Patent Publication Number: US-7218297-B2

Title: Light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation application of U.S. application Ser. No. 10/060,709, filed Jan. 29, 2002, now U.S. Pat. No. 6,788,003 now allowed, which claims the benefit of a foreign priority application filed in Japan as Serial No. 2001-019651 on Jan. 29, 2001. This application claims priority to each of these prior applications, and the disclosures of the prior applications are considered part of (and are incorporated by reference in) the disclosure of this application. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an OLED panel in which an organic light emitting device (OLED) formed on a substrate is enclosed between the substrate and a cover member. Also, the present invention relates to an OLED module in which an IC is mounted on the OLED panel. Note that, in this specification, the OLED panel and the OLED module are generically called light emitting devices. The present invention further relates to an electronic device using the light emitting device. 
   2. Description of the Related Art 
   An OLED emits light by itself, and thus, has high visibility. The OLED does not need a backlight necessary for a liquid crystal display device (LCD), which is suitable for a reduction of a light emitting device in thickness. Also, the OLED has no limitation on a viewing angle. Therefore, the light emitting device using the OLED has recently been attracting attention as a display device that substitutes for a CRT or the LCD. 
   The OLED includes a layer containing an organic compound in which luminescence generated by application of an electric field (electroluminescence) is obtained (organic light emitting material) (hereinafter, referred to as organic light emitting layer), an anode layer and a cathode layer. A light emission in returning to a base state from a singlet excitation state (fluorescence) and a light emission in returning to a base state from a triplet excitation state (phosphorescence) exist as the luminescence in the organic compound. The light emitting device of the present invention may use one or both of the above-described light emissions. 
   Note that, in this specification, all the layers provided between an anode and a cathode of the OLED are defined as the organic light emitting layers. The organic light emitting layers specifically include a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, an electron transporting layer and the like. The OLED basically has a structure in which an anode/a light emitting layer/a cathode are laminated in order. Besides this structure, the OLED may take a structure in which an anode/a hole injecting layer/a light emitting layer/a cathode are laminated in order or a structure in which an anode/a hole injecting layer/a light emitting layer/an electron transporting layer/a cathode are laminated in order. 
   In putting a light emitting device to practical use, a serious problem at present is a reduction in the luminance of the OLED, which is accompanied with deterioration of the organic light emitting material contained in the organic light emitting layer. 
   The organic light emitting material in the organic light emitting layer is easily affected by moisture, oxygen, light and heat, and the deterioration of the organic light emitting material is promoted by these substances. Specifically, speed of the deterioration of the organic light emitting layer is influenced by a structure of a device for driving the light emitting device, a characteristic of the organic light emitting material constituting the organic light emitting layer, a material for an electrode, conditions in a manufacturing process, a method of driving the light emitting device, and the like. 
   Even when a constant voltage is applied to the organic light emitting layer from a pair of electrodes, the luminance of the OLED is lowered due to the deterioration of the organic light emitting layer. Then, if the luminance of the OLED is lowered, an image displayed on the light emitting device becomes unclear. Note that, in this specification, a voltage applied to the organic light emitting layer from one pair of electrodes is defined as an OLED driving voltage (Vel). 
   Further, in a color display mode in which three kinds of OLEDs corresponding to R (red), G (green) and B (blue) are used, the organic light emitting material constituting the organic light emitting layer differs depending on the corresponding color of the OLED. If the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors, the luminance of the OLED differs depending on the color with the lapse of time. Thus, an image having a desired color can not be displayed on the light emitting device. 
   Furthermore, the luminance of the OLED has large temperature depending property, and thus, there has been a problem in that luminance of a display and a tone vary in accordance with the temperature in constant voltage drive. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above, and an object of the present invention is therefore to provide a light emitting device in which a change of luminance of an OLED is suppressed and a desired color display can be stably performed even when an organic light emitting layer is somewhat deteriorated or when an environmental temperature is varied. 
