Patent Publication Number: US-7592984-B2

Title: Display device and electronic device

Description:
TECHNICAL FIELD 
   The present invention relates to a display device comprising a light emitting element. More particularly, the invention relates to portable information equipment such as mobile phones and PDAs. 
   BACKGROUND ART 
   In recent years, a display device using a self-light emitting element typified by an electroluminescence (EL) element and the like has been researched and developed to replace a liquid crystal display (LCD) using a liquid crystal element for a pixel. The display device using a self-light emitting element has the advantages of high image quality, wide viewing angle, and being thin and light because of requiring no backlight. Therefore, it is expected to be widely used as a display panel of a mobile phone or as a display device. 
   On the other hand, the portable information equipment is required to have high added value as the intended purpose thereof is diversified. Thus, the equipment provided with a sub-display screen on the back side of the main display screen has been provided recently. 
   Furthermore, it is possible to browse the Web pages on the portable information equipment and the application is improved. Therefore, the equipment whose screen can be switched between the horizontal direction and the vertical direction has been proposed. 
   In the portable information equipment provided with the sub-display screen besides the main display screen, not only the volume occupied by a module which includes a backlight or the like but also the volume occupied by a substrate which mounts a control IC or the like for driving the module is to be paid attention to. Particularly, in the recent portable information equipment, reduction in weight, thickness, and size is considerably advanced and trades off between a high added value. For example, when the portable information equipment which can display on both screens is fabricated by using a liquid crystal display, a display portion thereof is quite difficult to be thin since a backlight or the like has to be disposed between the two screens. 
   In addition, the number of pixels of a display device is generally different in the vertical and the horizontal directions. Accordingly, when switching between vertical and horizontal display, it is necessary for example that image signals of one frame are temporarily stored in a frame memory provided additionally, and then the format of the image signals are converted in accordance with the number of pixels in the vertical and the horizontal directions. 
   In accordance with the foregoing, the invention provides a display device which is thin and capable of displaying on both screens, and which can be made into a small module. 
   DISCLOSURE OF THE INVENTION 
   The invention takes the following measures to solve the above problem. 
   The invention provides a dual emission display device which uses a self-light emitting element typified by an EL element and the like for a pixel portion and which can emit light on both the upper and the bottom screens. A pixel is divided into a first region having a first light emitting element and a second region having a second light emitting element, for example. In the first light emitting element, a first electrode of an EL element is a transparent electrode and a second electrode thereof is a reflective electrode. In the second light emitting element, a first electrode of an EL element is a reflective electrode and a second electrode is a transparent electrode. That is, the first light emitting element and the second light emitting element emit light in the opposite direction to each other. 
   Since users usually do not look at both screens at the same time, a display screen may be selected as usage. For example, a switching element may be provided in any of the current paths supplied to an EL element so that current supply to the second light emitting element is interrupted when the first light emitting element emits light and current supply to the first light emitting element is interrupted when the second light emitting element emits light. 
   The invention provides a display device having a pixel portion in which pixels are arranged in matrix on a substrate. Each of the pixels comprises a first light emitting element and a second light emitting element. The first light emitting element emits light in only one direction perpendicular to a surface of the substrate on which the pixel portion is formed, whereas the second light emitting element emits light in only one direction which is opposite to the one direction and perpendicular to the surface of the substrate on which the pixel portion is formed. 
   The invention also provides a display device having a pixel portion in which pixels are arranged in matrix on a substrate. Each of the pixels comprises a first light emitting element and a second light emitting element. The first light emitting element emits light in only one direction perpendicular to a surface of the substrate on which the pixel portion is formed, whereas the second light emitting element emits light in only one direction which is opposite to the one direction and perpendicular to the surface of the substrate on which the pixel portion is formed. The display device further comprises a means for selecting either of the two directions in which the first light emitting element and the second light emitting element emit light, and a means for selecting both of the directions. 
   The invention also provides a display device having a pixel portion in which pixels are arranged in matrix on a substrate. Each of the pixels comprises a first light emitting element and a second light emitting element. The first light emitting element emits light in only one direction perpendicular to a surface of the substrate on which the pixel portion is formed, whereas the second light emitting element emits light in only one direction which is opposite to the one direction and perpendicular to the surface of the substrate on which the pixel portion is formed. The display device further comprises a source signal line driver circuit, a first gate signal line driver circuit and a second gate signal line driver circuit on the surface of the substrate on which the pixel portion is formed. A scan direction of the first gate signal line driver circuit is orthogonal to that of the second gate signal line driver circuit. 
