections. The pixel circuits PX 1  to PX 3  are the same as the pixel circuits PX; however, they are indicated as PX 1  to PX 3  for identification in later description. 
         [0045]    In  FIG. 1 , the number of data lines is 2, the number of gate lines is 4, and the number of pixel circuits PX is 6 (=3×2), for simplicity of explanation. However, in an actual image display device, both the numbers of data and gate lines are over several hundreds. For example, in the case of a color image display device with VGA resolution, the number of data lines is 1920 (=640×3(RGB)), the number of gate lines is 481, and the number of pixel circuits PX is 921600 (=640×3×480). That is, the number of data lines is equal to the number of pixel circuits in the horizontal direction, and therefore equal to the number of data lines in a conventional image display device. The number of gate lines is equal to the number of pixel circuits in the vertical direction plus one, and therefore nearly equal to the number of gate lines in the conventional image display device shown in  FIG. 9 . 
         [0046]    A pixel circuit PX is composed of eight TFTs, which are TFTs  11  to  14  constituting a static memory, a TFT  15  constituting a sampling switch, TFTs  16  and  17  constituting a selector circuit for selecting an AC voltage, and a TFT  18  constituting a reset switch for resetting the state of the static memory. The TFTs  12  and  14  to  18  are n-channel TFTs, and the TFTs  11  and  13  are p-channel TFTs. 
         [0047]    It can also be considered that the static memory is composed of two inverters, which are an inverter having an input node az 1  (az 2 , or az 3 ) and an output node a 1  (a 2 , or a 3 ) composed of the TFTs  11  and  12 , and an inverter having an input node a 1  (a 2 , or a 3 ) and an output node aZ 1  (aZ 2 , or aZ 3 ) composed of the TFTs  13  and  14 . 
         [0048]    Thereby, the static memory has two stable states (bi-stable) in which the node az 1  is at a low level voltage when the node a 1  is at a high level voltage or the node az 1  is at a high level voltage when the node a 1  is at a low level voltage, and therefore can store 1 bit of information. The TFT  15  constituting the sampling switch is connected at its source electrode to the data line S 1  (or S 2 ), connected at its drain electrode to the node a 1  (a 2 , or a 3 ), and connected at its gate electrode to the gate line G 1  (G 2 , or G 3 ). 
         [0049]    The TFT  18  constituting the reset switch is connected at its source electrode to the wiring of a negative power supply voltage VSS, connected at its drain electrode to the node az 1  (az 2 , or az 3 ), and connected at its gate electrode to the gate line G 0  (G 1 , or G 2 ). The source electrodes of the TFTs  11  and  13  are connected to the wiring of a positive power supply voltage VDD for operating the static memory circuit, and the source electrodes of the TFTs  12  and  14  are connected to the wiring of a negative power supply voltage VSS for operating the static memory circuit. 
         [0050]    A liquid crystal cell LC has a pair of electrodes. One electrode is common to all pixels and is supplied with an AC square wave voltage Vcom. The other electrode which is a node b 1  (b 2 , or b 3 ) is connected to the drain electrodes of the TFTs  16  and  17  constituting the selector circuit. The gate electrodes of the TFTs  16  and  17  are connected to the node a 1  (a 2 , or a 3 ) and to the node az 1  (az 2 , or az 3 ), respectively. The source electrodes of the TFTs  16  and  17  are connected to the wiring of an AC square wave voltage VLCb of opposite phase to the AC square wave voltage Vcom and to the wiring of an AC square wave voltage VLCa in phase with the AC square wave voltage Vcom, respectively. 
         [0051]    With this connection, the selector circuit composed of the TFTs  16  and  17  have the function of selecting the AC square wave voltage VLCa or VLCb in accordance with the state of 1-bit data stored in the static memory circuit and supplying it to the liquid crystal cell LC. 
         [0052]      FIG. 2  is a timing chart of voltage waveforms supplied to pixel circuits PX and generated at the pixel circuits PX for the specific explanation of the operation of the pixel circuits PX. In  FIG. 2 , there are shown only the waveforms related to the three pixel circuits PX 1  to PX 4  which are connected to the data line S 1 . A timing chart when the pixel circuits PX perform data rewriting operation (RWRT) is shown at times t 0  to t 4 , and a timing chart when the pixel circuits PX perform static image display (DISP) is shown at times tF 0  to tF 4 . In  FIG. 2 , in order to make the timing chart easy to see, the length of the period from t 0  to t 4  is approximately the same as the length of the period from tF 0  to tF 4 . However, in reality, the time period from t 0  to t 4  is much shorter (e.g., less than a few microseconds) than the response time of the liquid crystal cell. The time period from tF 0  to tF 4  is approximately the same as or larger than the response time of the liquid crystal cell and, for example, is about a few tens of milliseconds. Thus, in reality, the scales differ by about four orders of magnitude. 