   Between a light emission with a constant OLED driving voltage and a light emission with a constant current flowing through the OLED, the present inventor directs an attention to the fact that a reduction of the luminance of the OLED due to deterioration is smaller in the latter. Note that the current flowing through the OLED is called an OLED driving current (Iel) in this specification. 
     FIG. 2  shows a change of the luminance of the OLED between a case where the OLED driving voltage is constant and a case where the OLED driving current is constant. As shown in  FIG. 2 , the change of the luminance due to deterioration is smaller in the OLED with the constant OLED driving current. This is because not only an inclination of a straight line L-I becomes small but also a curve I-V itself moves to the lower side when the OLED is deteriorated (see  FIGS. 18A and 18B ). 
   Thus, the present inventor devised a light emitting device with a simple structure in which an OLED driving voltage can be corrected such that an OLED driving current is always kept constant even if the OLED driving current is varied due to deterioration or the like. 
   Specifically, in the present invention, a pixel portion for measuring the OLED driving current is provided in the light emitting device besides a pixel portion for displaying an image. It is preferable that the monitor pixel portion can display some images in order to be effectively used as a display portion. However, it is not essential that the monitor pixel portion can perform an image display. Hereinafter, in order to clearly distinguish between the above-described two pixel portions, the pixel portion in which an image display is aimed is called the display pixel portion (first pixel portion) and the pixel portion in which the measurement of the OLED driving current is aimed is called the monitor pixel portion (second pixel portion) through this specification. 
   The display pixel portion and the monitor pixel portion have the same structures of their respective pixels, and can be described with the same circuit diagrams. With regard to OLEDs of a pixel of the display pixel portion (hereinafter referred to as display pixel or first pixel) and a pixel of the monitor pixel portion (hereinafter referred to as monitor pixel or second pixel), the OLED driving voltages at the time when the luminance is maximum are controlled by a variable power supply, and both the voltages are preferably kept to have equivalent values. 
   Note that the variable power supply indicates a power supply in which a voltage supplied to a circuit or an element is not constant but variable in this specification. 
   Further, the light emitting device of the present invention includes a first means for measuring the OLED driving current of the OLED of the monitor pixel portion (hereinafter referred to as monitor OLED or second OLED), a second means for calculating a voltage applied to the OLED based on the measured value, and a third means for actually controlling the voltage value. 
   Note that the second means may be a means for comparing the current measured value and a reference value, and the third means may be a means for controlling the variable power supply to shorten a difference between the measured value and the reference value and correcting the OLED driving voltages of the OLED of the display pixel (hereinafter referred to as display OLED or first OLED) and the monitor OLED in the case where the difference exists. 
   The monitor pixel portion is input with a video signal of a different system from that of a video signal to be input to the display pixel portion. However, both the video signals are the same in the point that the signals each include gradation information, and only the system of an image to be displayed differs between the signals. Hereinafter, the video signal to be input to the display pixel portion is referred to as the display video signal and the video signal to be input to the monitor pixel portion is referred to as the monitor video signal. 
   When the OLED driving current of the monitor OLED is measured, an image for monitor (hereinafter referred to as monitor image) is displayed on the monitor pixel portion in accordance with the monitor video signal. The monitor image may be either a static image or a dynamic image. Further, the same gradation may be displayed on all the pixels. Moreover, it is preferable that the monitor image in which an average value in time is substantially the same between the OLED drive currents of the display OLED and the monitor OLED is displayed such that the degree of deterioration becomes the same between the display OLED and the monitor OLED. 
   Note that the reference value of the current does not need to be fixed at the same value at all times. A plurality of monitor images with different reference current values are prepared, and the monitor image may be selected every monitor. Of course, several kinds of monitor images with the same reference current value may be prepared. 
   With the above-described structure, in the light emitting device of the present invention, the reduction of the luminance of the OLED can be suppressed even with the deterioration of the organic light emitting layer. As a result, a clear image can be displayed. 
