Abstract:
A light emitting device and an element substrate which are capable of suppressing variations in the luminance intensity of a light emitting element among pixels due to characteristic variations of a driving transistor without suppressing off-current of a switching transistor low and increasing storage capacity of a capacitor. According to the invention, a depletion mode transistor is used as a driving transistor. The gate of the driving transistor is fixed in its potential or connected to the source or drain thereof to operate in a saturation region with a constant current flow. A current controlling transistor which operates in a linear region is connected in series to the driving transistor, and a video signal for transmitting a light emission or non-emission of a pixel is inputted to the gate of the current controlling transistor through a switching transistor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a light emitting device comprising a plurality of pixels each having a light emitting element and a means for supplying current to the light emitting element. 
   2. Description of the Related Art 
   Since a light emitting element emits light by itself, it is highly visible and does not require a back light which is needed in a liquid crystal display device (LCD). Therefore, it is suitably applied to thin devices and not restricted in viewing angle. Because of these advantages, a light emitting device having a light emitting element has recently been drawing attentions as an alternative display device to a CRT and an LCD. It is to be noted that a light emitting element in this specification indicates an element whose luminance is controlled by current or voltage, and it includes an OLED (Organic Light Emitting Diode) or an MIM electron source element (electron discharge element) and the like which is used in an FED (Field Emission Display). 
   Also, a light emitting device of the invention includes a panel and a module obtained by mounting an IC or the like onto the panel. More generally, the invention relates to an element substrate which corresponds to the one before the completion of a panel in manufacturing steps of the light emitting device, and the element substrate comprises a plurality of pixels each having a means for supplying current to a light emitting element. 
   OLED which is one of the light emitting elements includes an anode layer, a cathode layer, and a layer containing an electric field light emitting material (hereinafter referred to as an electroluminescent layer) that generates luminescence (electroluminescence) when an electric field is applied thereto. The electroluminescent layer is provided between an anode and cathode, and it comprises a single or multiple layers. These layers may contain an inorganic compound. The electroluminescence in the electroluminescent layer includes a light emission (fluorescence) when a singlet exciting state returns to a ground state and a light emission (phosphorescence) when a triplet exciting state returns to a ground state. 
   Next, the configuration of a pixel of a general light emitting device and its drive will be described in brief. A pixel shown in  FIG. 9  comprises a switching transistor  900 , a driving transistor  901 , a capacitor  902 , and a light emitting element  903 . The gate of the switching transistor  900  is connected to a scan line  905 . Either the source or drain of the switching transistor  900  is connected to a signal line  904 , and the other is connected to the gate of the driving transistor  901 . The source of the driving transistor  901  is connected to a power supply line  906 , and the drain of the driving transistor  901  is connected to the anode of the light emitting element  903 . The cathode of the light emitting element  903  is connected to a counter electrode  907 . The capacitor  902  is provided for storing a potential difference between the gate and source of the driving transistor  901 . Also, the predetermined voltages are applied to the power supply line  906  and the counter electrode  907  from a power supply and each has a potential difference. 
   When the switching transistor  900  is turned ON by a signal from the scan line  905 , a video signal that is inputted to the signal line  904  is inputted to the gate of the driving transistor  901 . The potential difference between a potential of the inputted video signal and that of the power supply line  906  corresponds to a gate-source voltage Vgs of the driving transistor  901 . Thus, current is supplied to the light emitting element  903 , and the light emitting element  903  emits light by using the supplied current. 
   SUMMARY OF THE INVENTION 
   A transistor using polysilicon has high field effect mobility and large on-current. Therefore, it is suited for a light emitting device. However, the transistor using polysilicon has problems in that it is likely to have variations in characteristics due to a defect in a crystal grain boundary. 
   In the pixel shown in  FIG. 9 , when the magnitude of the drain current of the driving transistor  901  differs among pixels, the luminance intensity of the light emitting element  903  varies even with the same potential of a video signal. 
   As a means for controlling variations in drain current, there is a method for enlarging an L/W (L: channel length, W: channel width) of the driving transistor  901  as disclosed in Japanese Patent Application No. 2003-008719. The drain current Ids of the driving transistor  901  in a saturation region is expressed by the following formula 1.
 
 Ids=â ( Vgs−Vth ) 2 /2  (formula 1)
 
   It is apparent from the formula 1 that, the drain current Ids in the saturation region of the driving transistor  901  is easily fluctuated even by small variations in the gate-source voltage Vgs. Therefore, it is necessary to keep the gate-source voltage Vgs, which is stored between the gate and source of the driving transistor  901 , not to be varied while the light emitting element  901  emits light. Thus, storage capacity of the capacitor  902  which is disposed between the gate and source of the driving transistor  901  is required to be increased, and off-current of the switching transistor  900  is required to be suppressed low. 
