Electrophoretic display

An electrophoretic display using electrophoretic ink is configured by a transparent substrate, a common electrode, pixel electrodes, and thin-film transistors. An electrophoretic ink layer, which is arranged between the common electrode and pixel electrodes, is actualized by a linear arrangement of microcapsules each of which contains negatively charged white particles dispersed in a liquid having a specific color. All the pixel electrodes are simultaneously set to the low electric potential while the common electrode is set to the high electric potential so that the display content is erased from the entire area of the display surface at once, and then the pixel electrodes are driven respectively in response to display data while the common electrode is set to the low electric potential so that the display content is rewritten with a new one in response to the display data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to displays such as liquid crystal displays, and particularly to electrophoretic displays that display images using display media such as electrophoretic ink in accordance with electrophoretic effects.

2. Description of the Related Art

Conventionally, electrophoretic effects are well known among scientists and engineers, wherein charged particles dispersed in a fluid or liquid medium move under the influence of an electric field. As an example of the application of the electrophoretic effects, engineers try to realize displays by using charged pigment particles that are dispersed and contained in dyed solution arranged between a pair of electrodes. Under the influence of an electric field, the charged pigment particles are attracted to one of the electrodes, so that desired images will be displayed. The dyed solution in which charged pigment particles are dispersed is called electrophoretic ink, and the display using the electrophoretic ink is called an electrophoretic display (abbreviated as ‘EPD’).

Each of the charged pigment particles has a nucleus that corresponds to a rutile structure such as TiO2, for example. The nucleus is covered by a coating layer made of polyethylene, for example. As solvents, it is possible to use a solution dissolving ethylene tetrachloride, isoparaffin, and anthraquinone dye, for example. The charged pigment particles and the solvents each have different colors. For example, the charged pigment particles are white, while the solvents are blue, red, green, or black, for example. At least one of the electrodes is formed as a transparent electrode.

Applying an electric field to the electrophoretic ink externally, if the pigment particles are negatively charged, they move in a direction opposite to a direction of the electric field. Thus, the display produces a visual representation such that one surface of the display being observed through the electrophoretic ink seems to be colored in either the color of the solvent or the color of the charged pigment particles. By controlling the movement of charged pigment particles in each pixel, it is possible to represent visual information on the display surface of the display.

The solvent and the charged pigment particles both have approximately the same specific gravity. For this reason, even if the electric field disappears, the charged pigment particles can maintain their positions, which are fixed by the application of the electric field, for a relatively long time, which may range from several minutes to twenty minutes, for example. Because of the aforementioned property of the charged pigment particles of the electrophoretic ink, it is possible to anticipate low power consumption by the electrophoretic display. In addition, the electrophoretic display is advantageous because of the high contrast and very large viewing angle, which reaches approximately ±90 degrees. Generally speaking, a human observer is inevitably required to directly view colors of pigments and/or colors of dyes in the electrophoretic display. Whereas the liquid crystal display of the transmission type requires the human observer to view light from fluorescent tubes of the back light, the electrophoretic display can produce visually subtle colors and shades, which are gentle on the human eyes. In addition, the electrophoretic ink is inexpensive compared to liquid crystal. Further, the electrophoretic display does not need a back light. Therefore, it is anticipated that electrophoretic displays can be manufactured at the relatively low cost.

In spite of the aforementioned advantages, manufacturers could not actually produce electrophoretic displays for practical use because of low reliability in operation due to cohesion of charged pigment particles. However, recent advances in technology have shown that the reliability can be improved by using microcapsules filled with electrophoretic ink. Therefore, electrophoretic displays have recently suddenly become a focus of interest.

Various papers and monographs have been written with regard to concrete examples of displays using electrophoretic ink encapsulated in microcapsules. For example, it is possible to list two papers, namely, a first paper entitled “44.3L: A Printed and Rollable Bistable Electronic Display” that is written by P. Drzaic et al for the SID 98 DIGEST 1131, and a second paper entitled “53.3: Microencapsulated Electrophoretic Rewritable Sheet” that is written by H. Kawai et al for the SID 99 DIGEST 1102.

