Patent Publication Number: US-7714816-B2

Title: Display device, display module, electronic apparatus and driving method of the display device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an active matrix display device and a driving method thereof. The invention particularly relates to a display device having a switching element such as a thin film transistor (hereinafter referred to as a TFT) and a light-emitting element in each pixel, and a driving method thereof. In addition, the invention relates to an electronic apparatus using the display device and a driving method thereof. 
   2. Description of the Related Art 
   In recent years, technology to form a TFT has greatly progressed, and application development for an active matrix display device has been promoted. Particularly, since the field effect mobility (also called mobility) of a TFT using a polysilicon film as an active layer is higher than that of a TFT using a conventional amorphous silicon film, high speed operation is possible. Therefore, by using a driver circuit formed by using TFTs over the same substrate as pixels, control of the pixels can be performed. In a display device in which various circuits include TFTs over the same substrate as the pixels, various advantages such as reduction of manufacturing cost, downsizing, increase of yield, reduction of throughput are obtained. 
   Research has been activated on an active matrix EL display device having an electroluminescence element (hereinafter referred to as an EL element) which is a light-emitting element as a display element included in each pixel of a display device. An EL display device is also called an organic EL display (OELD: Organic EL Display) or an organic light-emitting diode (OLED: Organic Light-Emitting Diode). 
   In general, since light emission luminance of an EL element has a proportional relation with a current value flowing to the EL element, an EL display device using the EL element as a display element controls light emission luminance with the current value. As a method of a gray scale expression, in a configuration in which the EL element and the TFT (referred to as a driving TFT) are connected in series between two power supply lines, there is a method in which the driving TFT is operated in a saturation region, and a voltage between a gate and source of the driving TFT is changed to control the current value flowing to the EL element. In addition, there is a driving method in which the current value flowing to the EL element is constant, and light emission luminance is controlled by the time when a current flows to the EL element in a predetermined time to express a gray scale (see the following Patent Document 1). 
   [Patent Document 1] Japanese Patent Laid-Open No. 2001-5426 
   SUMMARY OF THE INVENTION 
   In a configuration in which an EL element (a light-emitting element) and a driving TFT (a driving transistor) are connected in series between two power supply lines that are held to have a predetermined potential difference, when the light-emitting element deteriorates, an operating point between the driving transistor and the light-emitting element has a possibility to become a linear region of the driving transistor. Therefore, it has been necessary to lower a potential of an electrode (hereinafter also called a counter electrode) not connected to the driving transistor between two electrodes of the light-emitting element so as not to operate the driving transistor in the linear region. Thus, a potential difference between a source of the driving transistor and the counter electrode had to be increased. 
   Description is made on the reason why the aforementioned potential difference between the source of the driving transistor and the counter electrode is required to be increased with reference to  FIGS. 1 and 2 . 
     FIG. 1  shows a pixel configuration of a basic organic EL display. In  FIG. 1 , reference numerals  101  and  102  denote TFTs,  103  denotes a capacitor,  104  denotes a light-emitting element,  105  denotes a counter electrode of the light-emitting element  104 ,  106  denotes a power supply line,  107  denotes a source signal line,  108  denotes a gate signal line, and  109  denotes a node Vm. The TFT  101  corresponds to the aforementioned driving transistor, and the TFT  101  and the light-emitting element  104  are connected in series between the power supply line  106  and the counter electrode  105 . 
     FIG. 2  shows a diagram showing a relation of operating points between the TFT  101  and the light-emitting element  104  of the pixel configuration of  FIG. 1 . In  FIG. 2 , reference numeral  201  shows a property of the TFT  101 ,  202  shows a property of the light-emitting element  104 ,  203  and  204  show properties of the light-emitting element  104  after having deteriorated,  205  shows an operating point between the  201  and the  202 ,  206  shows an operating point between the  201  and the  203 ,  207  shows an operating point between the  201  and the  204 ,  208  shows a pinch-off point,  209  shows a pinch-off curve,  210  shows a potential of the counter electrode  105 ,  211  shows a potential of the power supply line  106 ,  212  shows a current flowing between a source and drain of the TFT  101 ,  213  shows a current flowing to the light-emitting element  104 ,  214  shows a voltage between the source and drain of the TFT  101 , and  215  shows a voltage between a pair of electrodes of the light-emitting element  104 . 
     FIG. 2  shows changes of the operating point between the TFT  101  and the light-emitting element  104  when the light-emitting element  104  deteriorates under the condition that the voltage between the gate and source of the TFT  101  is set to be an arbitrary constant voltage. When the light-emitting element  104  deteriorates, the property of the light-emitting element  104  changes from the property  202  into the property  203  and the property  204 . In addition, the operating point is also changed from the operating point  205  into the operating point  206  and the operating point  207 . When the operating point changes from the saturation region to the linear region due to the deterioration of the light-emitting element  104 , the current value flowing to the light-emitting element  104  decreases sharply, thereby luminance of the light-emitting element  104  decreases sharply. Therefore, in order to prevent the operating point from being in the linear region due to the deterioration of the light-emitting element  104 , the potential difference between the counter electrode  105  and the power supply line  106  is required to be increased in advance. 
   As a method to increase the potential difference between the counter electrode  105  and the power supply line  106 , in the case of using a P-channel TFT as the driving transistor (TFT  101 ) as shown in  FIG. 1 , there is a method to lower the potential of the counter electrode  105 . This is because when the potential of the power supply line  106  increases, the potential difference between the gate and source of the driving TFT is changed so that the luminance control becomes difficult to perform. 
   In this manner, although the current value flowing to the light-emitting element and the luminance are hardly changed when the potential difference between the counter electrode  105  and the power supply line  106  is increased, only a voltage applied is increased; therefore, there is a problem that power consumption is increased. 
   In the invention, the aforementioned defect is solved and the operating point between the light-emitting element and the driving transistor is set to be close to the pinch-off point in accordance with the deterioration of the light-emitting element, so that the potential of the counter electrode  105  is not changed more than required and the low power consumption of the display device is realized. 
   A display device of the invention includes: a current source; a first wiring; a second wiring; a first light-emitting element; and a first transistor, one of a source and drain of the first transistor is electrically connected to the current source through the first wiring, and the other of the source and drain of the first transistor and a gate of the first transistor are electrically connected to the second wiring and one electrode of the first light-emitting element. 
   A display device of the invention includes: a current source; a first wiring; a second wiring; a first light-emitting element; and a first transistor, one of a source and drain of the first transistor and a gate of the first transistor are electrically connected to the current source through the first wiring, and the other of the source and drain of the first transistor is electrically connected to one electrode of the first light-emitting element and the second wiring. 
   A display device of the invention includes: a current source; a first wiring electrically connected to a first sampling circuit; a second wiring electrically connected to a second sampling circuit; a first light-emitting element; and a first transistor, one of a source and drain of the first transistor is electrically connected to the current source through the first wiring, and the other of the source and drain of the first transistor and a gate of the first transistor are electrically connected to the second wiring and one electrode of the first light-emitting element. 
   A display device of the invention includes: a current source; a first wiring electrically connected to a digital-analog converter circuit; a second wiring electrically connected to the digital-analog converter circuit; a first light-emitting element; and a first transistor, one of a source and drain of the first transistor is electrically connected to the current source through the first wiring, and the other of the source and drain of the first transistor and a gate of the first transistor are electrically connected to the second wiring and one electrode of the first light-emitting element. 
   A display device of the invention includes: a current source; a first wiring; a second wiring; a third wiring; a first light-emitting element; a second light-emitting element; a first transistor; a second transistor; a first sampling circuit holding a potential of the first wiring for a certain period and supplying the potential to the third wiring; a second sampling circuit holding a potential of the second wiring for a certain period; a digital-analog converter circuit in which a minimum output potential and a maximum output potential are determined by the potential held in the first sampling circuit and the potential held in the second sampling circuit; and a circuit supplying a signal in accordance with an output of the digital-analog converter circuit to a gate of the second transistor, one of a source and drain of the first transistor is electrically connected to the current source through the first wiring, the other of the source and drain of the first transistor and a gate of the first transistor are electrically connected to the second wiring and one electrode of the first light-emitting element, one of a source and drain of the second transistor is electrically connected to the third wiring, and the other of the source and drain of the second transistor is electrically connected to one electrode of the second light-emitting element. 