   It is quite difficult to suppress off-current of the switching transistor  900  low, to increase on-current thereof for charging large capacitance, and to achieve both of them in the formation process of the transistor. 
   Also, there is another problem that the gate-source voltage Vgs of the driving transistor  901  is varied due to the switching of the switching transistor  900 , and potential changes in the signal line, scan line, and the like. This derives from the parasitic capacitance on the gate of the driving transistor  901 . 
   In view of the foregoing problems, the invention provides a light emitting device and an element substrate which are not easily influenced by parasitic capacitance and capable of suppressing variations in luminance intensity of the light emitting element  903  among pixels due to characteristic variations of the driving transistor  901  without suppressing off-current of the switching transistor  900  low and increasing storage capacity of the capacitor  902 . 
   According to the invention, a depletion mode transistor is used as a driving transistor. The gate of the driving transistor is fixed in its potential or connected to the source or drain thereof to operate in a saturation region with a constant current flow. Also, a current controlling transistor which operates in a linear region is connected in series to the driving transistor. A video signal for transmitting a light emission or non-emission of a pixel is inputted to the gate of the current controlling transistor through a switching transistor. 
   Transistors other than the driving transistor are normal enhancement mode transistors here. 
   Since the current controlling transistor operates in a linear region, its source-drain voltage Vds is small, and small changes in a gate-source voltage Vgs of the current controlling transistor do not influence the current flowing in a light emitting element. Current flowing in the light emitting element is determined by the driving transistor which operates in a saturation region. 
   Current flowing in the light emitting element is not influenced even without increasing storage capacity of a capacitor which is disposed between the gate and source of the current controlling transistor or suppressing off-current of the switching transistor low. In addition, it is not influenced by the parasitic capacitance on the gate of the current controlling transistor either. Therefore, cause of variation is decreased, and image quality is thus enhanced to a great extent. 
   In addition, as there is no need to suppress off-current of the switching transistor low, manufacturing process of the transistor can be simplified, thus contributes greatly to the cost reduction and improvement in yield. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing an embodiment mode of the invention. 
       FIG. 2  is a diagram showing an embodiment mode of the invention. 
       FIG. 3  is a diagram showing an embodiment mode of the invention. 
       FIG. 4  is a diagram showing an embodiment mode of the invention. 
       FIG. 5  is a schematic view showing an external circuit and a panel. 
       FIG. 6  is a diagram showing the configuration example of a signal driver circuit. 
       FIG. 7  is an example showing a top plan view of the invention. 
       FIGS. 8A to 8D  are examples showing electronic apparatuses to which the invention is applied. 
       FIG. 9  is a diagram of an embodiment. 
       FIG. 10  is an example showing a top plan view of the invention. 
       FIGS. 11A and 11B  are examples showing cross-sectional structures of the invention. 
       FIG. 12  is an example showing the operation timing of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiment modes of the invention are described in detail with reference to the accompanying drawings below. 
   Embodiment Mode 1 
     FIG. 1  shows an embodiment mode of a pixel of the light emitting device of the invention. The pixel shown in  FIG. 1  comprises a light emitting element  104 , a transistor (switching transistor)  101  used as a switching element for controlling an input of a video signal to the pixel, a driving transistor  102  for controlling current flowing in the light emitting element  104 , and a current controlling transistor  103  for controlling a current supply to the light emitting element  104 . In addition, it is also possible to dispose in the pixel a capacitor  105  for storing a potential of a video signal. 
   The driving transistor  102  and the current controlling transistor  103  have the same conductivity. It is assumed that the driving transistor  102  is a depletion mode transistor, and the rest of the transistors are normal enhancement mode transistors. In the invention, the driving transistor  102  is operated in a saturation region and the current controlling transistor  103  is operated in a linear region. 
   The channel length (L) of the driving transistor  102  may be longer than its channel width (W), and L of the current controlling transistor  103  may be equal to or shorter than its W. Desirably, the ratio of L to W (L/W) of the driving transistor  102  is five or more. 