The aforementioned first paper describes that four types of layers are sequentially printed on a polyester film, that is, a transparent conductive plate, an encapsulated electrophoretic ink layer, a patterned conductive layer of silver or graphite, and an insulation film layer. In short, the first paper proposes a ‘flexible’ display in which a hole (or holes) is open on the insulating film to allow designation of an address (or addresses) for the patterned conductive layer and to allow provision of a lead line (or lead lines). The second paper proposes a rewritable sheet that operates based on the electrophoresis by using the microencapsulated electrophoretic ink, and it also proposes a method for writing information onto the sheet. In addition, it is possible to propose a display in which a surface of an active-matrix type array of elements such as the low-temperature processed polysilicon thin-film transistors (TFT) is coated with the electrophoretic ink. Thus, it is possible to provide the ‘visually subtle and gentle’ display that is also reduced in consumption of electricity.

FIG. 1shows a structure of the selected section of the electrophoretic display with respect to each pixel. The display uses two substrates111and112, which are fixed by bonding and are arranged opposite to each other. A common electrode113is formed just below the substrate112, under which a pixel electrode114is formed. An electrophoretic ink layer115containing plenty of microcapsules of electrophoretic ink is formed between the common electrode113and the pixel electrode114. The pixel electrode114is connected to a drain electrode117of a thin-film transistor (TFT)116in series. The TFT116plays a role as a switch. At least one of the common electrode113and pixel electrode114is made by a transparent electrode, which corresponds to a display surface to be visually observed by a person or human operator.

The TFT116contains a source layer119, a channel120, a drain layer121, and a gate insulation film122that are formed on an embedded insulation film118. In addition, it also contains a gate electrode123formed on the gate insulation film122, a source electrode124formed on the source layer119, and a drain electrode117formed on the drain layer121. Further, the TFT116is covered with an insulation film125and another insulation film126respectively.

Next, the internal structure and operation of the electrophoretic ink layer115will be described with reference toFIGS. 2Ato2C. The electrophoretic ink layer115is formed by a transparent binder211having light transmittance and plenty of microcapsules212. The microcapsules212are distributed uniformly in the inside of the binder211in a fixed state. The thickness of the electrophoretic ink layer115is 1.5 to 2 times as large as external diameters of the microcapsules212. As the material for the binder211, it is possible to use silicone resin and the like. Each microcapsule212is defined by a capsule body213that has a hollow spherical shape and transmits light. The inside of the capsule body213is filled with liquid (or solvent)214, in which negatively charged particles215are dispersed. Each of the charged particles215has a nucleus216that is coated with a coating layer217. Each charged particle215and the liquid214mutually differ from each other in color. That is, different colors are set to them respectively. For example, the charged particles215are white, while the liquid214is blue, red, green or black. Additionally, approximately the same specific gravity is set for both of the liquid214and charged particles215within the microcapsule212.

When an electric field is applied to the microcapsules212externally, the charged particles215move within the microcapsules212in directions opposite to the direction of the electric field. If the display surface of the display presently corresponds to an upper surface of the substrate112shown inFIG. 1, the charged particles215move upwards within the microcapsules212of the electrophoretic ink layer115, which is shown in FIG.2B. In that case, it is possible to observe the color (i.e., white) of the charged particles215that are floating upwards above the background color, which corresponds to the color (e.g., blue, red, green, or black) of the liquid214. In contrast, if the charged particles215move downwards due to the application of an electric field to the microcapsules212of the electrophoretic ink layer115shown inFIG. 1, the display allows only the color (e.g., blue, red, green, or black) of the liquid214to be observed, which is shown in FIG.2C. Once the charged particles215are moved in directions opposite to the direction of the electric field applied to the microcapsules212, they will likely maintain the same positions within the microcapsules212for a relatively long time after the electric field disappears because they have approximately the same specific gravity as that of the liquid214. That is, once the color of the charged particles215or the color of the liquid214appears on the display surface, it is maintained for several minutes or several tens of minutes. In short, the electrophoretic display has a memory for retaining colors of images. Therefore, by controlling the application of an electric field with respect to each of the pixels, it is possible to provide specific electric-field application patterns, by which information is to be displayed. Once the information is displayed on the display surface of the electrophoretic display, it is maintained on the display surface for a relatively long time.

However, the following problems are left unsolved in the electrophoretic display that is made by the combination of the electrophoretic ink and active-matrix type array of elements.