         [0053]    In  FIG. 2 , reference numerals G 0  to G 3  denote voltage signals supplied to the gate lines G 0  to G 3 ; S 1 , a voltage signal supplied to the data line S 1 ; a 1  to a 3  and a 1 Z to a 3   z,  voltage waveforms generated at the nodes a 1  to a 3  and the nodes az 1  to az 3 ; Vcom, VLCa, and VLCb, voltage waveforms of the supplied AC square wave signals; and b 1  to b 3 , voltage waveforms generated at the nodes b 1  to b 3 . The double hatched areas in the signal supplied to the data line S 1  signify that either a low level voltage or a high level voltage may appear. The double hatched areas in the voltage waveforms generated at the nodes a 1  to a 3 , az 1  to az 3 , and b 1  to b 3  signify an undetermined state because of dependence on the state prior to the rewriting operation. Symbols H and L denote a high level voltage and a low level voltage, and symbols V and t denote a voltage and time. 
         [0054]    Hereinafter, the data rewriting operation performed by the pixel circuits PX will be described. The gate lines G 0 , G 1 , G 2 , and G 3  are supplied with a positive pulse at times t 0 , t 1 , t 2 , and t 3 , respectively. The data line is supplied with voltages D 1 , D 2 , and D 3  corresponding to display image information at times t 1 , t 2 , and t 3 , respectively. In  FIG. 2 , by way of example, D 1  and D 3  are shown as signals of the low level voltage, and D 2  is shown as a signal of the high level voltage. However, in reality, the low level voltage and the high level voltage may change places in accordance with display image information. By configuring the scanning circuit VCIR shown in  FIG. 1  with a shift register circuit, the waveforms of the gate lines G 0  to G 3  can be easily generated. Further, by configuring the data driver circuit HCIR shown in  FIG. 1  with a shift register circuit and a latch circuit, externally inputted image information can be easily outputted to the data lines S 1  to S 2 . 
         [0055]    When a pulse is supplied to the gate line G 0  at time t 0 , the TFT  18  of the pixel circuit PX 1  is turned on. At this time, the TFT  18  is under the condition of  FIG. 10A  for generating a sink current I sync , so that it easily turns the node az 1  to the low level voltage. Accordingly, the inverter composed of the TFTs  11  and  12  of the pixel circuit PX 1  turns the node a 1  to the high level voltage. 
         [0056]    When a pulse is supplied to the gate line G 1  at time t 1 , the TFT  15  of the pixel circuit PX 1  and the TFT  18  of the pixel circuit PX 2  are turned on. The data line S 1  is supplied with the low level voltage. Since the TFT  15  of the pixel circuit PX 1  is under the condition of  FIG. 10A  for generating the sink current I sync , it easily turns the node a 1  to the low level voltage. Accordingly, the inverter composed of the TFTs  13  and  14  of the pixel circuit PX 1  turns the node az 1  to the high level voltage. The high level voltage at the node az 1  turns on the TFT  17 , so that the AC square wave voltage VLCa is outputted to the node b 1 . Since the TFT  18  of the pixel circuit PX 2  is under the condition of  FIG. 10A  for generating the sink current I sync , it easily turns the node az 2  to the low level voltage. Accordingly, the inverter composed of the TFTs  11  and  12  of the pixel circuit PX 2  turns the node a 2  to the high level voltage. 
         [0057]    When a pulse is supplied to the gate line G 2  at time t 2 , the TFT  15  of the pixel circuit PX 2  and the TFT  18  of the pixel circuit PX 3  are turned on. The data line S 1  is supplied with the high level voltage. Even though the TFT  15  of the pixel circuit PX 2  is turned on, since both the data line S 1  and the node a 2  are at the high level voltage, no current flows through the TFT  15  so that the node a 2  maintains the high level voltage. Accordingly, the inverter composed of the TFTs  13  and  14  of the pixel circuit PX 2  allows the node az 2  to maintain the low level voltage. The high level voltage at the node a 2  turns on the TFT  16 , so that the AC square wave voltage VLCb is outputted to the node b 2 . Since the TFT  18  of the pixel circuit PX 3  is under the condition of  FIG. 10A  for generating the sink current I sync , it easily turns the node az 3  to the low level voltage. Accordingly, the inverter composed of the TFTs  11  and  12  of the pixel circuit PX 3  turns the node a 3  to the high level voltage. 