   The invention also provides a display device having a pixel portion in which pixels are arranged in matrix on a substrate. Each of the pixels comprises a first light emitting element and a second light emitting element. The first light emitting element emits light in only one direction perpendicular to a surface of the substrate on which the pixel portion is formed, whereas the second light emitting element emits light in only one direction which is opposite to the one direction and perpendicular to the surface of the substrate on which the pixel portion is formed. The display device further comprises a means for selecting either of the two directions in which the first light emitting element and the second light emitting element emit light, and a means for selecting both of the directions. In addition, the display device comprises a source signal line driver circuit, a first gate signal line driver circuit and a second gate signal line driver circuit on the surface of the substrate on which the pixel portion is formed. A scan direction of the first gate signal line driver circuit is orthogonal to that of the second gate signal line driver circuit. 
   A sub-display screen of conventional portable information equipment is limited to a small size because of the space and the cost. However, according to the invention, a large screen can be mounted in the portable information equipment as a sub-display screen. Further, a function of switching between vertical and horizontal display can be easily mounted, leading to higher added value of the portable information equipment. 
   In addition, by utilizing a self-light emitting element which does not require a backlight, a quite thin and light display device can be fabricated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of a light emitting portion of the display device according to the invention. 
       FIG. 2  is a diagram showing a circuit configuration example of the display device according to the invention. 
       FIG. 3  is a diagram showing a circuit configuration example of the display device according to the invention. 
       FIG. 4  is a diagram showing a circuit configuration example of the display device according to the invention. 
       FIGS. 5(A) and 5(B)  are diagrams showing a circuit configuration example of the display device according to the invention. 
       FIGS. 6(A) and 6(B)  are diagrams showing an operating timing of the display device according to the invention. 
       FIGS. 7(A) and 7(B)  are block diagrams of a module and a display device which are mounted in portable information equipment. 
       FIGS. 8(A) and 8(B)  are views showing examples of application of the display device according to the invention. 
       FIG. 9  is a cross sectional view of a mobile phone using a liquid crystal. 
       FIG. 10  is a cross sectional view of a mobile phone using the display device according to the invention. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   The invention will be described in more detail hereinafter with reference to accompanying drawings. 
   Embodiment Mode 1 
     FIG. 2  shows an embodiment mode of the invention. Note that a thin film transistor (TFT) is used as a switching element and a driving element herein, though the invention is not exclusively limited to this. A MOS transistor, an organic transistor, a molecule transistor or the like may be employed as well. Since it is difficult to differentiate a source region and a drain region of a TFT in accordance with the structure or the operating condition, one of the two regions is referred to as a first electrode and the other thereof is referred to as a second electrode. 
   In  FIG. 2 , a region surrounded by a dotted line frame  200  shows one pixel, which comprises a source signal line  201 , a gate signal line  202 , a current supply line  203 , a switching TFT  204 , a first driving TFT  205 , a second driving TFT  206 , a first light emitting element  207 , and a second light emitting element  208 . Each pixel includes a first region in which light from the first light emitting element  207  can be obtained, and a second region in which light from the second light emitting element  208  can be obtained. 
   A gate electrode of the switching TFT  204  is electrically connected to the gate signal line  202 , a first electrode thereof is electrically connected to the source signal line  201 , and a second electrode thereof is electrically connected to gate electrodes of the first and the second driving TGTs  205  and  206 . A first electrode of the first driving TFT  205  is electrically connected to the current supply line  203 , and a second electrode thereof is electrically connected to a first electrode of the first light emitting element  207 . A first electrode the second driving TFT  206  is electrically connected to the current supply line  203 , and a second electrode thereof is electrically connected to a first electrode of the second light emitting element  208 . A second electrode of the first light emitting element  207  and a second electrode of the second light emitting element  208  are electrically connected to counter electrodes  209  and  210  respectively, each of which has a potential difference from the current supply line  203 . 