   The gate of the switching transistor  101  is connected to a scan line Gj (j=1 to y). Either the source or drain of the switching transistor  101  is connected to a signal line Si (i=1 to x), and the other is connected to the gate of the current controlling transistor  103 . The gate of the driving transistor  102  is connected to a power supply line Vi (i=1 to x). The driving transistor  102  and the current controlling transistor  103  are each connected to the power supply line Vi (i=1 to x) and the light emitting element  104  so that a current supplied from the power supply line Vi (i=1 to x) is supplied to the light emitting element  104  as a drain current of the driving transistor  102  and of the current controlling transistor  103 . In this embodiment mode, the source of the current controlling transistor  103  is connected to the power supply line Vi i=1 to x) and the drain of the driving transistor  102  is connected to a pixel electrode of the light emitting element  104 . 
   It is to be noted that the source of the driving transistor  102  may be connected to the power supply line Vi (i=1 to x), and the drain of the current controlling transistor  103  may be connected to the pixel electrode of the light emitting element  104 . 
   The light emitting element  104  comprises an anode, a cathode, and a light emitting layer interposed between the anode and cathode. As shown in  FIG. 1 , when the anode of the light emitting element  104  is connected to the driving transistor  102 , the anode is a pixel electrode and the cathode is a counter electrode. The counter electrode of the light emitting element  104  and the power supply line Vi (i=1 to x) are made to have a potential difference so that current flows into the light emitting element  104  in the forward bias direction. 
   One of the two electrodes of the capacitor  105  is connected to the power supply line Vi (i =1 to x), and the other is connected to the gate of the current controlling transistor  103 . The capacitor  105  is disposed so as to store a potential difference between the two electrodes of the capacitor  105  when the switching transistor  101  is not selected (off state). It is to be noted that although  FIG. 1  shows a configuration disposing the capacitor  105 , the invention is not limited to this and an alternative configuration without the capacitor  105  may be employed as well. 
   In  FIG. 1 , each of the driving transistor  102  and the current controlling transistor  103  is a P-type transistor, and the drain of the driving transistor  102  is connected to the anode of the light emitting element  104 . On the contrary, in the case where each of the driving transistor  102  and the current controlling transistor  103  is an N-type transistor, the source of the driving transistor  102  is connected to the cathode of the light emitting element  104 . In this case, the cathode of the light emitting element  104  is a pixel electrode and the anode thereof is a counter electrode. 
   Next, a driving method of the pixel shown in  FIG. 1  is described. The operation of the pixel shown in  FIG. 1  can be divided into a writing period and a data storage period. First, in the writing period, when the scan line Gj (j=1 to y) is selected, the switching transistor  101  whose gate is connected to the scan line Gj (j=1 to y) is turned ON. Then, a video signal which is inputted to the signal line Si (i=1 to x) is inputted to the gate of the current controlling transistor  103  through the switching transistor  101 . The driving transistor  102  is ON at all times as its gate is connected to the power supply line Vi (i=1 to x). 
   When the current controlling transistor  103  is turned ON by a video signal, current is supplied to the light emitting element  104  through the current supply line Vi (i=1 to x). At this time, the current controlling transistor  103  operates in a linear region, thus current flowing in the light emitting element  104  is determined by volt-ampere characteristics of the driving transistor  102  operating in a saturation region and the light emitting element  104 . The light emitting element  104  emits light at luminance corresponding to the magnitude of the supplied current. 
   Meanwhile, when the current controlling transistor  103  is turned OFF by a video signal, no current is supplied to the light emitting element  104 , thus it does not emit light. It is to be noted that according to the invention, it is possible to control current not to be supplied to the light emitting element  104  even when the driving transistor  102  is a depletion mode transistor since the current controlling transistor  103  is an enhancement mode transistor. 
   In the data storage period, the switching transistor  101  is turned OFF by controlling a potential of the scan line Gj (j=1 to y), thereby storing a potential of the video signal that is written in the writing period. In the writing period, when the current controlling transistor  103  is turned ON, a potential of a video signal is stored in the capacitor  105 , therefore, the current supply to the light emitting element  104  is kept on. On the contrary, when the current controlling transistor  103  is turned OFF in the writing period, a potential of a video signal is stored in the capacitor  105 , therefore, current is not supplied to the light emitting element  104 . 
   An element substrate of the invention corresponds to the one before the formation of a light emitting element in manufacturing steps of the light emitting device of the invention. 
   A transistor used in the light emitting device of the invention may be a transistor formed by using single crystalline silicon or an SOI, a thin film transistor using polycrystalline silicon or amorphous silicon, or a transistor using an organic semiconductor or a carbon nanotube. In addition, a transistor disposed in a pixel of the light emitting device of the invention may be a single gate transistor, a double gate transistor, or a multi-gate transistor having more than two gate electrodes. 