   A display device of the invention includes: a current source; a first wiring; a second wiring; a third wiring; a fourth wiring; a fifth wiring; a first light-emitting element; a second light-emitting element; a first transistor; a second transistor; a third transistor; a first sampling circuit holding a potential of the first wiring for a certain period and supplying the potential to the third wiring; a second sampling circuit holding a potential of the second wiring for a certain period; a digital-analog converter circuit in which a minimum output potential and a maximum output potential are determined by the potential held in the first sampling circuit and the potential held in the second sampling circuit; a source driver supplying a signal in accordance with an output of the digital-analog converter circuit to the fourth wiring; and a gate driver supplying a selection signal to the fifth wiring, one of a source and drain of the first transistor is electrically connected to the current source through the first wiring, the other of the source and drain of the first transistor and a gate of the first transistor are electrically connected to the second wiring and one electrode of the first light-emitting element, one of a source and drain of the second transistor is electrically connected to the third wiring, the other of the source and drain of the second transistor is electrically connected to one electrode of the second light-emitting element, one of a source and drain of the third transistor is electrically connected to the fourth wiring, the other of the source and drain of the third transistor is electrically connected to a gate of the second transistor, and a gate of the third transistor is electrically connected to the fifth wiring. 
   In the display device of the invention, the potential of the signal in accordance with the output of the digital-analog converter circuit is smaller than the potential of the first wiring. 
   In the display device of the invention, the first transistor and the second transistor are P-channel transistors. 
   In the display device of the invention, a channel width and a channel length of the first transistor are the same as a channel width and a channel length of the second transistor. 
   In the display device of the invention, the first transistor and the second transistor are formed over the same substrate as the second light-emitting element. 
   In the display device of the invention, an operating point of the first transistor and the first light-emitting element and an operating point of the second transistor and the second light-emitting element are a saturation region of the first transistor and a saturation region of the second transistor respectively. 
   In the display device of the invention, a structure of first light-emitting element is the same as a structure of the second light-emitting element. 
   In the display device of the invention, the first transistor is a normally off transistor. 
   More particularly, the display device of the invention has a plurality of monitor pixels, a monitor pixel power supply line, a plurality of pixels, a power supply line, and a signal line for determining a gate potential of the second transistor. Each of the plurality of monitor pixels has a first transistor and a first light-emitting element having a pair of electrodes. Each of the plurality of pixels has a second transistor and a second light-emitting element having a pair of electrodes. The monitor pixel power supply line is connected to one of a source and drain of the first transistor, the other of the source and drain of the first transistor is connected to one electrode of the first light-emitting element and a gate electrode of the first transistor. In addition, the power supply line is connected to one of a source and drain of the second transistor, the other of the source and drain of the second transistor is connected to one electrode of the second light-emitting element, and a potential from the signal line is given to a gate electrode of the second transistor. Here, each of a potential of the monitor pixel power supply line and a gate potential of the first transistor of the monitor pixel is sampled when a constant current is flowed into the first transistor and the first light-emitting element. The sampled gate potential of the first transistor is set to be a potential of the signal line included in the pixel and the sampled potential of the monitor pixel power supply line is set to be a potential of the power supply line included in the pixel; therefore, in accordance with the deterioration of the light-emitting element, an operating point between the second transistor and the second light-emitting element can always be set in a saturation region close to a pinch-off point of the second transistor so that the potential difference between the power supply line and the counter electrode can be prevented from being at excessive levels. 
   The potential sampled in the monitor pixel is described. A connecting point between the other of the source and drain of the first transistor and one electrode of the first light-emitting element of the monitor pixel is connected to the gate electrode of the first transistor. Therefore, the potential of the monitor pixel power supply line and the gate potential of the first transistor can be sampled when an operating point of the first transistor becomes close to the pinch-off point (Vds=Vgs−Vth). Vds shows a potential difference between the monitor pixel power supply line and one electrode of the first light-emitting element, Vgs shows a potential difference between the monitor pixel power supply line and the gate of the first transistor, and Vth shows a threshold voltage of the first transistor. Here, one electrode of the first light-emitting element and the gate electrode of the first transistor are connected to each other; thereby they have the same potential. That is, Vds and Vgs are the same potential. Therefore, the potential of the monitor pixel power supply line and the gate potential of the first transistor are sampled and fed back to the plurality of pixels in the display pixel region; thereby the second transistor can always be operated close to the pinch-off point when the second light-emitting element emits light at maximum luminance. That is, the gate potential of the first transistor is fed back to the signal line as a potential of the plurality of pixels in the display pixel region at maximum luminance. The potential of the monitor pixel power supply line is fed back to the signal line of the pixel and the power supply line of the pixel as a potential of the plurality of pixels in the display pixel region in non-light emission state. In this manner, the second transistor is always operated close to the pinch-off point when the second light-emitting element emits light at maximum luminance. 
   When a potential of the gate electrode of the first transistor is fed back to the signal line as the potential of the plurality of pixels at maximum luminance, in view of variation of the first transistor and the second transistor, a potential given to the signal line and the power supply line may be changed from the sampled potential so that an operating point of the second transistor becomes the saturation region side. 
   A configuration of a display device for performing display in the aforementioned driving method is described below. 
   (Structure 1) 
   The invention is a display device having a plurality of monitor pixels, a plurality of pixels, a first wiring, a second wiring, a third wiring, a fourth wiring, a fifth wiring, a sixth wiring, a constant current source, a first sampling circuit, a second sampling circuit, a digital-analog converter circuit, a source driver, and a gate driver. Each of the plurality of monitor pixels has a first P-channel transistor and a first light-emitting element having a pair of electrodes, each of the plurality of pixels has a second P-channel transistor, a third transistor, a capacitor having a pair of electrodes, and a second light-emitting element having a pair of electrodes. The constant current source is connected to the first wiring. The first wiring is connected to one of a source and drain of the first transistor. The other of the source and drain of the first transistor is connected to one electrode of the first light-emitting element, the second wiring, and a gate of the first transistor. The first wiring is connected to an input of the first sampling circuit. The second wiring is connected to an input of the second sampling circuit. An output of the first sampling circuit is connected to a power source of the digital-analog converter circuit and the fourth wiring. An output of the second sampling circuit is connected to the power source of the digital-analog converter circuit. The third wiring is connected to an input of the digital-analog converter circuit and a digital video signal is inputted thereto. An output of the digital-analog converter circuit is inputted to the source driver as a video signal. The fourth wiring is connected to one of a source and drain of the second transistor. The other of the source and drain of the second transistor is connected to one electrode of the second light-emitting element. A gate of the second transistor is connected to one electrode of the capacitor and one of a source and drain of the third transistor. The other of the source and drain of the third transistor is connected to the fifth wiring. The other electrode of the capacitor is connected to the fourth wiring. A gate of the third transistor is connected to the sixth wiring. The fifth wiring is connected to an output of the source driver while the sixth wiring is connected to an output of the gate driver. Potentials obtained by the first wiring and the second wiring are sampled with the first sampling circuit and the second sampling circuit. Each output of the first sampling circuit and the second sampling circuit is used as the power source of the digital-analog converter circuit, and a potential obtained thereby is outputted as a video signal from the fifth wiring through the source driver. 
   In addition, although a pixel configuration having two transistors and one capacitor in one pixel is shown, the invention is not limited to this. Any pixel configuration may be used as long as a driving method in which a voltage is outputted from the source driver and a potential of the power supply line is given to a source of the second transistor (driving transistor). For example, the pixel may have a configuration for correcting a threshold voltage of the driving transistor. 