The drive voltage (or potential difference) that is needed for changing over the display content depends upon the sizes (i.e., diameters) of the microcapsules, and it is estimated to be 1 V/ m or so. Generally, the microcapsules have prescribed diameters that range within several tens of microns, for example. In consideration of the prescribed diameters of the microcapsules, the drive voltage is estimated at 10V or so. Suppose that the drive voltage is set to 10V in the electrophoretic display, which is driven by the known drive method typically employed by liquid crystal displays. In addition, the constant voltage of 10V is applied to the common electrode113, while the prescribed voltage of 0V or 20V is applied to the pixel electrode114. That is, the prescribed voltage applied to the pixel electrode114(hereinafter, simply referred to as ‘pixel electrode drive voltage’) is set to 0V in order to increase the potential of the common electrode113to be higher than the potential of the pixel electrode114. The pixel electrode drive voltage is set to 20V in order to increase the potential of the pixel electrode114to be higher than the potential of the common electrode113. Switching over the pixel electrode drive voltage allows the electrophoretic display to rewrite its display content. Actually, the TFT116is used to switch over the pixel electrode drive voltage. In practice, however, if the electrophoretic display is driven as described above, the drive voltage is increased too high to ensure satisfactory reliability in the switching operation of the TFT116. In addition, the pixel electrode drive voltage of 20V is only the least estimated voltage. In other words, an electrophoretic display for practical use may have an increased pixel electrode drive voltage of 30V or more. If the pixel electrode drive voltage is increased very high, it becomes more difficult to ensure satisfactory reliability in the switching operation of the TFT.

Another typical drive method for use in liquid crystal displays is to vary the potential of the common electrode as well, which is normally called ‘common voltage swing’. Specifically, the pixel electrode drive voltage is set to 0V while the voltage applied to the common electrode (hereinafter, simply referred to as ‘common electrode drive voltage’) is set to 10V in order to increase the potential of the common electrode to be higher than the potential of the pixel electrode. Alternatively, the pixel electrode drive voltage is set to 10V while the common electrode drive voltage is set to 0V in order to increase the potential of the pixel electrode to be higher than the potential of the common electrode. Adequately switching over the pixel electrode drive voltage and common electrode drive voltage allows the electrophoretic display to rewrite its display content. Thus, it is possible to improve the reliability in the switching operation of the TFT.

The aforementioned drive method has a problem, which will be described below.

Suppose that the common electrode drive voltage is set to 10V while the pixel electrode drive voltage is set to 0V in order to rewrite the display content with respect to a certain pixel of the display. In order to prevent other pixels from being mistakenly rewritten in display content, the voltage of 10V should be applied to all other pixel electrodes of the display. Normally, the voltage is applied to the pixel electrodes by sequentially selecting transistors for the pixels. Therefore, it is difficult to perfectly match the voltage applying timing for the prescribed pixel electrode with the voltage applying timing for the common electrode. For this reason, a delay may be caused to occur between these timings. Due to such a delay, there is a possibility that an error will occur in rewriting the display content of the display. Although the appropriate voltage is applied to the pixel electrode at a good timing before the occurrence of an error in rewriting the display content, there is still a possibility that an error will occur in rewriting the display content because of the gradual reduction of the voltage applied to the pixel electrode, which is caused by electromagnetic leakage from the pixel transistor.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electrophoretic display of an active-matrix type that can be driven without error during rewriting of display contents for pixels, thus yielding highly reliable operation.

This invention provides an electrophoretic display using electrophoretic ink comprising a transparent substrate, a common electrode, pixel electrodes, and switching elements such as thin-film transistors. It is preferable to use low-temperature processed polysilicon thin-film transistors in consideration of the mobility (or portability) and the capability of incorporating drivers. In order to reduce the manufacturing cost, it is preferable that at least channels of the thin-film transistors are formed by organic films. An electrophoretic ink layer, which is arranged between the common electrode and pixel electrodes, is actualized by a linear arrangement of microcapsules each of which contains negatively charged white particles dispersed in a liquid having a specific color. Applying the prescribed voltage between the common electrode and pixel electrodes, the negatively charged particles move upwards or downwards within the microcapsules when being attracted to either the common electrode or the pixel electrodes under the influence of an electric field. Therefore, it is possible to observe the color of the negatively charged particles or the color of the liquid emerging on the display surface.

