Patent Publication Number: US-2017365226-A1

Title: Display device and method for driving the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display device, and particularly, a structure of a pixel including a liquid crystal and a backlight. 
     2. Description of the Related Art 
     A liquid crystal display device is formed by a combination of a liquid crystal panel in which a liquid crystal is sandwiched between a pair of substrates and a lighting system called a backlight on a rear side of the liquid crystal panel. As for the liquid crystal panel, a simple matrix mode and an active matrix mode using a thin film transistor (TFT) are known (refer to Patent Document 1: Japanese Published Patent Application No. H9-90404). In either mode, a voltage which is applied to electrodes sandwiching the liquid crystal is controlled, and the quantity of transmitted light of the backlight which illuminates the entire surface of the liquid crystal panel is adjusted, whereby an image is displayed. Therefore, for example, also in a case of black display in all the pixels, the backlight is always turned on and the power is kept being consumed. 
     Meanwhile, a liquid crystal display device using a light-emitting diode (LED) as a backlight is provided. The LED has advantages such as the long lifetime, no need of an inverter since it can be directly driven by a low-voltage direct current power source, low power consumption, and the like. As for such a backlight, a directly-below type and a sidelight type are known as a position of the LED. In a directly-below type, the LED is positioned directly below the liquid crystal panel, and the LED emits light uniformly on the surface using a diffusing plate or the like. In addition, in a sidelight type, the LED emits light from the side surface of the panel, and the light is supplied to the liquid crystal panel through a light-guiding plate or a diffusing plate; thus, the sidelight type can be formed thinly compared to a directly-below type, which is an advantage. 
     SUMMARY OF THE INVENTION 
     In a conventional transmission display device using a liquid crystal, light from a backlight is uniformly emitted to a rear side of the liquid crystal. Therefore, even when black is intended to be displayed, light leaks from a liquid crystal element, and completely black display cannot be performed, which leads to the reduced contrast. 
     It is an object of the present invention to provide a display device in which a problem of light leakage from a liquid crystal element in black display is overcome and the contrast is improved. 
     In the present invention, the above object is achieved by turning off a light-emitting element when a black gray scale is displayed. In addition, in the present invention, the above object is also achieved by a light-emitting element provided in each pixel and a function in a pixel circuit to individually control lighting and non-lighting of the light-emitting element depending on a gray scale to perform display. 
     One aspect of the present invention is a display device including a light-emitting element provided so as to overlap with a liquid crystal element in a pixel, where light-emission of the light-emitting element passes through the liquid crystal element. In this display device, the liquid crystal element is electrically connected to a data line to which a potential based on an analog signal is applied, a comparator which compares a potential of the data line with a reference potential is provided, and the light-emitting element is electrically connected to the comparator. 
     In the display device proposed in the present invention, a case is considered, where a liquid crystal element is driven by an analog data signal which is changed to be positive and negative. Conventionally, there has been a driving method in which an analog potential that is set in accordance with characteristics of the liquid crystal element is written as a data potential when a gray scale is displayed. In the liquid crystal element, transmitted light is controlled by a potential difference between a pixel electrode and a counter electrode, and a direction of voltage application is not particularly related. Therefore, a method for inverting a voltage value of a data signal to be positive and negative for input has been conventionally used from a problem of the lifetime of a liquid crystal or the like. At this time, there are positive and negative data potentials which perform the same gray scale display. 
     One aspect of the present invention is a display device including a pixel portion having a light-emitting element and a liquid crystal element provided so as to overlap with the light-emitting element; a data line electrically connected to the liquid crystal element, to which a potential based on an analog signal is applied; and a comparator electrically connected to the data line and the light-emitting element. In accordance with the present invention, in a display device using a liquid crystal, a backlight is made not to emit light when black is displayed; therefore, light leakage can be eliminated and the contrast can be improved. 
     Another aspect of the present invention is a display device including a pixel portion having a light-emitting element and a liquid crystal element provided so as to overlap with the light-emitting element; a data line electrically connected to the liquid crystal element, to which a potential based on an analog signal is applied; a first comparator electrically connected to the data line and the light-emitting element; a second comparator electrically connected to the data line and the light-emitting element; a first switch electrically connected to the data line and the first comparator; and a second switch electrically connected to the data line and the second comparator. In accordance with the present invention, in a display device using a liquid crystal, a backlight is made not to emit light when black is displayed; therefore, light leakage can be eliminated and the contrast can be improved. In addition, a function to control a lighting state of a light-emitting element based on each pixel is provided in a pixel circuit; therefore, a malfunction can be avoided, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. 
     Another aspect of the present invention is a method for driving a display device which includes a pixel portion having a light-emitting element and a liquid crystal element provided so as to overlap with the light-emitting element; a data line electrically connected to the liquid crystal element, to which a potential based on an analog signal is applied; and a comparator electrically connected to the data line and the light-emitting element, including the steps of comparing a potential of the data line with a reference potential in the comparator; and controlling light-emission of the light-emitting element based on a comparison result by the comparator. In accordance with the present invention, in a display device using a liquid crystal, a backlight is made not to emit light when black is displayed; therefore, light leakage can be eliminated and the contrast can be improved. 
     Another aspect of the present invention is a method for driving a display device which includes a pixel portion having a light-emitting element and a liquid crystal element provided so as to overlap with the light-emitting element; a data line electrically connected to the liquid crystal element, to which a potential based on an analog signal is applied; a first comparator and a second comparator each electrically connected to the data line and the light-emitting element; a first switch electrically connected to the data line and the first comparator; and a second switch electrically connected to the data line and the second comparator, including the steps of comparing a potential of the data line with a first reference potential in the first comparator by turning on the first switch when the potential of the data line has a negative value; comparing a potential of the data line with a second reference potential in the second comparator by turning on the second switch when the potential of the data line has a positive value; and controlling light-emission of the light-emitting element based on a comparison result by the first comparator or a comparison result by the second comparator. In accordance with the present invention, in a display device using a liquid crystal, a backlight is made not to emit light when black is displayed; therefore, light leakage can be eliminated and the contrast can be improved. In addition, a function to control a lighting state of a light-emitting element based on each pixel is provided in a pixel circuit; therefore, a malfunction can be avoided, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. 
     In addition, another aspect of the present invention is an electronic device having a feature that the above display device is included in a display portion. 
     In the present invention, “to be connected” also indicates “to be electrically connected”. Accordingly, in a structure disclosed in the present invention, other elements capable of electrical connection (such as other elements and a switch) may be arranged, in addition, between predetermined connected components. 