         [0058]    When a pulse is supplied to the gate line G 3  at time t 3 , the TFT  15  of the pixel circuit PX 3  is turned on. The data line S 1  is supplied with the low level voltage. Since the TFT  15  of the pixel circuit PX 3  is under the condition of  FIG. 10A  for generating the sink current I sync , it easily turns the node a 3  to the low level voltage. Accordingly, the inverter composed of the TFTs  13  and  14  of the pixel circuit PX 3  turns the node az 3  to the high level voltage. The high level voltage at the node az 3  turns on the TFT  17 , so that the AC square wave voltage VLCa is outputted to the node b 3 . 
         [0059]    As described above, data in the pixel circuits is rewritten only under the condition of  FIG. 10A , but is not rewritten under the condition of  FIG. 10B ; therefore, the high level voltage of the gate lines can be much the same as the power supply voltage of the pixel circuits, thus making it possible to reduce power required for the rewriting operation. 
         [0060]    Next, description will be made of the operation in which the pixel circuits PX display a static image. The voltage Vcom supplied to the common electrode of the liquid crystal cells LC is an AC square wave voltage whose polarity reverses every one frame period (tF 0 -tF 1 , tF 1 -tF 2 , tF 2 -tF 3 , tF 3 -tF 4 ). The voltage VLCa is an AC square wave voltage in phase with Vcom, and the voltage VLCb is an AC square wave voltage of opposite phase to Vcom. No signal is sent to the gate lines G 0  to G 3  and the data lines S 1  to S 2  suspended. 
         [0061]    In the pixel circuits PX 1  and PX 3  in which the signals D 1  and D 3  of the low level voltage are written during the rewriting period, since the AC square wave voltage VLCa is generated at the nodes b 1  and b 3 , the amplitude of the AC voltage applied to the liquid crystal cell LC becomes a relatively low voltage VL. On the other hand, in the pixel circuit PX 2  in which the signal D 2  of the high level voltage is written during the rewriting period, since the AC square wave voltage VLCb is generated at the node b 2 , the amplitude of the AC voltage applied to the liquid crystal cell LC becomes a relatively high voltage VH. 
         [0062]      FIG. 3  shows a general relationship between AC voltage amplitude applied to a liquid crystal cell LC and light reflectance (or transmittance). In this example, the liquid crystal cell LC is a normally white liquid crystal in which the light reflectance (or transmittance) becomes the maximum when the applied AC voltage amplitude Vac is zero. According to  FIG. 3 , in the pixel circuits PX 1  and PX 3  in which the relatively low voltage VL is applied to the liquid crystal cell LC, the reflectance becomes high so that white (WHT) is displayed. In the pixel circuit PX 2  in which the relatively high voltage VH is applied to the liquid crystal cell LC, the reflectance becomes low so that black (BLK) is displayed. 
         [0063]    Consequently, the pixel circuit in which the low level voltage is written during the rewriting period can maintain the white display during the display period, and the pixel circuit in which the high level voltage is written during the rewriting period can maintain the black display during the display period. 
         [0064]    Therefore, the circuit according to this embodiment of the invention shown in  FIG. 1  stores static-image data supplied from the data driver circuit HCIR into the pixel circuits PX, and thereby can continue to display the static image for a long time even while no signal is supplied to the gate lines or the data lines. 
         [0065]      FIG. 4  shows another configuration of the pixel circuit PX. In comparison with the pixel circuit PX shown in  FIG. 1 , the n-channel TFT  15  constituting the sampling switch and the n-channel TFT  18  constituting the reset switch are replaced with a p-channel TFT  15   b  and a p-channel TFT  18   b.  Further, the source electrode of the TFT  18   b  is connected to the wiring of the positive power supply voltage VDD. When the pixel circuit PX shown in  FIG. 4  is supplied with waveforms obtained by reversing the high level voltage and the low level voltage of the gate lines G 0  to G 3  and the data lines S 1  to S 2  in the supply waveforms shown in  FIG. 2 , it is possible to operate in the same way as the pixel circuit PX shown in  FIG. 1 . 
         [0066]      FIG. 5  is an exploded perspective view of the structure of the image display device according to the invention. There are formed over the surface of the glass substrate  1 , the data driver circuit HCIR formed with TFTs, the scanning circuit VCIR, and the display area  2  where pixel circuits PX are arranged in a matrix form. A film-like circuit board  23  (FPC: Flexible Printed Circuit) is attached to the glass substrate  1 , and external voltage signals and voltages required to drive circuits are supplied through the film-like circuit board  23 . 
         [0067]    Wiring  22  for connecting between the film-like circuit board  23 , the data driver circuit HCIR, the scanning circuit VCIR, and the display area  2  is formed using a metal wiring layer used in a TFT forming process. Display electrodes  24  are formed overlapping each pixel circuit PX, and a display electrode  24  is connected to the node b 1  (b 2 , or b 3 ) in the pixel circuit PX shown in  FIG. 1 . 