   According to the above-described configuration, a source-drain voltage Vds of the current controlling transistor  103  is small as the current controlling transistor  103  operates in a linear region, therefore, small changes in the gate-source voltage Vgs of the current controlling transistor  103  do not influence the current flowing in a light emitting element  104 . Current flowing in the light emitting element  104  is determined by the driving transistor  102  which operates in a saturation region. Current flowing in the light emitting element  104  is not influenced even without increasing storage capacity of the capacitor  105  which is disposed between the gate and source of the current controlling transistor  103  or suppressing off-current of the switching transistor  101  low. In addition, it is not influenced by the parasitic capacitance on the gate of the current controlling transistor  103  either. Therefore, cause of variation is decreased, and image quality is thus enhanced to a great extent. 
   Embodiment Mode 2 
   Described in this embodiment mode is a different configuration of a pixel of the light emitting device from that shown in  FIG. 1 . 
   The pixel shown in  FIG. 2  comprises a light emitting element  204 , a switching transistor  201 , a driving transistor  202 , a current controlling transistor  203 , and a transistor (erasing transistor)  206  for turning OFF the current controlling transistor  203  forcibly. In addition, it is also possible to dispose a capacitor  205  in addition to the aforementioned elements. 
   The driving transistor  202  and the current controlling transistor  203  have the same conductivity. The size, characteristics, and operating region of each transistor may be set in the same manner as Embodiment Mode 1. 
   The gate of the switching transistor  201  is connected to a first scan line Gaj (j=1 to y). Either the source or drain of the switching transistor  201  is connected to a signal line Si (i=1to x), and the other is connected to the gate of the current controlling transistor  203 . The gate of the erasing transistor  206  is connected to a second scan line Gej (j=1 to y). Either the source or drain of the erasing transistor  206  is connected to a power supply line Vi (i=1to x), and the other is connected to the gate of the current controlling transistor  203 . The gate of the driving transistor  202  is connected to the power supply line Vi (i=1 to x). The driving transistor  202  and the current controlling transistor  203  are each connected to the power supply line Vi (i=1 to x) and the light emitting element  204  so that a current supplied from the power supply line Vi (i=1 to x) is supplied to the light emitting element.  204  as a drain current of the driving transistor  202  and of the current controlling transistor  203 . In this embodiment mode, the source of the current controlling transistor  203  is connected to the power supply line Vi (i=1 to x) and the drain of the driving transistor  202  is connected to a pixel electrode of the light emitting element  204 . 
   It is to be noted that the source of the driving transistor  202  may be connected to the power supply line Vi (i=1 to x), and the drain of the current controlling transistor  203  may be connected to the pixel electrode of the light emitting element  204 . 
   The light emitting element  204  comprises an anode, a cathode, and a light emitting layer interposed between the anode and cathode. As shown in  FIG. 2 , when the anode of the light emitting element  204  is connected to the driving transistor  202 , the anode is a pixel electrode and the cathode is a counter electrode. The counter electrode of the light emitting element  204  and the power supply line Vi (i=1 to x) have a potential difference so that current flows into the light emitting element  204  in the forward bias direction. 
   One of the two electrodes of the capacitor  205  is connected to the power supply line Vi (i=1 to x), and the other is connected to the gate of the current controlling transistor  203 . 
   In  FIG. 2 , each of the driving transistor  202  and the current controlling transistor  203  is a P-type transistor, and the drain of the driving transistor  202  is connected to the anode of the light emitting element  204 . On the contrary, in the case where each of the driving transistor  202  and the current controlling transistor  203  is an N-type transistor, the source of the driving transistor  202  is connected to the cathode of the light emitting element  204 . In this case, the cathode of the light emitting element  204  is a pixel electrode and the anode thereof is a counter electrode. 
   The operation of the pixel shown in  FIG. 2  can be divided into a writing period, a data storage period, and an erasing period. The operations of the switching transistor  201 , the driving transistor  202 , and the current controlling transistor  203  in writing period and data storage period are the same as those in  FIG. 1 . 
   In erasing period, the second scan line Gaj (j=1 to y) is selected to turn ON the erasing transistor  206 , thus a potential of the power supply line Vi (i=1 to x) is supplied to the gate of the current controlling transistor  203  through the erasing transistor  206 . Therefore, the current controlling transistor  203  is turned OFF, and the light emitting element  204  can be forcibly brought into the state where no current is supplied. 
   Embodiment Mode 3 
   Described in this embodiment mode is a different configuration of a pixel of the light emitting device of the invention from those of Embodiment Modes 1 and 2. 