     It is to be noted that an element provided in a pixel is not limited to a specific light-emitting element. A light-emitting element provided in a pixel may be any light-emitting element such as an EL (Electroluminescence) element or an element used in a field emission display (FED), an SED (Surface-conduction Electron-emitter Display) that is one kind of FED, a plasma display panel (PDP), or a piezoelectric ceramic display. 
     In a transistor, it is difficult to distinguish between a source and a drain because of its structure. Further, there is also a case where levels of a potential are switched depending on an operation of a circuit. Therefore, in this specification, a source and a drain are each referred to as a first terminal or a second terminal without particular limitation. For example, when the first terminal is a source, the second terminal is a drain, and on the other hand, when the first terminal is a drain, the second terminal is a source. 
     In the present invention, a type of an applicable transistor is not limited. It is thus possible to apply a thin film transistor (TFT) using a non-single crystal semiconductor film typified by amorphous silicon and polycrystalline silicon, a transistor formed using a semiconductor substrate or an SOI (Silicon On Insulator) substrate, a MOS transistor, a junction type transistor, a bipolar transistor, a transistor using an organic semiconductor or a carbon nanotube, or other transistors. Further, a type of a substrate over which a transistor is arranged is not limited, and a transistor can be arranged over a single crystal substrate, an SOI substrate, a glass substrate, or the like. 
     In the present invention, a comparator may be an operation amplifier, a chopper inverter circuit, or the like, that is, any circuit can be used as long as a function as a comparator can be achieved. 
     In the present invention, when black is displayed in a display device using a liquid crystal, a backlight is made not to emit light; therefore, light leakage can be eliminated and the contrast can be improved. In addition, in the present invention, a backlight is arranged in each pixel and a function to control a lighting state of a light-emitting element based on each pixel is provided in a pixel circuit; therefore, a malfunction can be avoided, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. Moreover, a backlight in a portion where lighting is not needed can be individually turned off; therefore, power savings can be effectively achieved. 
     In a conventional liquid crystal display device, a plurality of pixels use the same backlight as a lint-emitting source. Therefore, a malfunction is generated, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. However, in the present invention, a backlight is provided in each pixel; therefore, a light-emitting element that is a backlight can be turned off when a black gray scale is displayed, and reduction in contrast due to light leakage from a liquid crystal element can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are diagrams each showing a relation between a range in which a light-emitting element emits light and a data potential in the present invention; 
         FIG. 2  is a circuit diagram schematically showing a pixel circuit of a display device according to the present invention; 
         FIG. 3  is a circuit diagram showing a pixel circuit of a display device according to the present invention; 
         FIGS. 4A to 4C  are circuit diagrams each showing an operation of a comparator provided in a pixel circuit of a display device according to the present invention; 
         FIG. 5  is a diagram showing a timing chart of a setting-operation of a pixel according to the present invention; 
         FIG. 6  is a circuit diagram schematically showing a pixel circuit of a display device according to the present invention; 
         FIG. 7  is a circuit diagram showing a pixel circuit of a display device according to the present invention; 
         FIG. 8  is a diagram showing a timing chart of a setting-operation of a pixel according to the present invention; 
         FIGS. 9A to 9H  are views each showing an example of an electronic device to which a display device according to the present invention is applied; 
         FIGS. 10A and 10B  are diagrams each showing an operation of a pixel circuit of a display device according to the present invention; 
         FIGS. 11A to 11C  are cross-sectional views each showing a structure of a display device related to Embodiment Mode 3; and 
         FIG. 12  is a block diagram showing a structure of a display device related to Embodiment Mode 3. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Hereinafter, Embodiment Modes of the present invention will be explained with reference to the accompanying drawings. It is to be noted that the present invention can be implemented in various modes, and it is easily understood by those skilled in the art that modes and details thereof can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention should not be interpreted as being limited to the following description of the embodiment modes. In all the drawings for explaining the embodiment modes, the same portions or portions having similar functions are denoted by the same reference numerals, and repeated explanation thereof will be omitted. 
     The upper limit or the lower limit of a range of a data voltage, which has a completely black gray scale, is defined as a reference potential, and lighting or non-lighting of a light-emitting element is selected by comparison of a data potential with the reference potential.  FIGS. 1A and 1B  each show a method for setting a reference potential.  FIG. 1A  shows a method for setting a reference potential in a liquid crystal having a normally white display mode (hereinafter referred to as a normally white liquid crystal). The normally white display mode is a polarized state in which light is transmitted in a state where no voltage is applied to a liquid crystal element. In a gray scale  101  of the liquid crystal element, the upper limit of a range  104  of a data voltage, in which completely black display is performed in a negative voltage region, is defined as a reference potential Vref 1 . In addition, the lower limit of a range  103  of a data voltage, in which completely black display is performed in a positive voltage region, is defined as a reference potential Vref 2 . 
     On the other hand, a normally black display mode is a polarized state in which light is transmitted in a state where a voltage is applied to a liquid crystal element. In a manner similar to the above,  FIG. 1B  shows a method for setting a reference potential in a liquid crystal having a normally black display mode (hereinafter referred to as a normally black liquid crystal). As can be seen in  FIG. 1B , in the normally black liquid crystal, a range  106  of a data voltage, in which completely black display is performed, exists only around 0 V. In a gray scale  101  of the liquid crystal element, the upper limit of the range  106  of a data voltage, in which completely black display is performed, is defined as a reference potential Vref 1 . In addition, the lower limit of the range  106  of a data voltage, in which completely black display is performed, is defined as a reference potential Vref 2 . 
     In  FIGS. 1A and 1B , it is necessary to provide, in a pixel circuit, a circuit which judges whether a data potential is in a range  105  of a data voltage, in which a light-emitting element emits light. In the present invention, a circuit which compares the data potential with the reference potential is used, and the light-emitting element is made to emit light in a case where the data potential is lower than Vref 2  or the data potential is higher than Vref 1 . 
     Embodiment Mode 1 
       FIG. 2  schematically shows one embodiment mode of a pixel included in a display device of the present invention. In a circuit shown in  FIG. 2 , a data line  201  and a scanning line  202  are provided; a transistor  203  which performs writing to a liquid crystal element  204  and a counter electrode  205  are provided; a light-emitting element  212  used as a backlight, and a first comparator  207  and a second comparator  209  are provided; and a first switch  213  for writing a data signal into the first comparator  207  and a second switch  214  for writing a data signal into the second comparator  209  are provided. A first reference potential  206  that is compared with a data potential is input to the first comparator  207 , and a second reference potential  208  that is compared with a data potential is input to the second comparator  209 . In addition, a third switch  210  and a fourth switch  211 , by which it is determined which of the first comparator  207  and the second comparator  209  is selected to transmit output to the light-emitting element  212 , are provided. 