   An image signal outputted to the source signal line  201  is inputted to the gate electrodes of the first and the second driving TFTs  205  and  206  when the switching TFT  204  is turned ON. The first and the second light emitting elements  207  and  208  are supplied with a current depending on the image signal, and emit light. As described above, light is emitted from the opposing surfaces of a substrate in the first region and the second region. 
   According to this configuration, the first and the second driving TFTs  205  and  206  control light emission or non-light emission of the first and the second light emitting elements  207  and  208 , respectively. However, as shown in  FIG. 3 , analog switches  311  and  312  which operate exclusively of each other may be provided between the current supply line  303  and the first electrodes of the first and the second driving TFTs  305  and  306 , respectively so that a display screen control signal controls ON/OFF of the analog switches  311  and  312 . When the analog switch  311  is turned ON, a current is supplied to the first light emitting element  307  and an image is displayed in the first region. On the other hand, the analog switch  312  which operates exclusively of the analog switch  311  is OFF during this period, and a current supply path to the second light emitting element  308  is interrupted. Thus, light is not emitted in the second region. Conversely, when the analog switch  312  is turned ON, a current is supplied to the second light emitting element  308  and an image is displayed in the second region. During this period, the analog switch  311  is OFF and a current supply path to the first light emitting element  307  is interrupted, therefore, light is not emitted in the first region. At this time, the display screen control signal may be outputted by some operation of users to switch a display screen, or a switching operation may be automatically performed according to the state of use (whether the equipment is folded or opened, for example). 
     FIG. 4  shows the case in which the analog switches  311  and  312  do not operate exclusively with each other, but they operate independently by using display screen control signals  1  and  2 . According to the configuration shown in  FIG. 4 , whether display is performed or not can be switched arbitrarily in both the first and the second regions. It is to be noted that in  FIG. 3 , reference numeral  301  denotes a source signal line,  302  denotes a gate signal line,  304  denotes a switching TFT, and  313  denotes an inverter. In  FIG. 4 , reference numerals  413  and  414  denote inverters, and the same portions are denoted by the same reference numerals as in  FIG. 3 . 
   As a method for displaying different images in the first and the second regions by using the configurations shown in  FIGS. 3 and 4 , there is a method, for example, in which display in the first region is performed during odd frames and display in the second region is performed during even frames. At this time, the display screen control signals are inverted per frame period so that the analog switches  311  and  312  are switched ON/OFF per frame. 
   Embodiment Mode 2 
   Described mainly in this embodiment mode is a circuit configuration. 
     FIG. 5A  shows an example of a configuration of the display device according to the invention. A pixel portion  501  is formed on a substrate  500 , and a source signal line driver circuit  502 , a first gate signal line driver circuit  503 , and a second gate signal line driver circuit  504  are formed in the peripheral portion of the pixel portion. A control signal input to each of the driver circuits and a current supply to a current supply line  505  are performed through a flexible printed circuit (FPC)  506 . A portion denoted by  510  in  FIG. 5A  corresponds to one pixel, which is shown in more detail in  FIG. 5B . 
   In  FIG. 5B , a portion surrounded by a dotted line frame  520  is a pixel which comprises a source signal line  521 , a first gate signal line  522 , a second gate signal line  523 , a current supply line  524 , a first switching TFT  525 , a second switching TFT  526 , a first driving TFT  527 , a second driving TFT  528 , a storage capacitor  529 , a first light emitting element  530 , and a second light emitting element  531 . Each pixel includes a first region in which light emission from the first light emitting element  530  can be obtained, and a second region in which light emission from the second light emitting element  531  can be obtained. 
   A gate electrode of the first switching TFT  525  is electrically connected to the first gate signal line  522 , a first electrode thereof is electrically connected to the source signal line  521 , and the second electrode thereof is electrically connected to a first electrode of the second switching TFT  526 . A gate electrode of the second switching TFT  526  is electrically connected to the second gate signal line  523 , and a second electrode thereof is electrically connected to each of gate electrodes of the first and the second driving TFTs  527  and  528 . A first electrode of the first driving TFT  527  is electrically connected to the current supply line  524 , and a second electrode thereof is electrically connected to a first electrode of the first light emitting element  530 . A first electrode of the second driving TFT  528  is electrically connected to the current supply line  524 , and a second electrode thereof is electrically connected to a first electrode of the second light emitting element  531 . A second electrode of the first light emitting element  530  and a second electrode of the second light emitting element  531  are electrically connected to counter electrodes  532  and  533  respectively, which have a potential difference from the current supply line  524 . The storage capacitor  529  is provided in order to store a gate-source voltage of the first and the second driving TFTs  527  and  528 . In  FIG. 5B , the storage capacitor  529  is disposed between the gate electrodes of the first and the second driving TFTs  527  and  528  and the current supply line  524 , though the connection point is not limited to this. 