   The pixel shown in  FIG. 3  comprises a light emitting element  304 , a transistor (switching transistor)  301  used as a switching element for controlling input of a video signal to the pixel, a driving transistor  302  for controlling a current flowing into the light emitting element  304 , a current controlling transistor  303  for controlling a current supply to the light emitting element  304 . In addition, it is also possible to dispose a capacitor  305  for storing a potential of a video signal as shown in the figure. 
   The driving transistor  302  and the current controlling transistor  303  have the same conductivity. The size, characteristics, and operating region of each transistor may be set in the same manner as those of Embodiment Mode 1. 
   The gate of the switching transistor  301  is connected to a scan line Gj (j=1 to y). Either the source or drain of the switching transistor  301  is connected to a signal line Si (i=1to x), and the other is connected to the gate of the current controlling transistor  303 . The gate of the driving transistor  302  is connected to the source thereof. The driving transistor  302  and the current controlling transistor  303  are each connected to a power supply line Vi (i=1 to x) and the light emitting element  304  so that a current supplied from the power supply line Vi (i=1 to x) is supplied to the light emitting element  304  as a drain current of the driving transistor  302  and of the current controlling transistor  303 . In this embodiment mode, the source of the current controlling transistor  303  is connected to the power supply line Vi (i=1 to x) and the drain of the driving transistor  302  is connected to a pixel electrode of the light emitting element  304 . 
   It is to be noted that the source and gate of the driving transistor  302  may be connected to the power supply line Vi (i=1 to x), and the drain of the current controlling transistor  303  may be connected to the pixel electrode of the light emitting element  304 . 
   The light emitting element  304  comprises an anode, a cathode, and a light emitting layer interposed between the anode and cathode. As shown in  FIG. 3 , when the anode of the light emitting element  304  is connected to the driving transistor  302 , the anode is a pixel electrode and the cathode is a counter electrode. The counter electrode of the light emitting element  304  and the power supply line Vi (i=1 to x) have a potential difference so that current flows into the light emitting element  304  in the forward bias direction. 
   One of the two electrodes of the capacitor  305  is connected to the power supply line Vi (i=1 to x), and the other is connected to the gate of the current controlling transistor  303 . The capacitor  305  is disposed so as to store a potential difference between the two electrodes of the capacitor  305  when the switching transistor  301  is not selected (off state). It is to be noted that although  FIG. 3  shows a configuration disposing the capacitor  305 , the invention is not limited to this and an alternative configuration without the capacitor  305  may be employed as well. 
   In  FIG. 3 , each of the driving transistor  302  and the current controlling transistor  303  is a P-type transistor, and the drain of the driving transistor  302  is connected to the anode of the light emitting element  304 . On the contrary, in the case where each of the driving transistor  302  and the current controlling transistor  303  is an N-type transistor, the source of the driving transistor  302  is connected to the cathode of the light emitting element  304 . In this case, the cathode of the light emitting element  304  is a pixel electrode and the anode thereof is a counter electrode. 
   The operation of the pixel shown in  FIG. 3  is the same as that shown in  FIG. 1 . 
   Embodiment Mode 4 
   Described in this embodiment mode is a different configuration of a pixel of the light emitting device of the invention from those of Embodiment Modes 1 to 3. 
   The pixel shown in  FIG. 4  comprises a light emitting element  404 , a switching transistor  401 , a driving transistor  402 , a current controlling transistor  403 , and a transistor (erasing transistor)  406  for erasing a potential of a written video signal. It is also possible to dispose a capacitor  405  in addition to the above elements. 
   The driving transistor  402  and the current controlling transistor  403  have the same conductivity. The size, characteristics, and operating region of each transistor may be set in the same manner as those of Embodiment Mode 1. 
   The gate of the switching transistor  401  is connected to a first scan line Gaj (j=1 to y). Either the source or drain of the switching transistor  401  is connected to a signal line Si (i=1to x), and the other is connected to the gate of the current controlling transistor  403 . The gate of the erasing transistor  406  is connected to a second scan line Gaj (j=1 to y). Either the source or drain of the erasing transistor  406  is connected to a second scan line Gaj (i=1to x), and the other is connected to the gate of the current controlling transistor  403 . The gate of the driving transistor  402  is connected to the source thereof. The driving transistor  402  and the current controlling transistor  403  are each connected to a power supply line Vi (i=1 to x) and the light emitting element  404  so that a current supplied from the power supply line Vi (i=1 to x) is supplied to the light emitting element  404  as a drain current of the driving transistor  402  and of the current controlling transistor  403 . In this embodiment mode, the source of the current controlling transistor  403  is connected to the power supply line Vi (i=1 to x) and the drain of the driving transistor  402  is connected to a pixel electrode of the light emitting element  404 . 