   In addition, although P-channel transistors are used for the first transistor and the second transistor, an N-channel transistor may be used. In the case where an N-channel transistor is used for the first transistor, the gate of the first transistor is not required to be connected to one electrode of the first light-emitting element but connected to the first wiring. 
   Moreover, a terminal connected to the fourth wiring of the capacitor may be connected anywhere as long as the terminal is held at a constant potential during the operation of the second transistor. For example, the terminal may be connected to the other electrode of the second light-emitting element or may be connected to other wirings. 
   (Structure 2) 
   The invention is a display device having a plurality of monitor pixels, a plurality of pixels, a first wiring, a second wiring, a third wiring, a fourth wiring, a fifth wiring, a sixth wiring, a constant current source, a first sampling circuit, a second sampling circuit, a source driver, and a gate driver. Each of the plurality of monitor pixels has a first P-channel transistor and a first light-emitting element having a pair of electrodes. Each of the plurality of pixels has a second P-channel transistor, a third transistor, a capacitor having a pair of electrodes, and a second light-emitting element having a pair of electrodes. The constant current source is connected to the first wiring. The first wiring is connected to one of a source and drain of the first transistor. The other of the source and drain of the first transistor is connected to one electrode of the first light-emitting element, the second wiring, and a gate of the first transistor. The first wiring is connected to an input of the first sampling circuit. The second wiring is connected to an input of the second sampling circuit. An output of the first sampling circuit is connected to a power source of a buffer portion of the source driver, a power source of a level shifter portion of the source driver, and the fourth wiring. An output of the second sampling circuit is connected to the power source of the buffer portion of the source driver and the power source of the level shifter portion of the source driver. The buffer portion and the level shifter portion correspond to a buffer portion and a level shifter portion in the source driver just before an output to each signal line respectively. The third wiring inputs a video signal to the source driver. The fourth wiring is connected to one of a source and drain of the second transistor. The other of the source and drain of the second transistor is connected to one electrode of the second light-emitting element. A gate of the second transistor is connected to one electrode of the capacitor and one of a source and drain of the third transistor. The other of the source and drain of the third transistor is connected to the fifth wiring. The other electrode of the capacitor is connected to the fourth wiring. A gate of the third transistor is connected to the sixth wiring. The fifth wiring is connected to an output of the source driver while the sixth wiring is connected to an output of the gate driver. Potentials obtained by the first wiring and the second wiring are sampled with the first sampling circuit and the second sampling circuit. Each output of the first sampling circuit and the second sampling circuit is used for the power sources of the buffer portion of the source driver and the level shifter portion of the source driver, and a potential obtained thereby is to be outputted as a video signal from the fifth wiring. 
   In addition, although a pixel configuration having two transistors and one capacitor in one pixel is shown, the invention is not limited to this. Any pixel configuration may be used as long as a driving method that a voltage is outputted from the source driver and a potential of the power supply line is given to a source of the second transistor (driving transistor) can be employed. For example, the pixel may have a configuration for correcting a threshold voltage of the driving transistor. A means for controlling the light-emitting element to emit no light in accordance with a signal which is different from the video signal may be provided. For example, such a configuration may be used that a transistor is provided in parallel with the capacitor, charge held in the capacitor is discharged by turning ON the transistor, the driving transistor is turned OFF, and the light-emitting element is turned to emit no light. 
   In addition, although P-channel transistors are used for the first transistor and the second transistor, an N-channel transistor may be used. In the case where an N-channel transistor is used for the first transistor, a gate of the transistor is not required to be connected to one electrode of the first light-emitting element but may be connected to the first wiring. 
   Moreover, the electrode of the capacitor connected to the fourth wiring may be connected anywhere as long as the electrode is held at a constant potential during the operation of the second transistor. For example, the electrode may be connected to the other electrode of the second light-emitting element or may be connected to other wirings. 
   Note that a state in which a voltage greater than the threshold voltage is applied between the gate and source of the transistor and a current flows between the source and drain of the transistor is called that the transistor is turned ON. In addition, a state in which a voltage less than or equal to the threshold voltage is applied between the gate and source of the transistor and a current does not flow between the source and drain of the transistor is called that the transistor is turned OFF. 
   In the invention, to be connected is a synonym to be electrically connected. Therefore, in the structure of the invention, in addition to a predetermined connection relation, other elements (for example, an element such as a switch, a transistor, a diode, or a capacitor) which enable an electrical connection therebetween may be arranged. 
   Although Structure 1 and Structure 2 show examples in which transistors are used as an example of a switching element, the invention is not limited to them. Either an electrical switch or a mechanical switch may be used for the switching element as long as it can control a current. As the switching element, a diode may be used or a logic circuit in which a diode and a transistor are combined may be used. 
   In addition, in the invention, the kinds of transistors applicable as a switching element are not limited, and a TFT using a non-single crystal semiconductor film typified by amorphous silicon and polycrystalline silicon, a MOS transistor formed by using a semiconductor substrate or an SOI substrate, a junction transistor, a bipolar transistor, a transistor using an organic semiconductor or a carbon nanotube, or other transistors can be applied. In addition, the kinds of substrates over which a transistor is formed are not limited, and a single crystalline substrate, an SOI substrate, a quartz substrate, a glass substrate, a resin substrate, or the like can be freely used. 
   Moreover, in the case where a source potential of a transistor is close to a power source on a low potential side, the transistor is desired to be an N-channel transistor. On the other hand, in the case where the source potential of the transistor is close to a power source on a high potential side, the transistor is desired to be a P-channel transistor. Such a structure can be used to increase the absolute value of a voltage between the gate and source of the transistor; therefore, the transistor is easy to be operated as a switch. Note that a CMOS switching element using both an N-channel transistor and a P-channel transistor may be used. 
   The invention can be applied to a display device using as a light-emitting element, an element of which a current flowing to a pair of electrodes and luminance are in a proportional relation with each other. For example, the invention can be applied to a display device using an EL element or a light-emitting diode as a light-emitting element. 
   The potential of the monitor pixel power supply line and the gate potential of the first transistor of the monitor pixel are sampled to be set as a potential of the power supply line of the pixel and a potential of the signal line of the pixel, respectively, and in accordance with the deterioration of the light-emitting element, the operating point between the second transistor and the second light-emitting element can always be set in the saturation region close to the pinch-off point of the second transistor. Therefore, the potential difference between the power supply line and the counter electrode can be prevented from being at excessive levels. In this manner, a display device with small power consumption and a long operating life can be provided. 
   In addition, since a voltage is used as a video signal, the invention can simplify a configuration of the driver circuit which inputs a video signal into the pixel. 
   In addition, the invention is effective not only for a case where the light-emitting element deteriorates, but also a case where a voltage-current property of the light-emitting element is changed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a pixel configuration of a conventional EL display device. 
       FIG. 2  is a diagram showing a pixel property of the EL display device of  FIG. 1 . 
       FIG. 3  is a diagram showing a pixel property of the invention. 
       FIG. 4  is a circuit diagram showing a pixel configuration of the invention. 
       FIG. 5  is a circuit diagram showing a pixel configuration of the invention. 
       FIGS. 6A and 6B  are diagrams each showing Embodiment 1 of the invention. 
       FIGS. 7A to 7C  are diagrams each showing Embodiment 2 of the invention. 
       FIG. 8  is a diagram showing Embodiment 3 of the invention. 
       FIGS. 9A to 9H  are views each showing an example of an electronic apparatus of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Although the present invention will be fully described by way of embodiment modes and embodiments with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. 
   Embodiment Mode 1 
   A display device of Structure 1 is described with reference to  FIG. 4 . 
   In  FIG. 4 , reference numerals  401 ,  402  and  411  denote TFTs,  403  denotes a capacitor,  404  and  412  denote light-emitting elements,  405  and  413  denote counter electrodes,  406  denotes a source signal line,  407  denotes a gate signal line,  409  denotes a source driver,  410  denotes a gate driver,  414  and  420  denote power supply lines,  415  denotes a sampling line,  416  denotes a constant current source,  417  and  418  denote sampling circuits,  419  denotes a digital-analog converter circuit,  421  denotes a monitor pixel region,  422  denotes a display pixel region, and  423  denotes a video signal line. 