All the pixel electrodes are simultaneously set to the low electric potential while the common electrode is set to the high electric potential so that the display content is erased from the entire area of the display surface at once. In this case, the display surface is entirely white because the negatively charged particles move upwards within the microcapsules when attracted to the common electrode. Then, the pixel electrodes are driven respectively in response to display data while the common electrode is set to the low electric potential so that the display content is rewritten with a new one in response to the display data. Due to the aforementioned processes, it is possible to ensure rewriting of the display content without error.

Because the negatively charged particles and the liquid are both approximately set to the same specific gravity, the negatively charged particles can maintain their positions within the microcapsules after the electric field disappears. Hence, it is possible for the display content to remain on the display surface for a relatively long time.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will be described in further detail by way of examples with reference to the accompanying drawings.

An electrophoretic display of this invention can be applied to electronic books, an example of which is shown in FIG.3. That is,FIG. 3shows a brief appearance of an electronic book31employing the electrophoretic display of this invention. The electronic book31is basically constructed by a frame32and an open/close cover33. A display34is installed in the frame32such that its display surface is exposed outside, and it is controlled by switches or buttons arranged in an area for operation controls35. Inside of the frame32, there are provided a controller36, a counter37and a memory38, which are shown in FIG.4. The display34provides a pixel array portion39that is formed by filling thin-film elements with electrophoretic ink, and peripheral circuits40whose circuit elements are integrated. The pixel array portion39and its peripheral circuits40are combined and unified together within the same unit of the display34. The peripheral circuits40contain scan drivers and data drivers that operate in accordance with the prescribed decoding system.

Next, the peripheral circuits40made by integrated circuits, which are unified together with the pixel array portion39, will be described with reference toFIGS. 5to7. The pixel array portion39is made in a matrix form defined by horizontal lines and vertical lines, namely scan lines and data lines. Therefore, the peripheral circuits40provide four drivers in connection with these lines of the pixel array portion39, which is shown in FIG.5. Specifically, a pair of data drivers51and52are connected to both ends of data lines, and a pair of scan drivers53and54are connected to both ends of scan lines.

FIG. 6shows a detailed circuit configuration for the data drivers51and52respectively. That is, each data driver is configured by a 9-bit decoder61, a level shifter62, a combination of buffers and AND gate switches63, and analog sample-hold thin-film transistors64. The decoder61is configured by three NAND gates and one NOR gate, which are connected with eighteen address signal lines. An output of the decoder61is connected to eight buffers by way of the level shifter62. Therefore, the decoder61outputs an address signal simultaneously to eight data lines by way of the eight buffers and their AND gate switches63respectively. Thus, eight analog sample-hold thin film transistors64are simultaneously switched over in response to the address signal. As a result, eight data are simultaneously and respectively transferred to eight hold capacitors. The aforementioned circuit configuration is suited to reduce operation speeds of the data drivers51and52.

FIG. 7shows a detailed circuit configuration for the scan drivers53and54respectively. That is, each scan driver comprises a 10-bit decoder71, an interlaced switch circuit72, a level shifter73, and an output buffer74. This circuit configuration is drawn in response to a simultaneous scan mode in which two lines are simultaneously scanned and/or a non-interlaced scan mode. In order to achieve scanning in these scan modes, three control signals A, B, and C are applied to the interlaced switch circuit72. By simultaneously scanning two lines, it is possible to increase resolution in a vertical direction on the display surface without raising the scan rate. Since each one pair of scan lines are simultaneously selected, combinations of paired scan lines are to be switched over between two states.

As described above, the data drivers51,52and the scan drivers53,54operate in accordance with the prescribed decoding system. Therefore, the pixel array portion39is merely required to control the electrophoretic ink layer115in such a manner that the display content is rewritten with respect to the pixel for which display data is to be updated. This guarantees reduction of power consumption by the electrophoretic display when it is used in an electronic book.

Next, a description will be given with respect to data update operations of the electrophoretic display with reference to FIG.1andFIGS. 2Ato2C. Suppose that the common electrode113is arranged in proximity to the display surface, and ‘white’ particles215being negatively charged are dispersed in ‘black’ liquid214within the microcapsules212as shown in FIG.2A. When the electric potential of the pixel electrode114is made ‘positive’ as compared with the common electrode113, the black color of the liquid214of the microcapsules212emerges on the display surface as shown in FIG.2C. In contrast, when the electric potential of the common electrode113is made ‘positive’ as compared with the pixel electrode114, the white color of the charged particles215emerges on the display surface as shown in FIG.2B.