     The components in the circuit shown in  FIG. 2  are connected as follows. A first terminal of the transistor  203  is connected to the data line  201 , and a first electrode of the liquid crystal element  204  is connected to a second terminal of the transistor  203 . A gate terminal of the transistor  203  is connected to the scanning line  202 . A second electrode of the liquid crystal element  204  is connected to the counter electrode  205 . Input of the first comparator  207  is connected to the data line  201  through the first switch  213 , and output of the first comparator  207  is connected to a first electrode of the light-emitting element  212  through the third switch  210 . Further, input of the second comparator  209  is connected to the data line  201  through the second switch  214 , and output of the second comparator  209  is connected to the first electrode of the light-emitting element  212  through the fourth switch  211 . It is to be noted that the first reference potential  206  which is compared with a data potential is applied to the first comparator  207 , and the second reference potential  208  is similarly applied to the second comparator  209 . 
     Next, an operation of the circuit will be briefly explained. A data potential written into the liquid crystal element  204  from the data line  201  is input to the second comparator  209  and the first comparator  207 . The first comparator  207  compares the data potential input from the data line with the first reference potential  206 . In addition, the second comparator  209  compares the data potential input from the data line with the second reference potential  208 . 
     The first switch  213  writes a signal from the data line  201  into the first comparator  207 , and the second switch  214  writes a signal from the data line  201  into the second comparator  209 . 
     By the third switch  210  and the fourth switch  211 , one of output of the first comparator  207  and output of the second comparator  209  is selected, and the output is transmitted to the light-emitting element  212 . Therefore, it is necessary to set an operation timing appropriately so as not to turn on the third switch  210  and the fourth switch  211  at the same time. 
       FIGS. 10A and 10B  show operations of the circuit shown in  FIG. 2 .  FIG. 10A  shows an operation in a case where a negative potential is loaded from the data line  201  to the comparator. The first switch  213  is turned on, a data potential is written into the first comparator  207 , the data potential is compared with the first reference potential  206  in the first comparator  207 , and the third switch  210  is turned on so that output of the first comparator  207  is output to the light-emitting element  212 . During this operation, the second switch  214  and the fourth switch  211  which are connected to input and output of the second comparator  209  respectively are turned off.  FIG. 10B  shows an operation in a case where a positive potential is loaded from the data line  201  to the comparator. The second switch  214  is turned on, a data potential is written into the second comparator  209 , the data potential is compared with the second reference potential  208  in the second comparator  209 , and the fourth switch  211  is turned on so that output of the second comparator  209  is output to the light-emitting element  212 . During this operation, the first switch  213  and the third switch  210  which are connected to input and output of the first comparator  207  respectively are turned off. 
     It is to be noted that the light-emitting element  212  may be an LED, an organic EL element, or the like. That is, any light-emitting element which can be controlled by a voltage, a current, or the like may be used. 
     It is to be noted that the switch can be an electrical switch or a mechanical switch. That is, any switch may be used as long as it can control a current flow, and the switch may be a transistor, a diode, or a logic circuit combining a transistor and a diode. Therefore, in a case where a transistor is used as the switch, polarity (conductivity type) thereof is not particularly limited because the transistor operates just as a switch. However, when a low off-state current is desired, a transistor of polarity with a lower off-state current is desirably used. For example, a transistor which is provided with an LDD region, a transistor which has a multi-gate structure, or the has a low off-state current. Further, it is desirable that an n-channel transistor be employed when a potential of a source terminal of a transistor operating as a switch is close to a potential of a low potential side power source (such as VSS, GND, pr 6 V), while a p-channel transistor be employed when the potential of the source terminal is close to a potential of a high potential side power source (such as VDD). This allows the transistor to operate as a switch with higher precision since an absolute value of a gate-source voltage can be increased. It is to be noted that a CMOS type switch using both an n-channel transistor and a p-channel transistor may also be used. 
     VDD is regarded as a high power source potential, and VSS is regarded as a low power source potential. Here, the high power source potential VDD is to be higher than the low power source potential VSS. 
       FIG. 3  shows an example of a detailed circuit diagram of the pixel shown in  FIG. 2 . The circuit shown in  FIG. 3  has a feature in that two CMOS chopper inverter circuits are used as comparators. In other words, a first comparator  326  and a second comparator  327  are used. The first comparator  326  includes a first capacitor element  318 ; a first transistor  317 ; a first inverter including a second transistor  319  and a third transistor  320 ; and a second inverter including a fourth transistor  321  and a fifth transistor  322 . The second comparator  327  includes a second capacitor element  314 ; a sixth transistor  313 ; and a third inverter including a seventh transistor  315  and an eighth transistor  316 . As a wiring, a first reference potential line  301 , a second reference potential line  303 , and a data line  302  are provided. In addition, a scanning line  306 , a first comparator initializing line  307 , a second comparator initializing line  305 , a comparator selecting line  304 , a first data loading line  331 , and a second data loading line  330  are provided. A ninth transistor  308  which writes a data voltage into a liquid crystal element  310  and a counter electrode  311  of the liquid crystal element are provided. A tenth transistor  312  and an eleventh transistor  329  which perform writing to the first comparator  326  are provided. A twelfth transistor  309  and a thirteenth transistor  328  which perform writing to the second comparator  327  are provided. Further, a fourteenth transistor  324  which transmits output of the first comparator  326  to a light-emitting element  325  and a fifteenth transistor  323  which transmits output of the second comparator  327  to the light-emitting element  325  are provided. It is to be noted that the second transistor  319  is a p-channel transistor and the third transistor  320  is an n-channel transistor; the fourth transistor  321  is a p-channel transistor and the fifth transistor  322  is an n-channel transistor; and the seventh transistor  315  is a p-channel transistor and the eighth transistor  316  is an n-channel transistor. 
     The components in  FIG. 3  are connected as follows. A first terminal of the ninth transistor  308 , a first terminal of the thirteenth transistor  328 , and a first terminal of the eleventh transistor  329  are connected to the data line  302 . A second terminal of the ninth transistor  308  is connected to a first electrode of the liquid crystal element  310 , and a gate terminal of the ninth transistor  308  is connected to the scanning line  306 . The counter electrode  311  is connected to a second electrode of the liquid crystal element  310 . A first terminal of the twelfth transistor  309  is connected to the second reference potential line  303 . A second terminal of the thirteenth transistor  328  and a second terminal of the twelfth transistor  309  are connected to a first electrode of the second capacitor element  314  in the second comparator  327 . A gate terminal of the thirteenth transistor  328  is connected to the second data loading line  330 , and a gate terminal of the twelfth transistor  309  is connected to the second comparator initializing line  305 . The seventh transistor  315  and the eighth transistor  316  form an inverter. A gate terminal of the seventh transistor  315  and a gate terminal of the eighth transistor  316  are connected to a second electrode of the second capacitor element  314  and a first terminal of the sixth transistor  313 . A second terminal of the seventh transistor  315  and a second terminal of the eighth transistor  316  are connected to a second terminal of the sixth transistor  313  and a first terminal of the fifteenth transistor  323 . A gate terminal of the sixth transistor  313  is connected to the second comparator initializing line  305 . 