   Further, in the color display mode in which three kinds of OLEDs corresponding to R (red), G (green) and B (blue) are used, monitor pixel portions corresponding to the respective colors may be provided, and the OLED driving current may be measured for every OLED of each color to thereby correct the OLED driving voltage. With this structure, the balance of luminance among the respective colors is prevented from being lost, and a desired color can be displayed even when the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors. 
   Further, a temperature of the organic light emitting layer is influenced by an outer temperature, heat generated by the OLED panel itself, or the like. Generally, when the OLED is driven at a constant voltage, the value of the flowing current changes in accordance with the temperature.  FIG. 3  shows a change of a voltage-current characteristic of the OLED when the temperature of the organic light emitting layer is changed. When the voltage is constant, if the temperature of the organic-light emitting layer becomes higher, the OLED driving current becomes larger. Since the relationship between the OLED driving current and the luminance of the OLED is substantially proportional, the luminance of the OLED becomes higher as the OLED driving current becomes larger. In  FIG. 2 , the constant voltage luminance shows a vertical period for about 24 hours. This is because a temperature difference between day and night is reflected. However, in the light emitting device of the present invention, the OLED driving current can always be kept constant by the correction of the OLED driving voltage even if the temperature of the organic light emitting layer is changed. Therefore, a constant luminance can be obtained without being influenced by the temperature change, and also, the increase in power consumption with the temperature rise can be prevented. 
   Moreover, a degree of the change of the OLED driving current in the temperature change generally differs depending on the kind of the organic light emitting material. Thus, in the color display, the luminances of the OLEDs of the respective colors may be separately changed in accordance with the temperature. However, in the light emitting device of the present invention, the constant luminance can be obtained without being influenced by the temperature change. Thus, the balance of luminance among the respective colors is prevented from being lost, and a desired color can be displayed. 
   Incidentally, the present invention is particularly effective for an active matrix light emitting device of digital time gradation drive, and is also effective for an active matrix light emitting device of analogue gradation drive. Further, the present invention can be applied to a passive light emitting device. 
   Furthermore, the monitor pixel portion can be effectively used in a display of icons, logos, patterns, indicators and the like, and this can eliminate waste. In addition, the monitor takes the same type as the pixel, whereby the deterioration of the pixel OLED can be caught with higher definition. Thus, the luminance correction can be performed with ease and with accuracy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a block diagram of a light emitting device of the present invention: 
       FIG. 2  shows a change of luminance due to deterioration in constant current drive or in constant voltage drive; 
       FIG. 3  shows a change of a current in accordance with a temperature of an organic light emitting layer; 
       FIG. 4  is a pixel circuit diagram of the light emitting device of the present invention; 
       FIG. 5  shows a change of a voltage in accordance with correction: 
       FIG. 6  is a block diagram of a correction circuit: 
       FIG. 7  shows a relationship between a deviation current and a correction voltage; 
       FIG. 8  is a pixel circuit diagram of a light emitting device of the present invention; 
       FIG. 9  is a diagram showing a method of driving the light emitting device of the present invention; 
       FIGS. 10A and 10B  are block diagrams of driver circuits: 
       FIGS. 11A to 11C  show an appearance of the light emitting device of the present invention; 
       FIG. 12  shows an appearance of the light emitting device of the present invention: 
       FIGS. 13A to 13D  show a method of manufacturing the light emitting device of the present invention; 
       FIGS. 14A to 14C  show the method of manufacturing the light emitting device of the present invention; 
       FIGS. 15A and 15B  show the method of manufacturing the light emitting device of the present invention; 
       FIGS. 16A and 16B  show a method of manufacturing the light emitting device of the present invention; 
       FIGS. 17A to 17H  show electronic equipment using the light emitting device of the present invention: and 
       FIGS. 18A and 18B  show changes of a voltage-current characteristic and a current-luminance characteristic of an OLED due to deterioration. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the structure of the present invention will be described. 
     FIG. 1  is a block diagram of the structure of an OLED panel of the present invention. Reference numeral  101  denotes a display pixel portion in which a plurality of display pixels  102  are formed in matrix. Reference numeral  103  denotes a monitor pixel portion in which a plurality of monitor pixels  104  are formed in matrix. Further, reference numerals  105  and  106  denote a source line driver circuit and a gate line driver circuit, respectively. 