   Operation of the circuit is explained. Note that although the number of pixels is m×n pixels in this specification, any method can be adopted for converting the format of image signals. Thus, the case in which m=n is satisfied is taken as an example for simplicity. Explanation is made with reference to  FIGS. 5A ,  5 B and  6 . 
   In the case of performing a first display, namely a normal display in which the pixel portion includes m×n pixels, the second switching TFT  526  is turned ON in the full screen by the second gate signal line driver circuit  504 . According to this, the pixel is controlled only by the first switching TFT  525  and the first and the second driving TFTs  527  and  528 . Then, an image is displayed by driving the source signal line driver circuit  502  and the first gate signal line driver circuit  503  by a normal method. As shown in  FIG. 6A , the order of writing an image signal to the pixel is ( 1 ,  1 ) ( 2 ,  1 ) . . . (m,  1 ), ( 1 ,  2 ) ( 2 ,  2 ) . . . (m,  2 ), . . . , ( 1 , n) ( 2 , n) . . . (m, n). 
   A case of performing a second display, namely switching between vertical and horizontal display on a screen, is described next.  FIG. 6B  is obtained by rotating a pixel of  FIG. 6A  clockwise by 90°. According to the display device of the invention, the input order of an image signal need not be changed. Accordingly, the order of writing to pixels shown in  FIG. 6B  is ( 1 , n) ( 1 , n−1) . . . ( 1 ,  1 ), ( 2 , n) ( 2 , n−1) . . . ( 2 ,  1 ), . . . , (m, n) (m, n−1) . . . (m,  1 ). 
   Therefore, the source signal line driver circuit  502  operates slower than usual during performing the second display, and outputs a sampling pulse per one horizontal period. Therefore, image signals of one horizontal period are outputted in sequence for each source signal line. On the other hand, the first gate signal line driver circuit  503  operates faster than usual, and outputs a gate signal line selective pulse per one dot sampling period. According to this, the first switching TFT  525  in each pixel is ON during one dot sampling period only, and an image signal outputted to the source signal line  521  is written at that time. Further, the second gate signal line driver circuit  504  operates in synchronism with the source signal line driver circuit  502 . That is, when a sampling pulse is outputted from the source signal line driver circuit  502  and an image signal is sequentially outputted to the source signal line  521  of a certain column, the second gate signal line  523  of that column is selected, and all of the second switching TFTs  526  connected to the selected second gate signal line  523  are turned ON. Thus, image signals can be written to that column only. 
   According to the aforementioned operations, writing of image signals to the pixels can be performed. As a result, switching between vertical and horizontal display can be achieved without using a frame memory, leading to reduction in size of a module. 
   Note that, switching of a control signal of each driver circuit can be performed by some operation of a user to switch between vertical and horizontal display, or the switching operation can be performed automatically depending on states of use (whether equipment is folded or opened, for example). 
   EMBODIMENT 
   Embodiment 1 
   A pixel configuration of the display device according to the invention is described with reference to  FIG. 1 . 
   In  FIG. 1 , a base film  6001  is formed on a substrate  6000 , and a first driving TFT  6002  and a second driving TFT  6021  are formed on the base film  6001 . 
   The first driving TFT  6002  comprises an active layer  6003 , a gate electrode  6005 , and a gate insulating film  6004  which is sandwiched between the active layer  6003  and the gate electrode  6005 . It is to be noted that although the gate electrode  6005  is formed of two layers having the upper layer and the lower layer with different widths in  FIG. 1 , it is not limited to this and may be formed of a single layer or multiple layers. 
   In addition, the first driving TFT  6002  is covered with a first interlayer insulating film  6006 , and a second interlayer insulating film  6007  and a third interlayer insulating film  6008  are formed in this order over the first interlayer insulating film  6006 . 