     A first terminal of the tenth transistor  312  is connected to the first reference potential line  301 . A gate terminal of the tenth transistor  312  is connected to the first comparator initializing line  307 . A second terminal of the eleventh transistor  329  and a second terminal of the tenth transistor  312  are connected to a first electrode of the first capacitor element  318  in the first comparator  326 . A gate terminal of the eleventh transistor  329  is connected to the first data loading line  331 . The second transistor  319  and the third transistor  320  form an inverter. A gate terminal of the second transistor  319  and a gate terminal of the third transistor  320  are connected to a second electrode of the first capacitor element  318  and a first terminal of the first transistor  317 . In addition, a second terminal of the second transistor  319  and a second terminal of the third transistor  320  are connected to a second terminal of the first transistor  317  and gate terminals of the fourth transistor  321  and the fifth transistor  322  which form another inverter. A second terminal of the fourth transistor  321  and a second terminal of the fifth transistor  322  are connected to a first terminal of the fourteenth transistor  324 . A gate terminal of the first transistor  317  is connected to the first comparator initializing line  307 . 
     A gate terminal of the fifteenth transistor  323  and a gate terminal of the fourteenth transistor  324  are connected to the comparator selecting line  304 . A second terminal of the fifteenth transistor  323  and a second terminal of the fourteenth transistor  324  are connected to a first electrode of the light-emitting element  325 . 
     Then, an operation of this pixel circuit will be explained. The first comparator  326  compares a potential Vref 1  of the first reference potential line  301  with a potential of the data line  302  when a negative voltage is applied to the liquid crystal element  310 . If the potential of the data line  302  is higher than Vref 1 , a power source voltage VDD is output. On the other hand, if the potential of the data line  302  is lower than Vref 1 , 0 V is output. The second comparator  327  compares a potential Vref 2  of the second reference potential line  303  with a potential of the data line  302  when a positive voltage is applied to the liquid crystal element  310 . If the potential of the data line  302  is lower than Vref 2 , a power source voltage VDD is output. On the other hand, if the potential of the data line  302  is higher than Vref 2 , 0 V is output. A detailed operation in the comparator will be explained later. In this embodiment mode, a CMOS chopper inverter circuit is used as a comparator; however, any circuit may be used as long as its function is achieved. 
     The fifteenth transistor  323  is a switch for determining whether the output of the second comparator  327  is output to the light-emitting element  325 . Similarly, the fourteenth transistor  324  is a switch for determining whether the output of the first comparator  326  is output to the light-emitting element  325 . For example, in the case of the normally white liquid crystal, a signal may be given to the comparator selecting line  304  so that the fifteenth transistor  323  is turned on when the potential of the data line  302  is positive, and the fourteenth transistor  324  is turned on when the potential of the data line  302  is negative. In addition, in the case of the normally black liquid crystal, a signal may be given to the comparator selecting line  304  so that the fifteenth transistor  323  is turned on when the potential of the data line  302  is negative, and the fourteenth transistor  324  is turned on when the potential of the data line  302  is positive. A period during which either the fifteenth transistor  323  or the fourteenth transistor  324  is turned on by a signal from the comparator selecting line  304  is regarded as a period during which the scanning line  306  is active, a data potential is loaded to the pixel circuit, and either the first comparator  326  or the second comparator  327  is initialized. The fifteenth transistor  323  and the fourteenth transistor  324  are required to have different polarity (conductivity type) from each other so that they are not turned on at the same time. A detailed operation timing of each transistor will be described later. 
     When a signal is input to the second comparator initializing line  305 , the twelfth transistor  309  is turned on, and Vref 2  is loaded to initialize the second comparator  327 . When a signal is input to the first comparator initializing line  307 , the tenth transistor  312  is turned on, and Vref 1  is loaded to initialize the first comparator  326 . It is necessary to set timings appropriately to input signals to the first comparator initializing line  307  and the second comparator initializing line  305  so that the twelfth transistor  309  and the tenth transistor  312  are not turned on at the same time. 
     The thirteenth transistor  328  and the eleventh transistor  329  write a data potential, which is written into the liquid crystal element  310 , into the initialized comparators. The thirteenth transistor  328  writes a data potential into the second comparator  327  when the data potential is positive, and the eleventh transistor  329  writes a data potential into the first comparator  326  when the data potential is negative. A timing which turns on either the thirteenth transistor  328  or the eleventh transistor  329  is required to be synchronized with a timing which makes the scanning line  306  active and turns on the ninth transistor  308  which performs writing to the liquid crystal element. Details of the timings will be described later. 
       FIGS. 4A and 4B  are diagrams each explaining an operation of the first comparator  326  shown in  FIG. 3 . Each element shown in  FIGS. 4A and 4B  corresponds to the element shown in  FIG. 3 . A first switch  401 , a second switch  403 , and a third switch  405  show operations of the eleventh transistor  329 , the tenth transistor  312 , and the first transistor  317 , respectively. In addition, a reference potential  402  corresponds to Vref 1 . A first inverter  406  corresponds to the inverter including the second transistor  319  and the third transistor  320 . Further, a second inverter  407  corresponds to the inverter including the fourth transistor  321  and the fifth transistor  322 . An input voltage Vin indicates an input voltage from the data line  302 . 
     First, as shown in  FIG. 4A , the second switch  403  and the third switch  405  are turned on, and the first switch  401  is turned off, whereby the comparator is initialized. At this time, a potential of a logic threshold value (hereinafter denoted by Vth_inv) of the first inverter  406  is applied to a point b. When the reference potential  402  is Vref 1 , Vref 1  is applied to a point a. Thus, a voltage of Vref 1 −Vth_inv is retained between both electrodes of a capacitor  404 . It is to be noted that the logic threshold value of the inverter is defined as a voltage which makes an input voltage and an output voltage of the inverter equal to each other. The comparator is required to be initialized because the logic threshold value of the inverter possibly varies from one pixel to another pixel due to variation in size or threshold value of the transistor included in the inverter. Initialization of the comparator can eliminate an adverse effect due to the logic threshold value of the inverter, which varies from one pixel to another pixel. 