   The display pixel portion  101  and the monitor pixel portion  103  may be formed on the same substrate or formed on different substrates. Note that, although the source line driver circuit  105  and the gate line driver circuit  106  are formed on the substrate on which the display pixel portion  101  and the monitor pixel portion  103  are formed in  FIG. 1 , the present invention is not limited to this structure. The source line driver circuit  105  and the gate line driver circuit  106  may be formed on the substrate different from the substrate on which the pixel portion  101  or the monitor pixel portion  103  is formed, and may be connected to the pixel portion  101  or the monitor pixel portion  103  through a connector such as an FPC. Further, one source line driver circuit  105  and one gate line driver circuit  106  are provided in  FIG. 1 , but the present invention is not limited to this structure. The number of source line driver circuits  105  and the number of gate line driver circuits  106  may be arbitrarily set by a designer. 
   Further, in  FIG. 1 , source lines S 1  to Sx, power supply lines V 1  to Vx and gate lines G 1  to Gy are provided in the display pixel portion  101 . Then, a source line S(x+1), a power supply line V(x+1) and the gate lines G 1  to Gy are provided in the monitor pixel portion  103 . The number of source lines and the number of power supply lines are not always the same. Further, in addition to these lines, different lines may be provided. Also in  FIG. 1 , an example in which only pixels of one line having the source line S(x+1) are provided in the monitor pixel portion  103  is shown. However, the light emitting device of the present invention is not limited to this structure. Pixels of plural lines having a plurality of source lines may be provided in the monitor pixel portion  103 . The number of pixels provided in the monitor pixel portion  103  can be appropriately selected by a designer. 
   Display OLEDs  107  are provided in the respective display pixels  102 . Further, monitor OLEDs  108  are provided in the respective monitor pixels  104 . The display OLED  107  and the monitor OLED  108  each have an anode and a cathode. In this specification, in the case where the anode is used as a pixel electrode (first electrode), the cathode is called an opposing electrode (second electrode) while, in the case where the cathode is used as a pixel electrode, the anode is called an opposing electrode. 
   The pixel electrode of each of the display OLEDs  107  is connected to one of the power supply lines V 1  to Vx through one TFT or a plurality of TFTs. The power supply lines V 1  to Vx are all connected to a display variable power supply  109 . The opposing electrodes of the display OLEDs  107  are all connected to the display variable power supply  109 . Note that the opposing electrodes of the display OLEDs  107  may be connected to the display variable power supply  109  through one element or a plurality of elements. 
   On the other hand, the pixel electrode of each of the monitor OLEDs  108  is connected to the power supply line V(x+1) through one TFT or a plurality of TFTs. The power supply line V(x+1) is connected to a monitor variable power supply  110  through an ammeter  111 . The opposing electrodes of the monitor OLEDs  108  are all connected to the monitor variable power supply  110 . Note that the opposing electrodes of the monitor OLEDs  108  may be connected to the monitor variable power supply  110  through one element or a plurality of elements. 
   Note that, in  FIG. 1 , the display variable power supply  109  and the monitor variable power supply  110  are connected such that the power supply line side is kept at a high potential (Vdd) while the opposing electrode side is kept at a low potential (Vss). However, the present invention is not limited to this structure, and the display variable power supply  109  and the monitor variable power supply  110  may be connected such that the current flown through the display OLED  107  and the monitor OLED  108  has a forward bias. 
   Further, a position where the ammeter  111  is provided is not necessarily located between the monitor variable power supply  110  and the power supply lines. The position may be located between the monitor variable power supply  110  and the opposing electrodes. 
   Reference numeral  112  denotes a correction circuit which controls the display variable power supply  109  and the monitor variable power supply  110  based on a current value (measured value) measured with the ammeter  111 . Specifically, the correction circuit  112  controls the voltage supplied to the opposing electrodes of the display OLEDs  107  and the power supply lines V 1  to Vx from the display variable power supply  109  and the voltage supplied to the opposing electrodes of the monitor OLEDs  108  and the power supply line V(x+1) from the monitor variable power supply  110 . 