   It is to be noted that the source of the driving transistor  402  may be connected to the power supply line Vi (i=1 to x), and the drain of the current controlling transistor  403  may be connected to the pixel electrode of the light emitting element  404 . 
   The light emitting element  404  comprises an anode, a cathode, and a light emitting layer interposed between the anode and cathode. As shown in  FIG. 4 , when, the anode of the light emitting element  404  is connected to the driving transistor  402 , the anode is a pixel electrode and the cathode is a counter electrode. The counter electrode of the light emitting element  404  and the power supply line Vi (i=1 to x) have a potential difference so that current flows into the light emitting element  404  in the forward bias direction. 
   One of the two electrodes of the capacitor  405  is connected to the power supply line Vi (i=1 to x), and the other is connected to the gate of the current controlling transistor  403 . 
   In  FIG. 4 , each of the driving transistor  402  and the current controlling transistor  403  is a P-type transistor, and the drain of the driving transistor  402  is connected to the anode of the light emitting element  404 . On the contrary, in the case where each of the driving transistor  402  and the current controlling transistor  403  is an N-type transistor, the source of the driving transistor  402  is connected to the cathode of the light emitting element  404 . In this case, the cathode of the light emitting element  404  is a pixel electrode and the anode thereof is a counter electrode. 
   The operation of the pixel shown in  FIG. 4  is the same as that shown in  FIG. 2 . 
   In addition, either an N-type transistor or a P-type transistor may be employed as a switching transistor and an erasing transistor used in the invention. 
   Embodiment 1 
   Described in this embodiment are a configuration of an active matrix display device to which the pixel configuration of the invention is applied and its drive. 
     FIG. 5  shows a block diagram of an external circuit and a schematic view of a panel. 
   An active matrix display device shown in  FIG. 5  comprises an external circuit  5004  and a panel  5010 . The external circuit  5004  comprises an A/D converter unit  5001 , a power supply unit  5002 , and a signal generator unit  5003 . The A/D converter unit  5001  converts an image data signal which is inputted as an analog signal into a digital signal (video signal), and supplies it to a signal driver circuit  5006 . The power supply unit  5002  generates power having a predetermined voltage from the power supplied from a battery or an outlet, and supplies it to the signal driver circuit  5006 , scan driver circuits  5007 , an OLED  5011 , the signal generator unit  5003 , and the like. The signal generator unit  5003  is inputted with power, an image signal, a synchronizing signal, and the like. Also, it generates a clock signal and the like for driving the signal driver circuit  5006  and the scan driver circuits  5007 . 
   A signal and power from the external circuit  5004  are inputted to an internal circuit and the like through an FPC and an FPC connection portion  5005  in the panel  5010 . 
   The pixel  5010  comprises a substrate  5008  mounting the FPC connection portion  5005 , the internal circuit, and the OLED  5011 . The internal circuit comprises the signal driver circuit  5006 , the scan driver circuits  5007 , a pixel portion  5009 . Although  FIG. 5  employs the pixel shown in Embodiment Mode 1, an alternative pixel configuration shown in other embodiment modes of the invention may be employed as well. 
   The pixel portion  5009  is disposed in the center of the substrate, and the signal driver circuit  5006  and the scan driver circuit  5007  are disposed on the periphery of the pixel portion  5009 . The OLED  5011  and a counter electrode of the OLED are formed over the pixel portion  5009 . 
     FIG. 6  shows a more detailed block diagram of the signal driver circuit  5006 . 
   The signal driver circuit  5006  comprises a shift register  6002  including a plurality of stages of D-flip flops  6001 , a data latch circuit  6003 , a latch circuit  6004 , a level shifter  6005 , a buffer  6006 , and the like. 
   It is assumed that a clock signal (S-CK), an inverted clock signal (S-CKB), a start pulse (S-SP), a video signal (DATA), and a latch pulse (LatchPulse) are inputted. 
   First, in accordance with a clock signal, an inverted clock signal, and a start pulse, a sampling pulse is sequentially outputted from the shift register  6002 . In accordance with the timing in which the sampling pulse is inputted to the data latch circuit  6003 , a video signal is sampled and thus stored. This operation is sequentially performed from the first column. 