     Next, as shown in  FIG. 4B , the second switch  403  and the third switch  405  are turned off, and the first switch  401  is turned on. At this time, a potential at the point a is equivalent to an input voltage Vin. A potential at the point b falls from the potential at the point a by the voltage retained between the both electrodes of the capacitor  404  and becomes Vin−(Vref 1 −Vth_inv), which is represented by the following expression (1). 
         V in+ V th_inv− V ref1   (1)
 
     It is found that a potential at a point c that is output of the first inverter  406  (referred to as Vc) is determined depending on which of the potential at the point b (referred to as Vb) represented by the expression (1) and Vth_inv is high. In the case of Vb&gt;Vth_inv, 0 V is output as Vc. At this time, logic is further inverted by the second inverter  407  so that a power source voltage VDD is output as an output voltage Vout. On the other hand, in the case of Vb&lt;Vth_inv, VDD is output as Vc, and logic is further inverted by the second inverter  407  so that 0 V is output as Vout. As described above, it is found that VDD is output in the case of Vin&gt;Vref 1 , and 0 V is output in the case of Vin&lt;Vref 1 . 
       FIG. 4C  is a diagram explaining an operation of the second comparator  327 . The operation of the second comparator  327  is different from that of the first comparator  326  in that the inverter  407  is not provided, and the potential at the point c is equivalent to a value of the output voltage Vout. Each element shown in  FIG. 4C  corresponds to the element shown in  FIG. 3 . A first switch  401 , a second switch  403 , and a third switch  405  show operations of the thirteenth transistor  328 , the twelfth transistor  309 , and the sixth transistor  313 , respectively. In addition, a reference potential  402  corresponds to Vref 2 . An inverter  406  corresponds to the inverter including the seventh transistor  315  and the eighth transistor  316 . An input voltage Vin indicates an input voltage from the data line  302 . 
     Initialization of the comparator is the same as that shown in  FIG. 4A , and explanation thereof will thus be omitted. It is to be noted that, in the second comparator  327 , as the reference potential  402 , the potential Vref 1  input from the first reference potential line  301  is replaced with the potential Vref  2  input from the second reference potential line  303 . When the comparator is initialized, a voltage of Vref 2 −Vth_inv is retained between both electrodes of a capacitor  404 , and Vb=Vth_inv is achieved. Here, as shown in  FIG. 4C , the second switch  403  and the third switch  405  are turned off, and the first switch  401  is turned on. At this time, a potential at a point a is equivalent to an input voltage Vin, and Vb has a value represented by the following expression (2). As described above, in the case of Vin&gt;Vref 2 , 0 V is output as Vout. On the other hand, in the case of Vin&lt;Vref 2 , VDD is output as Vout. 
         V in+ V th_inv− V ref2   (2)
 
       FIG. 5  shows a timing of each control signal of the circuit shown in  FIG. 3 . That is, each signal line of the data line  302 , the scanning line  306 , the second comparator initializing line  305 , the first comparator initializing line  307 , the second data loading line  330 , the first data loading line  331 , and the comparator selecting line  304  will be explained. A signal of the data line  302  is an AC signal, and a potential of a data signal which is written into one pixel is inverted to be positive or negative in every writing operation. In periods b and e of  FIG. 5 , the scanning line  306  has a high potential, and writing is performed to the liquid crystal element, but the potential of the data line  302  in the period b and the potential of the data line  302  in the period e are inverted. 
     First, the following will explain a case where a data signal having a positive potential is written into the liquid crystal element. A writing operation of a data signal having a positive potential into the liquid crystal element is performed in the periods a, b, and c of  FIG. 5 . When a positive potential is written from the data line  302 , a high potential is applied to the second comparator initializing line  305  in the period a to initialize the second comparator  327 , and Vref 2  is also applied to the second capacitor element  314  to initialize the second comparator  327 . Next, a high potential is applied to the second data loading line  330  in the period b, the thirteenth transistor  328  is turned on, and a data potential which is written into the liquid crystal element  310  is written into the second comparator  327 . Further, a high potential is applied to the comparator selecting line  304  in the period c, the fifteenth transistor  323  is turned on, the fourteenth transistor  324  is turned off, and the output of the second comparator  327  is transmitted to the light-emitting element  325 . 
     Subsequently, the following will explain a case where a data signal having a negative potential is written into the liquid crystal element. A writing operation of a data signal having a negative potential into the liquid crystal element is performed in the periods d, e, and f of  FIG. 5 . When a negative potential is written from the data line  302 , a high potential is applied to the first comparator initializing line  307  in the period d to initialize the first comparator  326 , and Vref 1  is applied to the capacitor element  318  to initialize the first comparator  326 . Next, a high potential is applied to the first data loading line  331  in the period e, the eleventh transistor  329  is turned on, and a data potential which is written into the liquid crystal element is written into the first comparator  326 . Further, a low potential is applied to the comparator selecting line  304  in the period f, the fourteenth transistor  324  is turned on, the fifteenth transistor  323  is turned off, and the output of the first comparator  326  is transmitted to the light-emitting element  325 . 
     According to this embodiment mode, when black is displayed in a display device using a liquid crystal, a backlight is made not to emit light; therefore, light leakage can be eliminated and the contrast can be improved. In addition, according to this embodiment mode, a backlight is arranged in each pixel and a function to control a lighting state of a light-emitting element based on each pixel is provided in a pixel circuit; therefore, a malfunction can be avoided, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. Moreover, a backlight in a portion where lighting is not needed can be individually turned off; therefore, power savings can be effectively achieved. 
     Embodiment Mode 2 
     This embodiment mode will explain a configuration of a pixel circuit which is different from that of the above embodiment mode.  FIG. 6  schematically shows a circuit in which one comparator is removed from the circuit of Embodiment Mode 1. In the circuit shown in  FIG. 6 , a data line  601  and a scanning line  602  are provided. In addition, a transistor  603  which performs writing to a liquid crystal element  604  and a counter electrode  605  are provided. Further, a light-emitting element  612  used as a backlight is provided. A first switch  613  which writes a data signal into a comparator  608  is provided. A first reference potential  606  and a second reference potential  607  which are compared with a data potential are provided. In addition, a second switch  609  and a third switch  610 , by which it is determined which of positive and negative outputs is output from the comparator to the light-emitting element, are provided. 