   Each pixel has the capacitor  403 , the light-emitting element  404 , the TFT  401 , and the TFT  402 . In addition, each monitor pixel has the light-emitting element  412  and the TFT  411 . 
   The constant current source  416  is connected to the power supply line  414  and an input of the sampling circuit  417 , while the power supply line  414  is connected to one of a source and drain of the TFT  411 . The other of the source and drain of the TFT  411  is connected to a gate of the TFT  411 , the sampling line  415 , and one electrode of the light-emitting element  412 . The sampling line  415  is connected to an input of the sampling circuit  418 . An output of the sampling circuit  417  is connected to a power source of the digital-analog converter circuit  419  and the power supply line  420 . An output of the sampling circuit  418  is connected to the power source of the digital-analog converter circuit  419 . The video signal  423  is a digital video signal, and inputted to the digital-analog converter circuit  419 . An output of the digital-analog converter circuit  419  is inputted to the source driver  409  as a video signal. An output of the source driver  409  is connected to the source signal line  406 . An output of the gate driver  410  is connected to the gate signal line  407 . The power supply line  420  is connected to one of a source and drain of the TFT  401 . The other of the source and drain of the TFT  401  is connected to one electrode of the light-emitting element  404 . A gate of the TFT  401  is connected to one electrode of the capacitor  403  and one of a source and drain of the TFT  402 . The other of the source and drain of the TFT  402  is connected to the source signal line  406 . A gate of the TFT  402  is connected to the gate signal line  407 . 
   A connecting point between the TFT  401  and one electrode of the light-emitting element  404  is a node Vm  408 . 
   A driving method of  FIG. 4  is described.
         In the invention, although the display pixel region  422  to be used in displaying images and the monitor pixel region  421  for sampling a potential are separately provided, the invention is not limited to such a structure.       

   First, an operation of the monitor pixel region  421  is described. 
   In the monitor pixel region  421 , a potential at which an operating point of the TFT  411  and the light-emitting element  412  becomes a boundary between the saturation region and the linear region of the TFT  411  is sampled. 
   Moreover, the boundary between the saturation region and the linear region is called a pinch-off point. In the case of a P-channel TFT, the following formula is satisfied at the pinch-off point. 
   Vds=Vgs−Vth (Vds: a voltage between a source and drain, Vgs: a voltage between a source and gate, and Vth: a threshold voltage) 
   In the saturation region, the following formula is satisfied.
 
 Vds&lt;Vgs−Vth  
 
   In the linear region, the following formula is satisfied.
 
 Vds&gt;Vgs−Vth  
 
   In the monitor pixel region  421 , the other of the source and drain of the TFT  411  is connected to the gate of the TFT  411  and one electrode of the light-emitting element  412 , and a constant current flows between the source and drain of the TFT  411  and then flows into the light-emitting element  412 ; therefore, the operating point between the TFT  411  and the light-emitting element  412  is set to be a voltage close to the pinch-off point of the TFT  411 . The constant current source  416  is set in a direction in which a current flows from the power supply line  414  to the counter electrode  413 , and the TFT  411  is a P-channel TFT; therefore, an electrode of the TFT  411  that is connected to the power supply line  414  is a source, while the other electrode of the TFT  411  that is connected to the light-emitting element  412  is a drain. Since a voltage (drain voltage) between the source and drain of the TFT  411  and a voltage (gate voltage) between the source and gate thereof are equivalent to each other by the connection relation of the aforementioned monitor pixel region  421 , in the case of normally on (in the case where a threshold voltage is positive), the TFT  411  operates in the linear region, while in the case of normally off (in the case where the threshold voltage is negative), the TFT  411  operates in the saturation region. That is, the operating point between the TFT  411  and the light-emitting element  412  is extremely close to the pinch-off point or equal to the pinch-off point. 
   A current value of the constant current source  416  is a value obtained by adding a current value corresponding to the maximum luminance at which the light-emitting element  404  of the display pixel region  422  is expected to emit light, to only the number of the light-emitting elements  412  of the monitor pixel region  421 . For example, when the current value corresponding to the maximum luminance at which the light-emitting element  404  of the display pixel region  422  is expected to emit light is set to be Ipix, in the case where the number of the light-emitting elements  412  of the monitor pixel region  421  is n, a current value flowing from the constant current source  416  is n×Ipix. 
   Next, a sampling method of a potential of the monitor pixel region  421  is described. 
   A potential of the power supply line  414  and a potential of the sampling line  415  are sampled. The potential of the power supply line  414  becomes a source potential of the TFT  411 , while the potential of the sampling line  415  becomes a gate potential and drain potential of the TFT  411 . In addition, as described above, the operating point between the TFT  411  and the light-emitting element  412  is extremely close to the pinch-off point of the TFT  411  or equal to the pinch-off point. 
   The power supply line  414  is connected to the input of the sampling circuit  417 . The potential of the power supply line  414  is sampled in the sampling circuit  417 , and the sampling circuit  417  outputs a potential corresponding to the sampled potential. Moreover, any configuration may be used for this sampling circuit  417 , and the sampling circuit  417  is not limited to a particular configuration. In addition, the sampling circuit  417  is not always required and a configuration without the sampling circuit  417  may be used. 
   The sampling line  415  is connected to the input of the sampling circuit  418 . The potential of the sampling line  415  is sampled in the sampling circuit  418 , and the sampling circuit  418  outputs a potential corresponding to the sampled potential. Moreover, any configuration may be used for this sampling circuit  418  and the sampling circuit  418  is not limited to a particular configuration. In addition, the sampling circuit  418  is not always required and a configuration without the sampling circuit  418  may be used. 
   The outputs of the sampling circuit  417  and the sampling circuit  418  are connected to the power source of the digital-analog converter circuit  419 . By using the outputs of the sampling circuit  417  and the sampling circuit  418  for the power source of the digital-analog converter circuit  419 , a potential between an output potential of the sampling circuit  417  and an output potential of the sampling circuit  418  can be outputted by the digital-analog converter circuit  419 . In addition, the potential outputted by the digital-analog converter circuit  419  is controlled by the video signal  423  connected to an input of the digital-analog converter circuit  419 . A general circuit configuration may be used for the digital-analog converter circuit  419 . In addition, the digital-analog converter circuit  419  of the invention is not limited to the digital-analog converter circuit shown in this embodiment mode, and any configuration may be used as long as such a circuit is used that an output potential is determined in accordance with the outputs of the sampling circuit  417  and the sampling circuit  418 . 
   Next, operations of the display pixel region  422 , the source driver  409 , and the gate driver  410  are described. 
   The output of the sampling circuit  417  is connected to the power supply line  420  to output the potential of the power supply line  414  of the monitor pixel region  421 . Here, a configuration of the source driver  409  is not limited, and such a circuit configuration that an output potential of the digital-analog converter circuit  419  is outputted to the source signal line  406  may be used. In addition, a circuit configuration of the gate driver  410  is not limited, and such a configuration that scans the gate signal line  407  may be used. 
   In the display pixel region  422 , a current is supplied from the power supply line  420  to the light-emitting element  404  through the TFT  401 . This current is controlled by a voltage (gate voltage) between the gate and source of the TFT  401 , and a gate potential of the TFT  401  is supplied from the source signal line  406  through the TFT  402  which is selected by the gate signal line  407  to be turned ON. In addition, since the potential supplied by this source signal line  406  is held in the capacitor  403 , the gate potential of the TFT  401  is held for a while even when the TFT  402  selected by the gate signal line  407  to be turned ON becomes OFF. 
   Here, the potential supplied by the source signal line  406  is a potential having a value between the potential of the power supply line  414  and the potential of the sampling line  415  of the monitor pixel region  421 . A potential supplied by the power supply line  420  is the potential of the power supply line  414  of the monitor pixel region  421 . In addition, the potential of the power supply line  414  and the potential of the sampling line  415  of the monitor pixel region  421  have a potential relation which allows the light-emitting element  412  to emit light at maximum luminance, and the operating point at maximum luminance is close to the pinch-off point of the TFT  411 . 