   Incidentally, the ammeter  111 , the display variable power supply  109 , the monitor variable power supply  110  and the correction circuit  112  may be formed on the substrate different from the substrate on which the display pixel portion  101  and the monitor pixel portion  103  are formed, and may be connected to the display pixel portion  101  and the monitor pixel portion  103  through a connector or the like. If possible, the above-described components may be formed on the same substrate as the display pixel portion  101  and the monitor pixel portion  103 . 
   Further, in a color display mode, a display variable power supply, a monitor variable power supply, a correction circuit and an ammeter may be provided for each color, and an OLED driving voltage may be corrected in the OLED of each color. Note that, at this time, the correction circuit may be provided for each color, or the common correction circuit may be provided for the OLEDs of plural colors. 
     FIG. 4  shows the detailed structure of the monitor pixel  104 . Note that the display pixel  102  has the same device connection structure as the monitor pixel  104 . 
   The monitor pixel  104  in  FIG. 4  has the source line S(x+1), the gate line Gj (j=1 to y,) the power supply line V(x+1), a switching TFT  120 , a driving TFT  121 , a capacitor  122  and the monitor OLED  108 . The pixel structure shown in  FIG. 4  is just one example, and the number of lines and elements of the pixel, the kind thereof and the connection are not limited to those in the structure shown in  FIG. 4 . The light emitting device of the present invention may take any structure provided that the OLED driving voltage of the OLED of each pixel can be controlled by the variable power supply. 
   In  FIG. 4 , a gate electrode of the switching TFT  120  is connected to the gate line Gj. One of a source region and a drain region of the switching TFT  120  is connected to the source line S(x+1), and the other is connected to a gate electrode of the driving TFT  121 . Then, one of a source region and a drain region of the driving TFT  121  is connected to the power supply line V(x+1), and the other is connected to the pixel electrode of the monitor OLED  108 . The capacitor  122  is formed between the gate electrode of the driving TFT  121  and the power supply line V(x+1). 
   In the monitor pixel  104  shown in  FIG. 4 , the potential of the gate line Gj is controlled by the gate line driver circuit  106 , and the source line S(x+1) is input with a monitor video signal by the source line driver circuit  105 . When the switching TFT  120  is turned on, the monitor video signal input to the source line S(x+1) is input to the gate electrode of the driving TFT  121  through the switching TFT  120 . Then, when the driving TFT  121  is turned on in accordance with the monitor video signal, the OLED driving voltage is applied between the pixel electrode and the opposing electrode of the monitor OLED  108  by the monitor variable power supply  110 . Thus, the monitor OLED  108  emits light. 
   While the monitor OLED  108  is emitting light, a current is measured with the ammeter  111 . The measured value as data is sent to the correction circuit  112 . The period for the measurement of the current differs depending on a performance of the ammeter  111 , and the period needs to have the length equal to or longer than that of the period during which the measurement can be performed. Further, with the ammeter  111 , the average value or the maximum value of the current flowing in the measurement period is made to be read. 
   In the correction circuit  112 , the measured value of the current and a set current value (reference value) are compared. Then, in the case where there is some difference between the measured value and the reference value, the correction circuit  112  controls the monitor variable power supply  110  and the display variable power supply  109 , and corrects the voltage between the power supply line V(x+1) and the opposing electrode of the monitor OLED  108  and the voltage between the power supply lines V 1  to Vx and the opposing electrodes of the display OLEDs  107 . Thus, the OLED driving voltages in the display OLED  107  and the monitor OLED  108  are corrected, and an OLED driving current with a desired size flows. 
   Note that the OLED driving voltage may be corrected by controlling the potential at the power supply line side or may be corrected by controlling the potential at the opposing electrode side. Further, the OLED driving voltage may be corrected by controlling both the potential at the power supply line side and the potential at the opposing electrode side. 