     The components in the circuit shown in  FIG. 6  are connected as follows. A first terminal of the transistor  603  is connected to the data line  601 , and a first electrode of the liquid crystal element  604  is connected to a second terminal of the transistor  603 . A gate terminal of the transistor  603  is connected to the scanning line  602 . A first terminal of the first switch  613  is connected to the data line  601 . A second electrode of the liquid crystal element  604  is connected to the counter electrode  605 . The comparator  608  is connected to a second terminal of the first switch  613 . The second switch  609  and an inverter  611  are connected to output of the comparator  608 . The output of the comparator  608  is connected to the light-emitting element  612  through the second switch  609 , and the third switch  610  is connected to output of the inverter  611  and the light-emitting element  612 . The first reference potential  606  and the second reference potential  607  which are compared with a data potential are connected to the comparator  608 . 
     Next, an operation of the circuit will be briefly explained. A data potential written into the liquid crystal element  604  is also input to the comparator  608  through the first switch  613 . The comparator  608  compares the data potential input from the data line  601  with a voltage of the first reference potential  606  or the second reference potential  607 . When a negative voltage is applied to the liquid crystal element  604  in a normally white liquid crystal, a value of the data potential input from the data line  601  is compared with a value of the first reference potential  606 . At that time, the third switch  610  is turned on, and the output of the comparator  608 , in which logic is inverted by the inverter  611 , is transmitted to the light-emitting element  612 . If the potential of the data line  601  is higher than the first reference potential  606 , a power source voltage VDD is output to the light-emitting element  612 . On the other hand, if the potential of the data line  601  is lower than the first reference potential  606 , 0 V is output. The comparator  608  compares the second reference potential  607  with the potential of the data line  601  when a positive voltage is applied to the liquid crystal element  604  in the normally white liquid crystal. At that time, the second switch  609  is turned on, and the output of the comparator is transmitted to the light-emitting element  612 . If the potential of the data line  601  is lower than the second reference potential  607 , a power source voltage VDD is output to the light-emitting element  612 . On the other hand, if the potential of the data line  601  is higher than the second reference potential  607 , 0 V is output. 
     It is to be noted that the light-emitting element  612  may be an LED, an organic EL element, or the like. That is, any light-emitting element which can be controlled by a voltage, a current, or the like may be used. 
       FIG. 7  shows an example of a detailed circuit diagram of the pixel shown in  FIG. 6 . In the circuit shown in  FIG. 7 , a data line  702  and a scanning line  704  are provided. A first transistor  709  which performs writing to a liquid crystal element  711  and a counter electrode  712  are provided. The circuit shown in  FIG. 7  has a feature in that one CMOS chopper inverter circuit is used as a comparator. In other words, a comparator  722  is used. The comparator  722  includes a capacitor element  726 ; a second transistor  714 ; and a first inverter including a third transistor  715  and a fourth transistor  716 . As a wiring, a first reference potential line  701 , a second reference potential line  703 , and the data line  702  are provided. In addition, the scanning line  704 , a comparator initializing line  705 , a second reference potential selecting line  704 , a first reference potential selecting line  707 , a first light-emitting element driving signal selecting line  708 , a second light-emitting element driving signal selecting line  725 , and a data-loading line  724  are provided. A fifth transistor  710 , a sixth transistor  723 , and a seventh transistor  713  which perform writing to the comparator  722  are provided. An eighth transistor  719  and a ninth transistor  720  which transmit output of the comparator to a light-emitting element are provided. In addition, a second inverter including a tenth transistor  717  and an eleventh transistor  718  is provided. Further, a light-emitting element  721  used as a backlight is provided. It is to be noted that the tenth transistor  717  is a p-channel transistor and the eleventh transistor  718  is an n-channel transistor. 
     The components in the circuit shown in  FIG. 7  are connected as follows. A first terminal of the first transistor  709  and a first terminal of the sixth transistor  723  are connected to the data line  702 . A second terminal of the first transistor  709  is connected to a first electrode of the liquid crystal element  711 , and a gate terminal of the first transistor  709  is connected to the scanning line  704 . A second electrode of the liquid crystal element  711  is connected to the counter electrode  712 . A first terminal of the seventh transistor  713  is connected to the first reference potential line  701 . A first terminal of the fifth transistor  710  is connected to the second reference potential line  703 . A second terminal of the seventh transistor  713 , a second terminal of the sixth transistor  723 , and a second terminal of the fifth transistor  710  are connected to a first electrode of the capacitor element  726  in the comparator  722 . A gate terminal of the sixth transistor  723  is connected to the data loading line  724 , and a gate terminal of the first transistor  709  is connected to the scanning line  704 . A gate terminal of the fifth transistor  710  is connected to the second reference potential selecting line  706 , and a gate terminal of the seventh transistor  713  is connected to the first reference potential selecting line  707 . The first inverter includes the third transistor  715  and the fourth transistor  716 . Gate terminals of these two transistors are connected to a second electrode of the capacitor element  726  and a first terminal of the second transistor  714 . A second terminal of the third transistor  715  and a second terminal of the fourth transistor  716  are connected to a second terminal of the second transistor  714 , a first terminal of the eighth transistor  719 , and gate terminals of the tenth transistor  717  and the eleventh transistor  718  included in the second inverter. A second terminal of the tenth transistor  717  and a second terminal of the eleventh transistor  718  are connected to a first terminal of the ninth transistor  720 . A gate terminal of the second transistor  714  is connected to the comparator initializing line  705 . A gate terminal of the eighth transistor  719  is connected to the second light-emitting element driving signal selecting line  725 , and a gate terminal of the ninth transistor  720  is connected to the first light-emitting element driving signal selecting line  708 . A second terminal of the eighth transistor  719  and a second terminal of the ninth transistor  720  are connected to a first electrode of the light-emitting element  721 . 
     Then, an operation of the circuit shown in  FIG. 7  will be explained. When the second reference potential selecting line  706  has a high potential, the fifth transistor  710  is turned on and transmits a potential Vref 2  of the second reference potential line  703  to the comparator  722 . In addition, when the first reference potential selecting line  707  has a high potential, the seventh transistor  713  is turned on and transmits a potential Vref 1  of the first reference potential line  701  to the comparator  722 . When the comparator initializing line  705  has a high potential, the second transistor  714  is turned on, and the comparator  722  is initialized. An operation in the comparator  722  is similar to that of the comparator  326 . 
     When the data loading line  724  has a high potential, the sixth transistor  723  is turned on and writes a data potential which is written into the liquid crystal element  711  also into the initialized comparator  722 . When the second light-emitting element driving signal selecting line  725  has a high potential, the eighth transistor  719  is turned on and transmits output of the comparator  722 , which is obtained when Vref 2  is compared with a potential of the data line  702 , to the light-emitting element  721 . When the first light-emitting element driving signal selecting line  708  has a high potential, the ninth transistor  720  is turned on and transmits output of the comparator  722 , which is obtained when Vref 1  is compared with a potential of the data line  702 , to the light-emitting element  721 . It is to be noted that, in this embodiment mode, a CMOS chopper inverter circuit is used as a comparator; however, any circuit may be used as long as its function is achieved. 