   An operating point between the TFT  401  and the light-emitting element  404  is close to the pinch-off point when a potential of the source signal line  406  is the potential of the sampling line  415 , and even when the potential of the source signal line  406  approaches the potential of the power supply line  414 , the operating point moves from the pinch-off point to the more saturation region side. This is described with reference to  FIG. 3 . 
   Reference numeral  301  denotes a property of the TFT  401 ,  302  denotes a property of the TFT  401  with increased Vgs,  303  denotes a property of the TFT  401  with further increased Vgs,  304  denotes a property of the light-emitting element  404 ,  305  denotes an operating point between the TFT  401  with the property  301  and the light-emitting element  404  with the property  304 ,  306  denotes an operating point between the TFT  401  with the property  302  of increased Vgs and the light-emitting element  404  with the property  304 ,  307  denotes an operating point between the TFT  401  with the property  303  of further increased Vgs and the light-emitting element  404  with the property  304 ,  308  denotes a pinch-off curve,  309  denotes a potential of the counter electrode  405 ,  310  denotes a potential of the power supply line  420 ,  311  denotes a current flowing between the source and drain of the TFT  401 , and  312  denotes a current flowing to the light-emitting element  404 . 
   Potentials at the operating points of  305 ,  306  and  307  correspond to the potential of the node Vm  408  shown in  FIG. 4 . 
   An intersection between the pinch-off curve  308  and the property  301  of the TFT  401 , the property  302  of the TFT  401  with increased Vgs, or the property  303  of the TFT  401  with further increased Vgs, correspond to the pinch-off point. When Vgs of the TFT  401  is increased, the operating point moves to the saturation region side further. In this embodiment mode, since a potential relation which can minimize the Vgs is determined in the monitor pixel region  421 , the operating point between the TFT  401  and the light-emitting element  404  does not become the linear region. 
   Moreover, a size (channel width, channel length, or the like) and a property (mobility, threshold voltage, or the like) of the TFT  411  included in the monitor pixel region  421  are desired to be the same or close to a size and the property of the TFT  401  included in the display pixel region  422 . In addition, an aperture ratio, a shape or the like of the light-emitting element  412  included in the monitor pixel region  421  is desired to be the same or close to an aperture ratio, and a shape or the like of the light-emitting element  404  included in the display pixel region  422 . 
   In this embodiment mode, as a method to express a luminance gray scale, the output of the digital-analog converter circuit  419  is controlled by the video signal  423  inputted to the digital-analog converter circuit  419 . In this manner, the gate voltage of the TFT  401  is adjusted by changing the potential of the source signal line  406 . As a result, a current value flowing to the light-emitting element  404  is changed to express the luminance gray scale. 
   In addition, in this embodiment mode, although P-channel TFTs are used for the TFT  411  and the TFT  401 , an N-channel TFT may be used. In the case of using an N-channel TFT for the TFT  411 , the gate of the TFT  411  may be connected to one of the source and drain of the TFT  411  (that is, connected to the power supply line  414 ), and the current from the constant current source  416  may flow in the direction from the counter electrode  413  to the power supply line  414 . At this time, a direction of the light-emitting element  412  is also inverted. 
   Embodiment Mode 2 
   A display device of Structure  2  is described with reference to  FIG. 5 . 
   In  FIG. 5 , reference numerals  501 ,  502  and  511  denote TFTs,  503  denotes a capacitor,  504  and  512  denote light-emitting elements,  505  and  513  denote counter electrodes,  506  denotes a source signal line,  507  denotes a gate signal line,  509  denotes a source driver,  510  denotes a gate driver,  514  and  520  denote power supply lines,  515  denotes a sampling line,  516  denotes a constant current source,  517  and  518  denote sampling circuits,  519  denotes a monitor pixel region, and  521  denotes a display pixel region. 
   Each pixel has the capacitor  503 , the light-emitting element  504 , the TFT  501 , and the TFT  502 . In addition, each monitor pixel has the light-emitting element  512  and the TFT  511 . 
   The constant current source  516  is connected to the power supply line  514  and an input of the sampling circuit  517 . The power supply line  514  is connected to one of a source and drain of the TFT  511 . The other of the source and drain of the TFT  511  is connected to a gate of the TFT  511 , the sampling line  515 , and one electrode of the light-emitting element  512 . The sampling line  515  is connected to an input of the sampling circuit  518 . An output of the sampling circuit  517  is connected to a power source of a level shifter and a power source of a buffer of the source driver  509 . An output of the sampling circuit  518  is connected to the power source of the level shifter and the power source of the buffer of the source driver  509 . An output of the source driver  509  is connected to the source signal line  506 , while an output of the gate driver  510  is connected to the gate signal line  507 . The power supply line  520  is connected to one of a source and drain of the TFT  501 . The other of the source and drain of the TFT  501  is connected to one electrode of the light-emitting element  504 . A gate of the TFT  501  is connected to one electrode of the capacitor  503  and one of a source and drain of the TFT  502 . The other of the source and drain of the TFT  502  is connected to the source signal line  506 . A gate of the TFT  502  is connected to the gate signal line  507 . 
   A connecting point between the TFT  501  and one electrode of the light-emitting element  504  is a node Vm  508 . 
   A driving method of  FIG. 5  is described. 
   In the invention, although the display pixel region  521  to be used in displaying images and the monitor pixel region  519  for sampling a potential are separately provided, the invention is not limited to such a structure. 
   First, an operation of the monitor pixel region  519  is described. 
   In the monitor pixel region  519 , such a voltage at which an operating point of the TFT  511  and the light-emitting element  512  becomes a boundary between the saturation region and the linear region of the TFT  511  is sampled. 
   Moreover, the boundary between the saturation region and the linear region is called a pinch-off point. In the case of a P-channel TFT, the following formula is satisfied at the pinch-off point. 
   Vds=Vgs−Vth (Vds: a voltage between a source and drain, Vgs: a voltage between a source and gate, and Vth: a threshold voltage) 
   In the saturation region, the following formula is satisfied.
 
 Vds&lt;Vgs−Vth  
 
   In the linear region, the following formula is satisfied.
 
 Vds&gt;Vgs−Vth  
 
   In the monitor pixel region  519 , the other of the source and drain of the TFT  511  is connected to the gate of the TFT  511  and one electrode of the light-emitting element  512 , and a constant current flows between the source and drain of the TFT  511  and then flows into the light-emitting element  512 ; therefore, the operating point between the TFT  511  and the light-emitting element  512  is set to be a voltage close to the pinch-off point of the TFT  511 . The constant current source  516  is set in a direction in which a current flows from the power supply line  514  to the counter electrode  513 , and the TFT  511  is a P-channel TFT; therefore, an electrode of the TFT  511  that is connected to the power supply line  514  is a source, while the other electrode of the TFT  511  that is connected to the light-emitting element  512  is a drain. Since a drain voltage and a gate voltage of the TFT  511  are equivalent to each other by the connection relation of the aforementioned monitor pixel region  519 , in the case of normally on (in the case where a threshold voltage is positive), the TFT  511  operates in the linear region, while in the case of normally off (in the case where the threshold voltage is negative), the TFT  511  operates in the saturation region. That is, the operating point between the TFT  511  and the light-emitting element  512  is extremely close to the pinch-off point or equal to the pinch-off point. 
   A current value of the constant current source  516  is a value obtained by adding a current value corresponding to the maximum luminance at which the light-emitting element  504  of the display pixel region  521  is expected to emit light, to only the number of the light-emitting elements  512  of the monitor pixel region  519 . For example, when the current value corresponding to the maximum luminance at which the light-emitting element  504  of the display pixel region  521  is expected to emit light is set to be Ipix, in the case where the number of the light-emitting elements  512  of the monitor pixel region  519  is n, a current value flowing from the constant current source  516  is n×Ipix. 
   Next, a sampling method of a potential of the monitor pixel region  519  is described. 