   Note that the first driving TFT  6002  and the second driving TFT  6021  have the same structure. 
   In  FIG. 1 , reference numerals  6010  and  6030  denote anodes of a first and a second light emitting elements respectively,  6011  denotes an electroluminescent layer, and  6012  denotes a cathode. An overlapping area of the anode  6010 , the electroluminescent layer  6011  and the cathode  6012  corresponds to a first light emitting element  6013 , and an overlapping area of the anode  6030 , the electroluminescent layer  6011  and the cathode  6012  corresponds to a second light emitting element  6023 . The first driving TFT  6002  controls a current supplied to the first light emitting element  6013 , and it is electrically connected to the first light emitting element  6013  directly or through other elements. On the other hand, the second driving TFT  6021  controls a current supplied to the second light emitting element  6023 , and it is electrically connected to the second light emitting element  6023  directly or through other elements. 
   Note that it is preferable to form the anode  6010  by using a highly reflective or light shielding material which has a high work function (preferably, more than 4.2 eV). Specifically, the anode  6010  is formed of a material such as gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chrome (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), and titanium nitride (TiN). Alternatively, a conductive film having a high work function may be formed on a reflective (or light shielding) film in order to obtain the anode  6010 . For example, a metal film such as Al is formed and indium tin oxide (ITO) or the like is laminated on the metal film. It is to be noted that the ITO formed on the Al film is only required to have a thickness enough to function as an anode of an EL, and is preferably thin so as to prevent deviation of light emitted from an EL layer due to the interference. On the other hand, the anode  6030  is preferably formed by using a material which has a high light transmittance. For example, ITO, zinc oxide (ZnO), indium zinc oxide (IZO), zinc oxide added with gallium, or the like may be employed. The cathode  6012  is preferably formed so as to have a low work function (preferably, 4.2 eV or less) and a high light transmittance. For example, a metal film such as Mg: Ag alloy is formed to be thin enough to transmit light (preferably, 10 nm or less) and ITO or the like is laminated on the metal film. As the thickness of a metal thin film is reduced, the resistance of the metal thin film is increased. However, ITO is not necessarily provided as long as the conductivity is high enough. Alternatively, a conductive material with a high light transmittance such as ITO may be used as the cathode  6012  regardless of a work function. In this case, in order to inject electrons more efficiently, a metal having a low work function such as alkali metal or alkaline earth metal, or an alloy including these metal elements is preferably added in the vicinity of an area in which the electroluminescent layer  6011  is in contact with the cathode  6012 . 
   The electroluminescent layer  6011  is formed of a single light emitting layer or a plurality of layers including a light emitting layer. 
   The anode  6010  and the anode  6030  are formed on the third interlayer insulating film  6008 . A resin film  6014  used as a bank is also formed on the third interlayer insulating film  6008 . 
   A protective film  6016  is formed over the resin film  6014  and the cathode  6012 . On the protective film  6016 , a reflective film  6024  is formed and has a function to reflect light emitted from the second light emitting element  6023  and transmit the light in the direction of the substrate side only. 
   Light emitted from the first light emitting element  6013  is reflected on the anode  6010 , and transmitted in the upper direction of the substrate only. 
   As described in this embodiment, by using a self-light emitting element as a display device, a thin display device having a thickness of 2 mm or less (preferably, 1 mm or less) and capable of displaying on both screens can be achieved, which was impossible with the conventional display device requiring a backlight. 
   Embodiment 2 
   When utilizing a display device for a display portion of electronic equipment such as a mobile phone, the display device is mounted in the equipment as a module  701  as shown in  FIG. 7A . The module  701  includes here a display device and a substrate connected thereto, which mounts a signal processing LSI for driving the display device, a memory or the like. 
     FIG. 7B  is a block diagram of the module  701 . The module  701  comprises a power source portion  711 , a signal control portion  712 , an FPC  713 , a display device  714 . The power source portion  711  generates power source with desired voltage values from power source supplied from an external battery, and supplies it to a source signal line driver circuit, a gate signal line driver circuit, a light emitting element or the like. An image signal and a synchronous signal are inputted to the signal control portion  712 . The signal control portion  712  converts various signals so as to be processed in the display device  714 , and generates a clock signal or the like for driving the source signal line driver circuit and the gate signal line driver circuit. 