The controller36shown inFIG. 4performs a display content rewriting process in accordance with a flowchart of FIG.8. At first, the controller36erases all display data that have been displayed over an entire area of the display surface. Concretely speaking, all the pixel electrodes are set to the same electric potential (e.g., ground potential or Vss), then, ‘positive’ voltages are applied to the common electrode. This makes the entire area of the display surface to be colored white; in other words, it is possible to erase the overall display content that has been displayed on the display surface. That is, the display34has the specific reference color that is white.

Next, the display34proceeds to rewriting old display contents with new ones on the display surface. That is, the controller36reads new display data from the memory38; then, it issues instructions to write the new display data to the electrophoretic ink layer115with respect to pixels of the pixel array portion39respectively. Specifically, the ground potential or Vss is applied to the common electrode113; then, the controller36proceeds to selection of pixels by means of the aforementioned drivers51to54of the prescribed decoding system, so that the new display data are to be rewritten with respect to the pixel electrodes114respectively. In order to display the white color on the entire area of the display surface, for example, low voltage corresponding to the same voltage (e.g., ground potential or Vss) of the common electrode113is applied to the pixel electrodes114. In order to display the black color on the entire area of the display surface, high voltage that is ‘positive’ as compared with the electric potential of the common electrode113is applied to the pixel electrodes114.

It is possible to propose another drive method for the purpose of the reduction of electricity consumed by the display34That is, the controller36selects only the pixels that are used to display the black color, to which display data are to be written, without selecting other pixels that are used to display the white color. This method can be easily realized by using the aforementioned drivers51to54of the prescribed decoding system. Using the electrophoretic ink, it is possible to actualize a display of a reflection type having a high contrast ratio. In addition, this display can reduce consumption of electricity because it does not require the back light.

The aforementioned process is executed every time the display34rewrites its display content on the display surface. As described above, the display using the electrophoretic ink can be easily applied to electronic books because it has memory for retaining its display content for a while. The aforementioned drive method is preferable for use in electronic books using electrophoretic displays.

In consideration of the mobility (or portability) and the capability of incorporating drivers, it is preferable to use the low-temperature processed polysilicon thin-film transistors for the electrophoretic display of this invention. In addition, it is also preferable that at least channels of thin-film transistors are formed by organic films in order to further reduce the manufacturing cost.

The electrophoretic display of this invention is not necessarily limited to the aforementioned embodiment corresponding to the electronic book, hence, it is possible to propose a variety of modifications.

One modification can be proposed for the electrophoretic display to provide ‘colored’ visual representations instead of ‘monochrome’ visual representations. The monochrome display is actualized using a single thin-film transistor with respect to each pixel as shown in FIG.1. The color display is actualized using three thin-film transistors with respect to each pixel, which is shown in FIG.9. That is, an entire area of the electrophoretic ink layer115, which is arranged between the common electrode113and the pixel electrode114, is partitioned into three layers by intervention of two banks93respectively. Hence, the partitioned three layers are named a cyan-color electrophoretic ink layer115C, a magenta-color electrophoretic ink layer115M, and a yellow-color electrophoretic ink layer115Y, which are arranged below the common electrode113and in connection with the divided three portions of the pixel electrode114. The divided three portions of the pixel electrode114are respectively controlled in switching voltage by three thin-film transistors respectively. Therefore, a combination of the three electrophoretic ink layers115C,115M and115Y that are respectively controlled in switching voltage by a combination of the three thin-film transistors is used to form one color pixel for the electrophoretic display. By adequately arranging combinations of these electrophoretic ink layers over an entire area of the display surface, it is possible to form a ‘color’ electrophoretic display. Applying voltage to the respective ink layers is controlled in response to a display color pattern, so that the electrophoretic display is able to display a color image on its display surface.

The electrophoretic display of this invention is applicable to a variety of electronic devices having displays; therefore, the application of this invention is not necessarily limited to electronic books, an example of which is described as the foregoing embodiment. Next, descriptions will be given with respect to other examples of the electronic devices, each of which is able to use the electrophoretic display of this invention.

1. Mobile Computer

A brief description will be given with respect to an example of a personal computer of a mobile type (or portable type) to which the electrophoretic display of this invention is applied.FIG. 10shows an external appearance of a personal computer1100, which basically comprises a main unit1104providing a keyboard1102, and a display unit whose display100corresponds to the electrophoretic display.