   A potential of the power supply line  514  and a potential of the sampling line  515  are sampled. The potential of the power supply line  514  becomes a potential of a source side of the TFT  511 , while the potential of the sampling line  515  becomes a drain potential and a gate potential of the TFT  511 . In addition, as described above, the operating point between the TFT  511  and the light-emitting element  512  is extremely close to the pinch-off point of the TFT  511  or equal to the pinch-off point. 
   The power supply line  514  is connected to the input of the sampling circuit  517 . The potential of the power supply line  514  is sampled in the sampling circuit  517 , and the sampling circuit  517  outputs a potential corresponding to the sampled potential. Moreover, any configuration may be used for this sampling circuit  517  and the sampling circuit  517  is not limited to a particular configuration. In addition, the sampling circuit  517  is not always required and a configuration without the sampling circuit  517  may be used. 
   The sampling line  515  is connected to the input of the sampling circuit  518 . The potential of the sampling line  515  is sampled in the sampling circuit  518 , and the sampling circuit  518  outputs a potential corresponding to the sampled potential. Moreover, any configuration may be used for this sampling circuit  518  and the sampling circuit  518  is not limited to a particular configuration. In addition, the sampling circuit  518  is not always required and a configuration without the sampling circuit  518  may be used. 
   The outputs of the sampling circuit  517  and the sampling circuit  518  are connected to the power source of the level shifter and the power source of the buffer of the source driver  509 . 
   Next, operations of the display pixel region  521 , the source driver  509 , and the gate driver  510  are described. 
   The output of the sampling circuit  517  is connected to the power supply line  520  to output the potential of the power supply line  514  of the monitor pixel region  519 . Here, a configuration of the source driver  509  is not limited, and such a configuration that output potentials of the sampling circuit  517  and the sampling circuit  518  are outputted to the source signal line  506  may be used. In addition, a configuration of the gate driver  510  is not limited, and such a configuration that scans the gate signal line  507  may be used. 
   In the display pixel region  521 , a current is supplied from the power supply line  520  to the light-emitting element  504  through the TFT  501 . This current is controlled by a voltage (gate voltage) between the gate and source of the TFT  501 , and a gate potential of the TFT  501  is supplied from the source signal line  506  through the TFT  502  which is selected by the gate signal line  507  to be turned ON. In addition, since the potential supplied by this source signal line  506  is held in the capacitor  503 , the gate potential of the TFT  501  is held for a while even when the TFT  502  selected by the gate signal line  507  to be turned ON becomes OFF. 
   Here, the potential supplied by the source signal line  506  is a potential having a value between the potential of the power supply line  514  and the potential of the sampling line  515  of the monitor pixel region  519 . A potential supplied by the power supply line  520  is the potential of the power supply line  514  of the monitor pixel region  519 . In addition, the potential of the power supply line  514  and the potential of the sampling line  515  of the monitor pixel region  519  have a potential relation which allows the light-emitting element  512  to emit light at maximum luminance, and the operating point at maximum luminance is close to the pinch-off point of the TFT  511 . 
   An operating point between the TFT  501  and the light-emitting element  504  is close to the pinch-off point when a potential of the source signal line  506  is the potential of the sampling line  515 , and even when the potential of the source signal line  506  approaches the potential of the power supply line  514 , the operating point moves from the pinch-off point to the more saturation region side by the aforementioned formula (the saturation region is obtained when Vds&lt;Vgs−Vth). 
   Moreover, a size (channel width, channel length, or the like) and a property (mobility, threshold voltage, or the like) of the TFT  511  included in the monitor pixel region  519  are desired to be the same or close to a size and the property of the TFT  501  included in the display pixel region  521 . In addition, an aperture ratio, a shape or the like of the light-emitting element  512  included in the monitor pixel region  519  is desired to be the same or close to an aperture ratio, a shape or the like of the light-emitting element  504  included in the display pixel region  521 . 
   In this embodiment mode, as a method to express a luminance gray scale, there is a method (time division gray scale) which controls time when a light-emitting element emits light. In that case, only two values of a signal voltage that is for turning ON the TFT  501  and a signal voltage that is for turning OFF the TFT  501  are outputted from the source driver  509  to the source signal line  506 . 
   In addition, in this embodiment mode, although P-channel TFTs are used for the TFT  511  and the TFT  501 , an N-channel TFT may be used. In the case of using an N-channel TFT for the TFT  511 , the gate of the TFT  511  may be connected to one of the source and drain of the TFT  511  (that is, connected to the power supply line  514 ), and the current from the constant current source  516  may flow in the direction from the counter electrode  513  to the power supply line  514 . At this time, a direction of the light-emitting element  512  is also inverted. 
   In Embodiment Mode 1 and Embodiment Mode 2, arrangement of the TFTs is described with reference to  FIGS. 4 and 5 . However, in the invention, the arrangement of the TFTs is not limited to the arrangement of  FIGS. 4 and 5 . TFTs can be arranged in an arbitrary position as long as the drive described in Embodiment Mode 1 and Embodiment Mode 2 is possible. For example, a TFT may be added in order to control the light-emitting element to emit no light with a signal which is different from a video signal or a TFT may be added in order to correct a threshold voltage of a driving TFT. 
   Moreover, in the invention, any circuit configuration may be used for the source drivers, the gate drivers, the sampling circuits, the digital-analog converter circuits, and the like shown in block diagrams as long as the drive described in Embodiment Mode 1 and Embodiment Mode 2 is possible. 
   In the invention, a known circuit can be used for a driver circuit which inputs a signal to a pixel. 
   Embodiment 1 
   An example in which a display device of the invention is actually made is described. 
     FIGS. 6A and 6B  are cross-sectional views of a pixel in a display device of Embodiment Mode 1 and Embodiment Mode 2. An example using a TFT is shown as a transistor arranged in the pixel of Embodiment Mode 1 and Embodiment Mode 2. 
   In  FIGS. 6A and 6B , reference numeral  1000  denotes a substrate,  1001  denotes a base film,  1002  denotes a semiconductor layer,  1102  denotes a semiconductor layer,  1003  denotes a first insulating film,  1004  denotes a gate electrode,  1104  denotes an electrode of a capacitor,  1005  denotes a second insulating film,  1006  denotes a source electrode or drain electrode,  1007  denotes a first electrode,  1008  denotes a third insulating film,  1009  denotes a light-emitting layer, and  1010  denotes a second electrode. In addition, reference numeral  1100  denotes a TFT,  1011  denotes a light-emitting element, and  1101  denotes the capacitor. 
   In  FIGS. 6A and 6B , the TFT  1100 , the capacitor  1101 , and the light-emitting element  1011  are typically shown as elements forming the pixel. Note that a monitor pixel can have a similar configuration. 
   A configuration of  FIG. 6A  is described. 
   As the substrate  1000 , for example, a glass substrate such as barium borosilicate glass or alumino borosilicate glass, a quartz substrate, a ceramic substrate, or the like can be used. Further, a substrate obtained by forming an insulating film over a surface of a metal substrate containing stainless steel or of a semiconductor substrate may be used. A substrate formed of a synthetic resin having flexibility such as plastic may be used as well. A surface of the substrate  1000  may be planarized by a method such as CMP. 
   As the base film  1001 , an insulating film such as silicon oxide, silicon nitride, or silicon nitride oxide can be used. By forming the base film  1001 , an alkaline metal such as sodium (Na) or an alkaline earth metal contained in the substrate  1000  can be prevented from diffusing into the semiconductor layer  1002  and affecting adversely on the properties of the TFT  1100 . In  FIG. 6A , although the base film  1001  has a monolayer structure, a stacked layer structure of two layers or more may be used as well. Note that in the case where impurity diffusion does not become a problem so much such as a quartz substrate, the base film  1001  is not necessarily required to be provided. 