       FIG. 8  shows a timing of each control signal of the circuit shown in  FIG. 7 . That is, each signal line of the data line  702 , the scanning line  704 , the comparator initializing line  705 , the second reference potential selecting line  706 , the first reference potential selecting line  707 , the data loading line  724 , the second light-emitting element driving signal selecting line  725 , and the first light-emitting element driving signal selecting line  708  will be explained. A signal of the data line  702  is an AC signal, and a potential of a data signal which is written into one pixel is inverted to be positive or negative in every writing operation. In periods b and e of  FIG. 8 , the scanning line  704  has a high potential, and writing is performed to the liquid crystal element, but the potential of the data line  702  in the period b and the potential of the data line  702  in the period e are inverted. 
     First, the following will explain a case where a data signal having a positive potential is written into the liquid crystal element. A writing operation of a data signal having a positive potential into the liquid crystal element is performed in the periods a, b, and c of  FIG. 8 . When a positive potential is written from the data line  702 , a high potential is applied to the comparator initializing line  705  in the period a to initialize the comparator  722 . At the same time, a high potential is applied to the second reference potential selecting line  706 , and Vref 2  is applied to the capacitor element  726 . Next, a high potential is applied to the data loading line  724  in the period b, the sixth transistor  723  is turned on, and a data potential which is written into the liquid crystal element  711  is also written into the comparator  722 . Further, a high potential is applied to the second light-emitting element driving signal selecting line  725  in the period c, the eighth transistor  719  is turned on, and output of the comparator  722  is transmitted to the light-emitting element  721 . 
     Subsequently, the following will explain a case where a data signal having a negative potential is written into the liquid crystal element. A writing operation of a data signal having a negative potential into the liquid crystal element is performed in the periods d, and f of  FIG. 8 . When a negative potential is written from the data line  702 , a high potential is applied to the comparator initializing line  705  in the period d to initialize the comparator  722 . At the same time, a high potential is applied to the first reference potential selecting line  707 , and Vref 1  is applied to the capacitor element  726 . Next, a high potential is applied to the data loading line  724  in the period e, the sixth transistor  723  is turned on, and a data potential which is written into the liquid crystal element  711  is written into the comparator  722 . Further, a high potential is applied to the first light-emitting element driving signal selecting line  708  in the period f, the ninth transistor  720  is turned on, and the output of the comparator  722  is transmitted to the light-emitting element  721  through the tenth transistor  717  and the eleventh transistor  718  included in the second inverter. 
     In the periods a, b, d, and e of  FIG. 8 , both the first light-emitting element driving signal selecting line  708  and the second light-emitting element driving signal selecting line  725  have low potentials. This is because there is a period during which logic is indefinite after the comparator is initialized and until the output is determined. Since the light-emitting element is always turned off in this period, the period is preferably suppressed to be so short that it cannot be recognized by human eyes. 
     According to this embodiment mode, when black is displayed in a display device using a liquid crystal, a backlight is made not to emit light; therefore, light leakage can be eliminated and the contrast can be improved. In addition, according to this embodiment mode, a backlight is arranged in each pixel and a function to control a lighting state of a light-emitting element based on each pixel is provided in a pixel circuit; therefore, a malfunction can be avoided, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. Moreover, a backlight in a portion where lighting is not needed can be individually turned off; therefore, power savings can be effectively achieved. Further, in this embodiment mode, one comparator is removed from the circuit shown in Embodiment Mode 1, and thus, the number of transistors included in the circuit can be reduced, whereby an advantage such as a higher aperture ratio of the pixel can also be obtained. 
     Embodiment Mode 3 
       FIG. 11A  shows one mode of a display device of this embodiment mode. Over a glass substrate  1101 , a base film  1115  is formed, and a liquid crystal element driving transistor  1113  and a light-emitting element driving transistor  1114  are formed thereover. The liquid crystal element driving transistor  1113  includes a first impurity region  1117   a  and a second impurity region  1117   b.  A first channel region  1116   a  is formed between the first impurity region  1117   a  and the second impurity region  1117   b . A gate insulating film  1118  is formed over the first impurity region  1117   a,  the second impurity region  1117   b,  and the first channel region  1116   a.  A first gate electrode  1119   a  is formed over the gate insulating film  1118 . Similarly, the light-emitting element driving transistor  1114  includes a third impurity region  1117   c  and a fourth impurity region  1117   d.  A second channel region  1116   b  is formed between the third impurity region  1117   c  and the fourth impurity region  1117   d.  The gate insulating film  1118  is formed over the third impurity region  1117   c,  the fourth impurity region  1117   d,  and the second channel region  1116   b.  A second gate electrode  1119   b  is formed over the gate insulating film  1118 . Over the liquid crystal element driving transistor  1113  and the light-emitting element driving transistor  1114 , a first interlayer film  1109  is formed, and a first electrode  1111   a,  a second electrode  1111   b,  a third electrode  1111   c,  a fourth electrode  1111   d,  and a first wiring  1120  are formed. An anisotropic conductive particle  1112  is provided over each of the third electrode  1111   c  and the first wiring  1120  and a light-emitting diode  1110  is provided thereover. When the light-emitting diodes  1110  is a white light-emitting diode, color display can be performed by combining the light-emitting diode  1110  and a color filter. Alternatively, the light-emitting diodes  1110  emitting light of red (R), green (G), and blue (B) may be each provided in a pixel to perform color display. Also in this case, the light-emitting diode may be combined with a color filter to improve color purity. 
     Further, a second interlayer film  1108  is formed over the first interlayer film  1109 . A pixel electrode  1107  is formed over the second interlayer film  1108 , and a liquid crystal layer  1306  is provided thereover. An alignment film  1121  is provided over the liquid crystal layer  1306 , a counter electrode  1105  is formed thereover, and a light-shielding layer  1103  and a color filter  1104  are provided thereover. A glass substrate  1102  that is a counter substrate is provided over the light-shielding layer  1103  and the color filter  1104 . A polymer dispersed liquid crystal is preferably used for the liquid crystal layer  1306 . As shown in  FIG. 11B , the polymer dispersed liquid crystal is a liquid crystal in which liquid crystals  1322  are dispersed in a polymer  1321 . When an electric field is applied by dispersion of the microparticle liquid crystals  1322  in the polymer  1321  as shown in  FIG. 11C , the liquid crystals are aligned in the polymer  1321 , and an alignment film is not necessary. Further, a polarizing plate is not necessary, either. Therefore, the absorption of light can be significantly reduced, and a bright screen can be obtained. 