   When the storage of a video signal is completed in the data latch circuit  6003  on the last stage, a latch pulse is inputted during a horizontal retrace period, and the video signal stored in the data latch circuit  6003  is transferred to the latch circuit  6004  all at once. Then, it is level-shifted in the level shifter  6005 , and adjusted in the buffer  6006  so as to be outputted to signal lines S 1  to Sn all at once. At this time, an H-level or an L-level signal is inputted to pixels in the row selected by the scan driver circuits  5007 , thereby controlling a light emission or non-emission of the OLED  5011 . 
   Although the active matrix display device shown in this embodiment comprises the panel  5010  and the external circuit  5004  each formed independently, they may be integrally formed on the same substrate. Also, although the display device employs. OLED in this embodiment, other light emitting elements can be employed as well. In addition, the level shifter  6005  and the buffer  6006  may not necessarily be provided in the signal driver circuit  5006 . 
   Embodiment 2 
   Described in this embodiment is a top plan view of the pixel shown in  FIG. 2 .  FIG. 7  shows a top plan view of a pixel of this embodiment. 
   Reference numeral  7001  denotes a signal line,  7002  denotes a power supply line,  7004  denotes a first scan line, and  7003  denotes a second scan line. In this embodiment, the signal line  7001  and the power supply line  7002  are formed of the same conductive film, and the first scan line  7004  and the second scan line  7003  are formed of the same conductive film. Reference numeral  7005  denotes a switching transistor, and a part of the first scan line  7004  functions as its gate electrode. Reference numeral  7006  denotes an erasing transistor, and a part of the second scan line  7003  functions as its gate electrode. Reference numeral  7007  denotes a driving transistor, and  7008  denotes a current controlling transistor. An active layer of the driving transistor  7007  is curved so that its L/W becomes larger than that of the current controlling transistor  7008 . Reference numeral  7009  denotes a pixel electrode, and light is emitted in its overlapped area (light emitting area)  7010  with a light emitting layer and a cathode (neither of them is shown). 
   It is to be noted that the top plan view of the invention shown in this embodiment is only an example, and the invention is, needless to say, not limited to this. 
   Embodiment 3 
   Described in this embodiment is an example of a top plan view of the pixel shown in  FIG. 2 , which is different from that shown in  FIG. 7 .  FIG. 10  shows a top plan view of a pixel of this embodiment. 
   Reference numeral  10001  denotes a signal line,  10002  denotes a power supply line,  10004  denotes a first scan line, and  10003  denotes a second scan line. In this embodiment, the signal line  10001  and the power supply line  10002  are formed of the same conductive film, and the first scan line  10004  and the second scan line  10003  are formed of the same conductive film. Reference numeral  10005  denotes a switching transistor, and a part of the first scan line  10004  functions as its gate electrode. Reference numeral  10006  denotes a erasing transistor, and a part of the second scan line  10003  functions as its gate electrode. Reference numeral  10007  denotes a driving transistor, and  10008  denotes a current controlling transistor. An active layer of the driving transistor  10007  is curved so that its L/W becomes larger than that of the current controlling transistor  10008 . Reference numeral  10009  denotes a pixel electrode, and light is emitted in its overlapped area (light emitting area)  10010  with a light emitting layer and a cathode (neither of them is shown). 
   It is to be noted that the top plan view of this embodiment is only an example, and the invention is, needless to say, not limited to this. 
   Embodiment 4 
   Described in this embodiment is a cross-sectional structure of a pixel. 
     FIG. 11A  shows a cross-sectional view of a pixel in which a driving transistor  11021  is a P-type transistor and light emitted from a light emitting element  11022  is transmitted to an anode side  11023 . In  FIG. 11A , the anode  11023  of the light emitting element  11022  is electrically connected to the driving transistor  11021 , and a light emitting layer  11024  and a cathode  11025  are laminated on the anode  11023  in this order. As for the cathode  11025 , known material can be used as long as it is a conductive film having a small work function and reflecting light. For example, Ca, Al, CaF, MgAg, AlLi, and the like are desirably used. The light emitting layer  11024  may comprise a single layer or multiple layers. When it comprises multiple layers, a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injection layer are sequentially laminated in this order on the cathode  11023 . It is to be noted that not all of the above layers are necessarily provided. The anode  11023  may be formed of a transparent conductive film which transmits light, such as the one comprising ITO or the one in which indium oxide is mixed with zinc oxide (ZnO) of 2 to 20%. 
   The overlapped portion of the anode  11023 , the light emitting layer  11024 , and the cathode  11025  corresponds to the light emitting element  11022 . In the case of the pixel shown in  FIG. 11A , light emitted from the light emitting element  11022  is transmitted to the anode  11023  side as shown by an outline arrow. 