   As the semiconductor layer  1002  and the semiconductor layer  1102 , a crystalline semiconductor film or an amorphous semiconductor film can be used. A crystalline semiconductor film can be obtained by crystallizing an amorphous semiconductor film. As a crystallization method, a laser crystallization method, a thermal crystallization method using RTA or an annealing furnace, a thermal crystallization method using a metal element to promote crystallization, or the like can be used. The semiconductor layer  1002  has a channel forming region and a pair of impurity regions to which an impurity element is added to impart conductivity type. Note that between the channel forming region and the pair of impurity regions, another impurity region to which the impurity element is added at a low concentration may be provided as well. The semiconductor layer  1102  can have a structure in which an impurity element is added entirely to impart conductivity type. 
   As the first insulating film  1003 , silicon oxide, silicon nitride, silicon nitride oxide or the like can be stacked in a monolayer or a plurality of layers. 
   As the gate electrode  1004  and the electrode  1104  of the capacitor, a monolayer structure or a stacked layer structure formed of one element selected from tantalum (Ta), tungsten (W), titanium (Ti), molybdenum (Mo), aluminum (Al), copper (Cu), chromium (Cr), or neodymium (Nd), or an alloy or a compound containing such elements, can be used. 
   The TFT  1100  is formed of the semiconductor layer  1002 , the gate electrode  1004 , and the first insulating film  1003  interposed between the semiconductor layer  1002  and the gate electrode  1004 . Although only the TFT  1100  connected to the first electrode  1007  of the light-emitting element  1011  is shown as a TFT forming a pixel in  FIG. 6A , a structure having a plurality of TFTs may be used as well. Further, although the TFT  1100  is described as a top-gate transistor in this embodiment, a bottom-gate transistor having a gate electrode below a semiconductor layer may be used, or a dual-gate transistor having gate electrodes above and below a semiconductor layer may be used as well. 
   The capacitor  1101  is formed of the first insulating film  1003  as a dielectric and the semiconductor layer  1102  and the electrode of the capacitor  1104  as a pair of electrodes which are opposed to each other with the first insulating film  1003  interposed therebetween. Note that as the capacitor  1101  included in the pixel, description is made on an example in which one of the pair of electrodes is the semiconductor layer  1102  formed at the same time as the semiconductor layer  1002  of the TFT  1100  while the other electrode thereof is the electrode of the capacitor  1104  formed at the same time as the gate electrode  1004  of the TFT  1100  in  FIG. 6A . However, the structure is not limited to this structure. 
   As the second insulating film  1005 , a monolayer or a stacked layer of an inorganic insulating film or an organic insulating film can be used. As the inorganic insulating film, a silicon oxide film formed by CVD, a silicon oxide film formed by SOG (Spin On Glass), or the like can be used while as the organic insulating film, a film such as polyimide, polyamide, BCB (benzocyclobutene), acrylic, a positive photosensitive organic resin, or a negative photosensitive organic resin can be used. 
   Further, as the second insulating film  1005 , a material composed of a skeleton formed by the bond of silicon (Si) and oxygen (O) can be used. An organic group containing at least hydrogen (such as an alkyl group or aromatic hydrocarbon) is used as a substituent of this material. Alternatively, a fluoro group may be used as the substituent. Further alternatively, both a fluoro group and an organic group containing at least hydrogen may be used as the substituent. 
   As the source electrode or drain electrode  1006 , a monolayer structure or a stacked layer structure formed of one element selected from aluminum (Al), nickel (Ni), carbon (C), tungsten (W), molybdenum (Mo), titanium (Ti), platinum (Pt), copper (Cu), tantalum (Ta), gold (Au), or manganese (Mn), or an alloy or a compound containing such elements can be used. 
   One or both of the first electrode  1007  and the second electrode  1010  may be a light-transmissive electrode. As the light-transmissive electrode, indium tin oxide (ITO), zinc oxide (ZnO), zinc oxide doped with gallium (GZO), or other light-transmissive conductive oxide materials can be used. In addition, ITO containing silicon oxide (hereinafter referred to as ITSO), ITO containing titanium oxide (hereinafter referred to as ITTO), ITO containing molybdenum oxide (hereinafter referred to as ITMO), ITO doped with titanium, molybdenum, or gallium, or a material formed by mixing indium oxide containing silicon oxide with zinc oxide (ZnO) as a target by 2 to 20 wt % may be used as well. 
   The other of the first electrode  1007  and the second electrode  1010  may be formed of a material having no light-transmitting property. For example, an alkaline metal such as lithium (Li) or cesium (Cs), an alkaline earth metal such as magnesium (Mg), calcium (Ca), or strontium (Sr), an alloy including these (Mg:Ag, Al:Li, Mg:In or the like), a compound of these (calcium fluoride, calcium nitride), or a rare earth metal such as ytterbium (Yb) or erbium (Er) can be used. 
   A material similar to that of the second insulating film  1005  can be used to form the third insulating film  1008 . The third insulating film  1008  is formed on the periphery of the first electrode  1007  so as to cover an end portion of the first electrode  1007 , and has a function to separate the light-emitting layer  1009  in adjacent pixels. 
   The light-emitting layer  1009  is formed of a monolayer or a plurality of layers. In the case of forming of a plurality of layers, these layers are classified, in view of carrier transport properties, into a hole injecting layer, a hole transporting layer, a light-emitting layer, an electron transporting layer, an electron injecting layer, or the like. Note that boundaries of each layer are not required to be clear, and there are some cases where materials forming respective layers are partially mixed; therefore, interfaces are not defined clearly. An organic material or an inorganic material can be used for each layer. As the organic material, any of a polymeric material, a middle molecular weight material, and a low molecular weight material can be used. 
   The light-emitting element  1011  includes the first electrode  1007 , the second electrode  1010 , and the light-emitting layer  1009  between the first and second electrodes. One of the first electrode  1007  and the second electrode  1010  corresponds to an anode, and the other thereof corresponds to a cathode. When a voltage higher than a threshold voltage is applied between the anode and the cathode in forward bias direction, a current flows from the anode to the cathode, therefore, the light-emitting element  1011  emits light. 
   Description is made on a structure of  FIG. 6B . Note that the same reference numerals are used for the same portions as  FIG. 6A , and description thereof is omitted. 
     FIG. 6B  is a structure including a fourth insulating film  1108  between the second insulating film  1005  and the third insulating film  1008  in  FIG. 6A . 
   Moreover, the source electrode or drain electrode  1006  and the first electrode  1007  are connected through a connection electrode  1106  in a contact hole provided in the insulating film  1108 . 
   The fourth insulating film  1108  can have a similar structure to the second insulating film  1005 . The connection electrode  1106  can have a similar structure to the source electrode or drain electrode  1006 . 
   This embodiment can be implemented by freely combining with embodiment modes. 
   Embodiment 2 
   In this embodiment, description is made on a structure for sealing a display device with reference to  FIGS. 7A to 7C .  FIG. 7A  is a top plan view of a display panel formed by sealing a display device, and each of  FIGS. 7B and 7C  is a cross sectional view along a line A-A′ of  FIG. 7A . Note that  FIGS. 7B and 7C  are examples for performing sealing by different methods. 
   In  FIGS. 7A to 7C , a display portion  1302  having a plurality of pixels is arranged over a substrate  1301 , and to surround them, a sealing material  1306  is provided to stick a sealing material  1307 . As for a pixel configuration, structures of embodiment modes or Embodiment 1 can be used. 
   In a display panel in  FIG. 7B , the sealing material  1307  in  FIG. 7A  corresponds to a counter substrate  1321 . The sealing material  1306  is used as an adhesive layer and a light-transmissive counter substrate  1321  is stuck thereto. The substrate  1301 , the counter substrate  1321 , and the sealing material  1306  form a closed space  1322 . A color filter  1320  and a protective film  1323  to protect the color filter are provided to the counter substrate  1321 . Light emitted from a light-emitting element arranged in the display portion  1302  is emitted outside through the color filter  1320 . The closed space  1322  is filled with an inert resin, a liquid, or the like. Note that as the resin to fill the closed space  1322 , a light-transmissive resin in which a moisture absorption material is dispersed may be used as well. Further, the sealing material  1306  and a material for filling the closed space  1322  may be the same material, and adhesion of the counter substrate  1321  and sealing of the display portion  1302  may be performed at the same time. 