2. Cellular Phone

A brief description will be given with respect to an example of a cellular phone to which the electrophoretic display of this invention is applied.FIG. 11shows an external appearance of a cellular phone1200, which basically comprises numeric keys and function keys1202, an earpiece1204, a mouthpiece1206, and a small-size display100that corresponds to the electrophoretic display.

3. Digital Still Camera

A brief description will be given with respect to an example of a digital still camera to which the electrophoretic display of this invention is applied.FIG. 12shows a backside appearance of a digital still camera1300and its connections to external devices.

Normal cameras are designed to provide exposures on films in response to optical images of photographed subjects. In contrast, the digital still camera1300uses an image pickup device such as a CCD (i.e., Charge Coupled Device) to pick up an optical image of a photographed subject, based on which image pickup signals representing a photographed image are produced by photoelectric conversion. A display100corresponding to the electrophoretic display is arranged at a prescribed area of a backside surface of a case1302of the digital still camera1300. This display100displays pictures that are produced based on image pickup signals for a viewer. That is, the display100acts as a viewfinder for a photographer. A light sensing unit is embedded in the backside of the case1302to provide various parts for optics such as optical lenses and a CCD.

When a photographer pushes a shutter button1306while watching a photographed subject that is displayed on a screen of the display100, the CCD produces image pickup signals, representing a photographed image, which are transferred and stored in a memory on a circuit board1308embedded in the backside of the case1302at its right area. At a right side of the case1302, the digital still camera1300provides video signal output terminals1312and an input/output terminal1314for data communication. Therefore, it is possible to connect the digital still camera1300with a television monitor1430by way of cables connected to the video signal output terminals1312. In addition, it is possible to connect the digital still camera1300with a personal computer1440by way of a data communication cable connected to the input/output terminal1314. By adequately operating switches and controls (not shown) of the digital still camera1300, it is possible to output image pickup signals, which are once stored in the memory on the circuit board1308, to a television signal reproduction circuit of the television monitor1430or a main unit of the personal computer1440.

4. Electronic Paper

A brief description will be given with respect to an example of a ‘flexible’ electronic paper to which the electrophoretic display of this invention is applied.FIG. 13shows an external appearance of an electronic paper1400, which basically comprises a rewritable sheet1401that has similar touch and flexibility of conventional papers, and a display100corresponding to the electrophoretic display.

FIG. 14shows an external appearance of an electronic notebook1402in which a number of electronic papers1400are bound together with a note cover1403. The note cover1403provides a display data input device (not shown) that is used to input display data from the external device. In response to the display data, it is possible to change or update the display content with respect to each of the electronic papers1400bound together with the note cover1403.

We have listed various examples for the application of the electrophoretic display such as the electronic book ofFIG. 3, the personal computer ofFIG. 10, the cellular phone ofFIG. 11, the digital still camera ofFIG. 12, and the electronic paper of FIG.13. Of course, it is possible to list other examples such as the liquid crystal display television set, videotape recorder of the viewfinder type or monitor type, car navigation device, pager, electronic picket notebook, electronic calculator, word processor, workstation, television phone, POS terminal, and other devices having touch panels. The electrophoretic display of this invention can be used as displays for the aforementioned devices.

As described heretofore, this invention has a variety of technical features and effects, which are described below.

(1) The electrophoretic display of this invention is designed to simultaneously erase the overall display content over the entire area of the display surface in order to change the old display content with new one. Herein, the old display content is completely rewritten with the new display content. This is realized by a unique drive method for use in the active-matrix type electrophoretic display of this invention, by which it is possible to improve the reliability in rewriting display contents on the display surface of the electrophoretic display.
(2) To write the new display content, this invention introduces a prescribed relationship of potentials between the common electrode and pixel electrode with respect to each of pixels, wherein an absolute value of the electric potential of the common electrode is made lower than an absolute value of the electric potential of the pixel electrode. This eliminates the necessity for maintaining the pixel electrode at the high electric potential after the writing operation, which yields reduction of the risk in occurrence of write errors.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the claims. The entire disclosures of Japanese Patent Application No. 2000-263565 filed Aug. 31, 2000 and Japanese Patent Application No. 2001-233811 filed Aug. 1, 2001 are incorporated herein by reference.