     FIG. 11B  shows a cross-sectional view of a pixel in which a driving transistor  11001  is an N-type transistor and light emitted from a light emitting element  11002  is transmitted to an anode side  11005 . In  FIG. 11B , a cathode  11003  of the light emitting element  11002  is electrically connected to the driving transistor  11001 , and a light emitting layer  11004  and an anode  11005  are laminated on the cathode  11003  in this order. As for the cathode  11005 , known material can be used as long as it is a conductive film having a small work function and reflecting light. For example, Ca, Al, CaF, MgAg, AlLi, and the like are desirably used. The light emitting layer  11004  may comprise a single layer or multiple layers. When it comprises multiple layers, a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injection layer are sequentially laminated in this order on the cathode  11003 . It is to be noted that not all the above layers are necessarily provided. The anode  11005  may be formed of a transparent conductive film which transmits light, such as the one comprising ITO or the one in which indium oxide is mixed with zinc oxide (ZnO) of 2 to 20%. 
   The overlapped portion of the anode  11003 , the light emitting layer  11004 , and the cathode  11005  corresponds to the light emitting element  11002 . In the case of the pixel shown in  FIG. 11B , light emitted from the light emitting element  11002  is transmitted to the anode  11003  side as shown by an outline arrow. 
   It is to be noted that although shown in this embodiment is the one in which a driving transistor is electrically connected to a light emitting element, a current controlling transistor may be interposed between the driving transistor and the light emitting element. 
   Embodiment 5 
   Described in this embodiment is an example of the drive timing where the pixel configuration of Embodiment Mode 2 is employed. 
     FIG. 12A  shows an example using a digital time gray scale method for a 4-bit gray scale display. In data storage periods Ts 1  to Ts 4 , the ratio of the time length is assumed to be Ts 1 :Ts 2 :Ts 3 :Ts 4  =2 3 :2 2 :2 1 :2 0 =4:2:1. 
   The operation is described next. First, in a writing period Tb 1 , the first scan line is selected from the first row in sequence, thereby turning ON the switching transistor. Next, a video signal is inputted to each pixel from a signal line, thereby controlling a light emission or non-light emission of each pixel according to a potential of the signal. Once the video signal is written, that row proceeds to the data storage period Ts 1  immediately. The same operation is performed up to the last row, and thus a period Ta 1  terminates. Subsequently, a writing period Tb 2  is started from the row in which the data storage period Ts 1  is complete in sequence. 
   In the sub-frame period having the shorter data storage period than the writing period (corresponds to a period Ta 4  here), an erasing period  2102  is provided so that a next writing period is not started immediately after the data storage period. In the erasing period, a light emitting element is forced to be in a non-emission state. 
   Taken as an example here is the case of expressing a 4-bit gray scale display, however the number of bits and gray scales is not limited to this. In addition, light emission is not necessarily performed from Ts 1  to Ts 4  in sequence. It may be performed at random, or divided into a plurality of periods. 
   Embodiment 6 
   The display device of the invention can be used in display portions of various electronic apparatuses. In particular, the display device of the invention is desirably applied to a mobile device that requires low power consumption. 
   Electronic apparatuses using the display device of the invention include a portable information device (a cellular phone, a mobile computer, a portable game machine, an electronic book, and the like), a video camera, a digital camera, a goggle display, a display device, a navigation system, and the like. Specific examples of these electronic apparatuses are shown in  FIGS. 8A to 8D . 
     FIG. 8A  shows a display device which includes a housing  8001 , an audio output portion  8002 , a display portion  8003 , and the like. The display device of the invention can be used for the display portion  8003 . Note that, the display device includes all the information display devices for personal computers, television broadcast reception, advertisement displays, and the like. 
     FIG. 8B  shows a mobile computer which includes a main body  8101 , a stylus  8102 , a display portion  8103 , operation keys  8104 , an external interface  8105 , and the like. The display device of the invention can be used for the display portion  8103 . 
     FIG. 8C  shows a game machine which includes a main body  8201 , a display portion  8202 , operation keys  8203 , and the like. The display device of the invention can be used for the display portion  8202 . 
     FIG. 8D  shows a cellular phone which includes a main body  8301 , an audio output portion  8302 , a display portion  8304 , operation switches  8305 , an antenna  8306 , and the like. The display device of the invention can be used for the display portion  8304 . 
   As described above, an application range of the invention is so wide that the invention can be applied to electronic apparatuses in various fields. 
   Although the invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the invention hereinafter defined, they should be constructed as being included therein.