   In a display panel shown in  FIG. 7C , the sealing material  1307  in  FIG. 7A  corresponds to a sealing material  1324 . The sealing material  1306  is used as an adhesive layer and the sealing material  1324  is stuck thereto. The substrate  1301 , the sealing material  1306 , and the sealing material  1324  form a closed space  1308 . An absorbent  1309  is provided in a depressed portion of the sealing material  1324  in advance, and inside the closed space  1308 , the absorbent  1309  absorbs moisture, oxygen, or the like to keep a clean atmosphere and functions to suppress deterioration of the light-emitting element. This depressed portion is covered with a cover material  1310  with a fine mesh. Although air and moisture are passed through the cover material  1310 , they are not passed though the absorbent  1309 . Note that the closed space  1308  may be filled with a rare gas such as nitrogen or argon, and can be filled with a resin or a liquid as long as it is inert. 
   Over the substrate  1301 , provided is an input terminal portion  1311  for transmitting a signal to the display portion  1302  and the like, and transmitted is a signal such as a video signal to the input terminal portion  1311  through an FPC (Flexible Printed Circuit)  1312 . In the input terminal portion  1311 , a wiring formed over the substrate  1301  is electrically connected to a wiring provided over the FPC  1312  by using a resin (anisotropic conductive resin: ACF) in which a conductor is dispersed. 
   Over the substrate  1301  over which the display portion  1302  is formed, a driver circuit to input a signal to the display portion  1302  may be integrally formed. A driver circuit to input a signal to the display portion  1302  may be formed with an IC chip and connected onto the substrate  1301  by COG (Chip On Glass), or an IC chip may be arranged over the substrate  1301  by using TAB (Tape Auto Bonding) or a printed board. 
   This embodiment can be implemented by freely combining with embodiment modes and Embodiment 1. 
   Embodiment 3 
   The invention can be applied to a display module in which a circuit to input a signal to a display panel is mounted onto a display panel. 
     FIG. 8  shows a display module in which a display panel  1200  and a circuit board  1204  are combined. 
   In  FIG. 8 , shown is an example in which a control circuit  1205 , a signal division circuit  1206 , and the like are formed over the circuit board  1204 . A circuit formed over the circuit board  1204  is not limited to this. Any circuit which generates a signal to control a display panel may be formed as well. 
   Signals outputted from these circuits formed over the circuit board  1204  are inputted to the display panel  1200  through a connection wiring  1207 . 
   The display panel  1200  has a display portion  1201 , a source driver  1202 , and a gate driver  1203 . A structure of the display panel  1200  can have a similar structure to a structure described in Embodiment 2 or the like.  FIG. 8  shows an example in which the source driver  1202  and the gate driver  1203  are formed over the same substrate as the display portion  1201 . However, the display module of the invention is not limited to this. Only the gate driver  1203  may be formed over the same substrate as the display portion  1201 , and a source driver may be formed over a circuit board. Both a source driver and a gate driver may be formed over a circuit board as well. 
   Display portions of various electronic apparatuses can be formed with such display modules incorporated therein. 
   This embodiment can be implemented by freely combining with embodiment modes, and Embodiments 1 and 2. 
   Embodiment 4 
   As an electronic apparatus using the display module of the invention, there are a camera such as a video camera and a digital still camera, a goggle type display (a head mounted display), a navigation system, an audio reproducing device (a car audio, an audio component and the like), a personal computer, a game machine, a portable information terminal (a mobile computer, a mobile phone, a portable game machine, an electronic book, or the like), an image reproducing device provided with a recording medium reading portion (specifically, a device which reproduces a recording medium such as a Digital Versatile Disc (DVD), and is provided with a display capable of displaying the reproduced image), and the like. In particular, for a portable information terminal of which a display is often looked from an oblique direction, the range of a viewing angle is emphasized; therefore, it is desired to use a self-luminous display device. The invention is particularly effective on a portable information apparatus in which reduction of power consumption is an essential task. 
   Specific examples of electronic apparatuses are described in  FIGS. 9A to 9H . Note that electronic apparatuses described here are just illustrative, and the invention is not limited to these applications. 
     FIG. 9A  shows a display including a housing  2001 , a support base  2002 , a display portion  2003 , speaker portions  2004 , a video input terminal  2005 , and the like. The display module of the invention can be used for the display portion  2003 . Note that the display includes all display devices for displaying information such as a display device for a personal computer, for TV broadcast reception, and for advertisement display. 
     FIG. 9B  shows a digital still camera including a main body  2101 , a display portion  2102 , an image receiving portion  2103 , operation keys  2104 , an external connection port  2105 , a shutter  2106 , and the like. The display module of the invention can be used for the display portion  2102 . 
     FIG. 9C  shows a personal computer including a main body  2201 , a housing  2202 , a display portion  2203 , a keyboard  2204 , an external connection port  2205 , a pointing pad  2206 , and the like. The display module of the invention can be used for the display portion  2203 . 
     FIG. 9D  shows a mobile computer including a main body  2301 , a display portion  2302 , a switch  2303 , operation keys  2304 , an infrared port  2305 , and the like. The display module of the invention can be used for the display portion  2302 . 
     FIG. 9E  shows a portable image reproducing device (specifically, a DVD reproducing device) provided with a recording medium reading portion, including a main body  2401 , a housing  2402 , a display portion A  2403 , a display portion B  2404 , a recording medium (DVD and the like) reading portion  2405 , an operation key  2406 , a speaker portion  2407 , and the like. The display portion A  2403  mainly displays image data while the display portion B  2404  mainly displays text data. However, the display module of the invention can be used for the display portion A  2403  and the display portion B  2404 . Note that an image reproducing device provided with a recording medium includes a game machine and the like. 
     FIG. 9F  shows a goggle type display (a head mounted display) including a main body  2501 , a display portion  2502 , an arm portion  2503 , and the like. The display module of the invention can be used for the display portion  2502 . 
     FIG. 9G  shows a video camera including a main body  2601 , a display portion  2602 , a housing  2603 , an external connection port  2604 , a remote control receiving portion  2605 , an image receiving portion  2606 , a battery  2607 , a sound input portion  2608 , operation keys  2609 , and the like. The display module of the invention can be used for the display portion  2602 . 
   Here,  FIG. 9H  shows a mobile phone including a main body  2701 , a housing  2702 , a display portion  2703 , a sound input portion  2704 , a sound output portion  2705 , an operation key  2706 , an external connection port  2707 , an antenna  2708 , and the like. The display module of the invention can be used for the display portion  2703 . Note that the display portion  2703  displays white text on a black background; therefore, the power consumption of the mobile phone can be suppressed further. 
   Note that when the light emission luminance of a light-emitting element increases in the future, outputted light including image data can be enlarged and projected by a lens or the like to be used for a front projector or a rear projector. 
   Further, the aforementioned electronic apparatuses display data distributed through a telecommunication line such as the Internet or a CATV (cable television) in many cases, and particularly, an opportunity to display video data has been increasing. The response speed of a light-emitting material is extremely high; therefore, the display module of the invention is preferable for displaying video data. 
   Moreover, the display device of the invention consumes electricity in a light-emitting portion; therefore, it is desirable to display data so as to minimize the light-emitting portion. Accordingly, in the case of using the display module for a display portion which mainly displays text data such as a portable information terminal, particularly a mobile phone or an audio reproducing device, it is desirable to drive so as to form text data with a light-emitting portion while a non-light-emitting portion is used as a background. 
   As set forth above, the applicable range of the invention is extremely wide; therefore, the invention can be used for electronic apparatuses of various fields. 
   This embodiment can be implemented by freely combining with embodiment modes and Embodiments 1 to 3. 
   This application is based on Japanese Patent Application serial no. 2005-101152 filed in Japan Patent Office on Mar. 31, in 2005, the entire contents of which are hereby incorporated by reference.