   Note that the power source portion  711  and the signal control portion  712  are formed separately from the display device  714  in the module  701  shown in this embodiment, they may be integrally formed on a substrate. 
   Embodiment 3 
     FIGS. 8A and 8B  show examples in which the display device of the invention is applied to a mobile phone which is one of the representative portable information equipment. Since the display device can display on each side of a substrate, a housing  800  can be reduced in thickness even when forming display portions on both screens as shown in  FIG. 8A . 
   As an example of use, when the equipment is opened, a first display screen  801  is used as a main display screen and the screen is operated by operating buttons  802 . The size of a second display screen  802  which is used when the equipment is folded has been limited because of the limited space. However, according to the invention, the second display screen  803  having the same display size as the first display screen  801  can be mounted so as to be used for checking e-mail, Web pages, or the like. When the equipment is folded, the operation is performed by operating buttons  804 . 
   Recently, a mobile phone and the like provided with a digital camera has been widely used. In taking a picture facing a lens  805 , the picture can be taken while monitoring on the second display screen  803  which has a large display region. 
   As shown in  FIG. 8B , a user can switch between vertical and horizontal display at will. As described in Embodiment Mode 2, the switching between vertical and horizontal display can be performed automatically depending on the application or arbitrarily in accordance with the operation by a user. 
   It is needless to say that the mobile phone is shown as a representative example here, though the invention is not limited to this and can be easily applied to various equipment such as a PDA, a sub-note PC, and an electronic dictionary. 
   Embodiment 4 
     FIG. 9  shows an example of a mobile phone which can display on both screens by using a conventional liquid crystal display device.  FIG. 10  shows an example of a mobile phone which can display on both screens by using the display device of the invention. Note that the same portions are denoted by the same reference numerals in order to clearly show differences between the two examples. 
   A mobile phone shown in  FIG. 9  comprises a first housing  1101  and a second housing  1108 , and can be folded. The first housing  1101  includes a first display device (for main display)  1102  and a second display device (for sub-display)  1103 , which are controlled by display controllers  1104  and  1105  respectively. The first housing  1101  further comprises a speaker  1106  and an antenna  1107 . 
   The second housing  1108  comprises a main body driving module  1109 , an operating button module  1110 , a microphone  1112 , and a battery  1113 . The first housing  1101  and the second housing  1108  are connected to each other with a hinge  1111 . 
   At this time, each of the first display device  1103  and the second display device  1102  includes a backlight. Reference numeral  1151  denotes a first display region and  1152  denotes a second display region. The thickness of a housing including the display devices, that is the first housing  1101  is denoted by T 1 . 
   A mobile phone shown in  FIG. 10  also comprises a first housing  1131  and the second housing  1108 . The structure of the second housing  1108  is the same as the conventional example shown in  FIG. 9 , therefore, the explanation is omitted here. In a display device  1132  of the invention, a self-light emitting element which is capable of emitting light in both directions is sandwiched between two substrates, and the display device  1132  is controlled by a single display controller  1133 . Reference numeral  1161  denotes a first display region and  1162  denotes a second display region. The thickness of a housing including the display device, that is the first housing  1131  is denoted by T 2 . 
   When comparing the thickness of the first housings in the two examples, the first housing the display device of the invention can be made much thinner. In the conventional example, the main display screen and the sub-display screen are provided by using two display devices each including a backlight, and thus T 1  is increased in the case of displaying on both screens. On the other hand, since the display device of the invention adopts a self-light emitting element, it can display on both screens without backlight and can be drastically reduced in thickness as compared to the conventional example. 
   Further, when comparing the size of the second display regions in the two examples, the second display region using the display device of the invention is more suitable for a large screen. 
   As set forth above, the display device of the invention can significantly contribute to reduction in size and multifunction of a portable information terminal such as a mobile phone. 
   INDUSTRIAL APPLICABILITY 
   A sub-display screen of conventional portable information equipment is limited to a small size because of the space and the cost. However, according to the invention, a large screen can be mounted in the portable information equipment as a sub-display screen. Further, a function of switching between vertical and horizontal display can be easily mounted, leading to higher added value of the is portable information equipment. 
   In addition, by utilizing a self-light emitting element which does not require a backlight, a quite thin and lightweight display device can be fabricated.