     FIG. 5  shows a change of the OLED driving voltage of the OLED of each color in the case where the potential at the power supply line side is controlled in a color light emitting device. In  FIG. 5 , Vr indicates the OLED driving voltage before correction in a display OLED (R) for R, and Vr o  indicates the OLED driving voltage after correction. Similarly, Vg indicates the OLED driving voltage before correction in a display OLED (G) for G, and Vg o  indicates the OLED driving voltage after correction. Vb indicates the OLED driving voltage before correction in a display OLED (B) for B, and Vb o  indicates the OLED driving voltage after correction. 
   In case of  FIG. 5 , the potentials of the opposing electrodes (opposing potentials) are fixed at the same level in all of the display OLEDs. The OLED driving current is measured for every display OLED of each color, and the potential of the power supply line (power supply potential) is controlled by the display variable power supply, whereby the OLED driving voltage is corrected. 
   Incidentally, two variable power supplies, that is, the display variable power supply corresponding to the display pixel portion and the monitor variable power supply corresponding to the monitor pixel portion are used in  FIG. 1 , but the present invention is not limited to this structure. One variable power supply may be substituted for the display variable power supply and the monitor variable power supply. 
   In the light emitting device of the present invention, with the above-described structure, there can be obtained the same change of luminance as that obtained when the OLED driving current in  FIG. 2  is made constant. 
   According to the present invention, with the above-described structure, the reduction of the luminance of the OLED can be suppressed even if the organic light emitting layer is deteriorated. As a result, a clear image can be displayed. Further, in case of the light emitting device with the color display in which the OLEDs corresponding to respective colors are used, the balance of luminance among the respective colors is prevented from being lost, and a desired color can be displayed even when the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors. 
   Further, the change of the luminance of the OLED can be suppressed even if the temperature of the organic light emitting layer is influenced by the outer temperature, the heat generated by the OLED panel itself, or the like. Also, the increase in power consumption with the temperature rise can be prevented. Further, in case of the light emitting device with the color display, the change of the luminance of the OLED of each color can be suppressed without being influenced by the temperature change. Thus, the balance of the luminance among the respective colors is prevented from being lost, and a desired color can be displayed. 
   EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described. 
   Embodiment 1 
   In this embodiment, the detailed structure of a correction circuit of a light emitting device of the present invention is described. 
     FIG. 6  is a block diagram of the structure of the correction circuit in this embodiment. A correction circuit  203  includes an A/D converter circuit  204 , a memory for measured value  205 , a calculation circuit  206 , a memory for reference value  207  and a controller  208 . 
   A current value (measured value) measured with an ammeter  201  is input to the A/D converter circuit  204  of the correction circuit  203 . In the A/D converter circuit  204 , an analogue measured value is converted into a digital one. Digital data of the converted measured value is input to the memory for measured value  205  to be held. 
   On the other hand, digital data of the reference value of an OLED driving current is held in the memory for reference value  207 . In the calculation circuit  206 , the digital data of the measured value held in the memory for measured value  205  and the digital data of the reference value held in the memory for reference value  207  are read out to be compared. 
   Then, in accordance with the comparison between the digital data of the measured value and the digital data of the reference value, a monitor variable power supply  202  and a display variable power supply  209  are controlled in order to make the value of the current actually flowing through the ammeter  201  close to the reference value. More specifically, the monitor variable power supply  202  and the display variable power supply  209  are controlled, whereby the voltage between the power supply lines V 1  to Vx and the opposing electrodes of the display OLEDs and the voltage between the power supply line V(x+1) and the opposing electrode of the monitor OLED are corrected. As a result, the OLED driving voltages in the display OLED and the monitor OLED are corrected, and thus, the OLED driving current with a desired size flows. 
   When it is assumed that the current difference between the measured value and the reference value is a deviation current and that the voltage of the amount for change in accordance with the correction between the power supply lines V 1  to Vx and the opposing electrodes is a correction voltage, the relationship between the deviation current and the correction voltage is illustrated in  FIG. 7 , for example. In  FIG. 7 , the correction voltage is changed with a constant size every time when the deviation current is changed with a constant width. 