     The components in  FIG. 11A  are connected as follows. In the liquid crystal element driving transistor  1113 , the first impurity region  1117   a  is connected to the first electrode  1111   a,  and the second impurity region  1117   b  is connected to the second electrode  1111   b.  The second electrode  1111   b  is connected to the pixel electrode  1107 . In the light-emitting element driving transistor  1114 , the third impurity region  1117   c  is connected to the third electrode  1111   c,  and the fourth impurity region  1117   d  is connected to the fourth electrode  1111   d.  The third electrode  1111   c  is connected to a first electrode  1122  of the light-emitting diode  1110  through the anisotropic conductive particle  1112 . A second electrode  1123  of the light-emitting diode  1110  is connected to the first wiring  1120  through the anisotropic conductive particle  1112 . It is to be noted that other substances such as solder may be used instead of the anisotropic conductive particle  1112  without limitation to the anisotropic conductive particle, as long as the substances can electrically connect the components. 
       FIG. 12  shows a block diagram of the display device of this embodiment mode. The display device shown in  FIG. 12  includes a pixel portion  1201  including a plurality of pixels each having a light-emitting element, a scanning line driving circuit  1202  selecting each pixel, a data line driving circuit  1203  controlling input of a data signal to a selected pixel, and a comparator driving circuit  1204  controlling a comparator in each pixel over a substrate  1200 . 
     According to the display device of this embodiment mode, when black is displayed in a display device using a liquid crystal, a backlight is made not to emit light; therefore, light leakage can be eliminated and the contrast can be improved. In addition, a backlight is arranged in each pixel and a function to control a lighting state of a light-emitting element based on each pixel is provided in a pixel circuit; therefore, a malfunction can be avoided, in which, when a backlight is turned off for a pixel displaying black, all other pixels also display black. Moreover, a backlight in a portion where lighting is not needed can be individually turned off; therefore, power savings can be effectively achieved. 
     Embodiment Mode 4 
     Electronic devices using the display device of the present invention include a camera such as a video camera or a digital camera, a goggle type display (head mounted display), a navigation system, an audio reproducing device (such as a car audio stereo or an audio component set), a laptop personal computer, a game machine, a portable information terminal (such as a mobile computer, a mobile phone, a portable game machine, or an electronic book), an image reproducing device provided with a storage medium (specifically, a device for reproducing a storage medium such as a digital versatile disc (DVD) and having a display for displaying the reproduced image), and the like. Specific examples of such electronic devices are shown in  FIGS. 9A to 9H . 
       FIG. 9A  shows a television device, which includes a housing  901 , a supporting base  902 , a display portion  903 , speaker portions  904 , a video input terminal  905 , and the like. The present invention can be applied to a display device which forms the display portion  903 . In accordance with the present invention, an image with the improved contrast can be provided. This television device can be used for receiving TV broadcast as well as for a monitor of a video game machine, computer, or the like. 
       FIG. 9B  shows a digital still camera, which includes a main body  906 , a display portion  907 , an image receiving portion  908 , operation keys  909 , an external connecting port  910 , a shutter release button  911 , and the like. The present invention can be applied to a display device which forms the display portion  907 . In accordance with the present invention, an image with the improved contrast can be provided. 
       FIG. 9C  shows a laptop personal computer, which includes a main body  912 , a housing  913 , a display portion  914 , a keyboard  915 , an external connecting port  916 , a pointing device  917 , and the like. The present invention can be applied to a display device which forms the display portion  914 . In accordance with the present invention, an image with the improved contrast can be provided. 
       FIG. 9D  shows a mobile computer, which includes a main body  918 , a display portion  919 , a switch  920 , operation keys  921 , an infrared port  922 , and the like. The present invention can be applied to a display device which forms the display portion  919 . In accordance with the present invention, an image with the improved contrast can be provided. 
       FIG. 9E  shows a portable image reproducing device provided with a storage medium (specifically, a DVD player), which includes a main body  923 , a housing  924 , a display portion A  925 , a display portion B  926 , a storage medium (such as DVD) reading portion  927 , operation keys  928 , a speaker portion  929 , and the like. The display portion A  925  mainly displays image data, while the display portion B  926  mainly displays text data. The present invention can be applied to display devices which form the display portion A  925  and the display portion B  926 . In accordance with the present invention, an image with the improved contrast can be provided. It is to be noted that the image reproducing device provided with a storage medium also includes a home-use game machine and the like. 
       FIG. 9F  shows a goggle type display (head mounted display), which includes a main body  930 , a display portion  931 , an arm portion  932 , and the like. The present invention can be applied to a display device which forms the display portion  931 . In accordance with the present invention, an image with the improved contrast can be provided. 
       FIG. 9G  shows a video camera, which includes a main body  933 , a display portion  934 , a housing  935 , an external connecting port  936 , a remote control receiving portion  937 , an image receiving portion  938 , a battery  939 , an audio input portion  940 , operation keys  941 , and the like. The present invention can be applied to a display device which forms the display portion  934 . In accordance with the present invention, an image with the improved contrast can be provided. 
       FIG. 9H  shows a mobile phone, which includes a main body  942 , a housing  943 , a display portion  944 , an audio input portion  945 , an audio output portion  946 , operation keys  947 , an external connecting port  948 , an antenna  949 , and the like. The present invention can be applied to a display device which forms the display portion  944 . It is to be noted that current consumption of the mobile phone can be suppressed by display of white text on a black background in the display portion  944 . In accordance with the present invention, an image with the improved contrast can be provided. 
     When a light-emitting material with high luminance is used, the display device can be applied to a front projector or a rear projector in which light including output image data is magnified and projected with a lens or the like. 
     In addition, since a light-emitting device consumes power in its light-emitting portion, it is desirable to display data so that the light-emitting portion is as small as possible. Thus, when a light-emitting device is used in a display portion of a portable information terminal such as a mobile phone or an audio reproducing device, which mainly displays text data, it is desirable to drive the device so that text data is formed by using a light-emitting portion with a non-light-emitting portion on the background. 
     The applicable range of the present invention is so wide that the present invention can be applied to electronic devices in various fields. In addition, the electronic device of this embodiment mode can employ a display device with any of the configurations shown in Embodiment Modes 1 and 2. 
     This application is based on Japanese Patent Application serial No. 2006-112533 filed in Japan Patent Office on Apr. 14, 2006, the entire contents of which are hereby incorporated by reference.