         [0068]    The glass substrate  1  and the other glass substrate  21  are bonded together with a several-μm thick liquid crystal (not shown) between them. The thickness of the liquid crystal can be maintained uniformly by distributing globular beads (not shown) over the glass substrate  1 . There is formed a transparent electrode  25  on the inside surface of the glass substrate  21 . The liquid crystal is held between the transparent electrode  25  and the metal electrode  24  of each pixel circuit PX, thus forming the liquid crystal cell LC. The transparent electrode  25  is connected to a connection terminal  26  provided outside the display area  2  over the glass substrate  1 , so that the AC square wave voltage Vcom is supplied through the film-like circuit board  23 . 
         [0069]    There are provided openings  27  at positions where the inside surface of the glass substrate  21  is superposed over the display electrodes  24 . A shading layer is applied to the area other than the openings  27 , thereby preventing light from being transmitted through the area other than the openings  27 . If color filters, namely, red, green, and blue filters (not shown) are provided in the openings  27 , it becomes possible for the image display device to display color images. 
         [0070]    A polarizing plate  28  and a retardation plate  29  are bonded to the other surface of the glass substrate  21  remote from the glass substrate  1 . The role of the polarizing plate  28  and the retardation plate  29  is to obtain a high light reflectance ratio between different AC voltage amplitudes VH and VL applied to the liquid crystal so that black or white is displayed. 
         [0071]      FIG. 6  shows an example of the layout of pixel circuits PX. In  FIG. 6 , there is shown the layout of about 2×2 pixel circuits in an area including the pixel circuits PX 2  and PX 3  shown in  FIG. 1 . The wirings of the voltages VDD, VSS, VLCa, and VLCb and the source and drain electrodes of transistors are formed by a polysilicon layer and connected in common with respect to a row of pixel circuits PX arranged in a horizontal direction. The gate lines G 0  to G 3  and the gate electrodes of transistors are formed by a gate metal layer. The data lines S 1  to S 2  and the remaining wiring are formed by a metal wiring layer. 
         [0072]    The display electrode  24  is formed overlapping most components of the pixel circuit and is connected to the metal wiring layer through a contact hole. The TFTs  11  to  18  are formed by overlapping wiring of the gate metal layer with wiring of the polysilicon layer. Polysilicon layer portions that are adjacent to the TFTs  11  and  13  are doped with boron so that the TFTs  11  and  13  function as p-channel TFTs. Polysilicon layer portions that are adjacent to the TFTs  12  and  14  to  18  are doped with phosphorus so that the TFTs  12  and  14  to  18  function as n-channel TFTs. 
         [0073]    The source electrode of the TFT  18  is connected to the power supply wiring VSS of an adjacent pixel circuit. For example, the TFT  18  constituting the pixel circuit PX 3  is connected to the wiring that supplies the power supply voltage VSS to the TFTs  12  and  14  constituting the static memory in the pixel circuit PX 2 . 
         [0074]      FIG. 7  shows a cross section structure along the bold dotted line A-A′ in  FIG. 6 . An insulating film  31  made of silicon oxide is formed on the glass substrate  1 . A polysilicon layer  32  is formed thereon. Further, a gate metal layer  34  is formed thereover with a gate insulating film  33  made of silicon oxide between them. 
         [0075]    The portion where the gate metal layer  34  overlaps the polysilicon layer  32  becomes the TFT  17 . Further, a metal wiring layer  36  is formed thereover with an interlayer insulating film  35  made of silicon oxide between them. A contact hole  37  is bored through the gate insulating film  33  and the interlayer insulating film  35  so that the metal wiring layer  36  is connected to the polysilicon layer  32 , or the metal wiring layer  36  is connected to the gate metal layer  34 . Further, a display electrode  24  is formed thereover with a planarization insulating layer  38  between them. A contact hole  39  is bored through the planarization insulating layer  38  so that the display electrode  24  is connected to the metal wiring layer  36 . In order to prevent corrosion, a transparent electrode  40  is overlapped and formed on the surface of the display electrode  24 . 
         [0076]      FIG. 8  shows a mobile electronic device to which the image display device according to the invention applied. A mobile electronic device  51  is equipped with an antenna  52 , a microphone  53 , a speaker  54 , an image sensor  55 , and an audio playback button  56 , as well as an image display device  50  according to the invention. Further, the mobile electronic device  51  incorporates a battery  57  for supplying power. The application of the image display device  50  according to the invention can reduce the power consumption of the mobile electronic device  51  and thereby prolong the operating time of the battery  57 , or can reduce the size of the mobile electronic device  51  by downsizing the battery  57 . 
         [0077]    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.