   Note that the relationship between the deviation current and the correction voltage may not necessarily conform to the graph shown in  FIG. 7 . It is only necessary that the deviation current and the correction voltage have a relationship such that the value of the current actually flowing through the ammeter becomes close to the reference value. For example, the relationship between the deviation current and the correction voltage may have linearity. Also, the deviation current may be proportional to the second power of the correction voltage. 
   Note that the structure of the correction circuit shown in this embodiment is just one example, and the present invention is not limited to this structure. It is only necessary that the correction circuit used in the present invention has the means for measuring the measured value and the reference value and the means for performing some calculation processing based on the measured value by means of the ammeter and correcting the OLED driving voltage. The voltage value of the monitor variable power supply and the voltage value of the display variable power supply may not necessarily have the same structure. It may be only necessary that a calculation processing method for the time when the deviation current becomes a value equal to or larger than a certain fixed value is prescribed instead of performing correction using the current reference value stored in the memory. 
   Embodiment 2 
   In this embodiment, the structure of a monitor pixel different from that in  FIG. 4  in the light emitting device of the present invention is described. 
     FIG. 8  shows the structure of the monitor pixel in this embodiment. In a monitor pixel portion of the light emitting device in this embodiment, monitor pixels  300  are provided in matrix. The monitor pixel  300  has a source line  301 , a first gate line  302 , a second gate line  303 , a power supply line  304 , a switching TFT  305 , a driving TFT  306 , an erasing TFT  309  and a monitor OLED  307 . 
   A gate electrode of the switching TFT  305  is connected to the first gate line  302 . One of a source region and a drain region of the switching TFT  305  is connected to the source line  301 , and the other is connected to a gate electrode of the driving TFT  306 . 
   A gate electrode of the erasing TFT  309  is connected to the second gate line  303 . One of a source region and a drain region of the erasing TFT  309  is connected to the power supply line  304 , and the other is connected to the gate electrode of the driving TFT  306 . 
   A source region of the driving TFT  306  is connected to the power supply line  304 , and a drain region of the driving TFT  306  is connected to a pixel electrode of the monitor OLED  307 . A capacitor  308  is formed between the gate electrode of the driving TFT  306  and the power supply line  304 . 
   The power supply line  304  is connected to a monitor variable power supply  311  through an ammeter  310 . Further, opposing electrodes of the monitor OLEDs  307  are all connected to the monitor variable power supply  311 . Note that, in  FIG. 8 , the monitor variable power supply  311  is connected such that the power supply line side is kept at a high potential (Vdd) and the opposing electrode side is kept at a low potential side (Vss). However, the present invention is not limited to this structure. It may be only necessary that the monitor variable power supply  311  is connected such that the current flowing through the monitor OLED  307  has a forward bias. 
   The ammeter  310  does not necessarily provided between the monitor variable power supply  311  and the power supply line  304 , and may be provided between the monitor variable power supply  311  and the opposing electrode. 
   Reference numeral  312  denotes a correction circuit which controls the voltage supplied to the opposing electrode and the power supply line  304  from the monitor variable power supply  311  based on the current value (measured value) measured in the ammeter  310 . 
   Note that the ammeter  310 , the monitor variable power supply  311  and the correction circuit  312  may be formed on the substrate different from the substrate on which the monitor pixel portion is formed, and may be connected to the monitor pixel portion through a connector or the like. If possible, the above-described components may be formed on the same substrate as the monitor pixel portion. 
   Further, in a color display mode, a monitor variable power supply, an ammeter and a correction circuit may be provided for each color, and an OLED driving voltage may be corrected in the OLED of each color. Note that, at this time, the correction circuit may be provided for each color, or the common correction circuit may be provided for the OLEDs of plural colors. 
   In the monitor pixel shown in  FIG. 8 , the potentials of the first gate line  302  and the second gate line  303  are controlled by different gate line driver circuits. The source line  301  is input with a monitor video signal by a source line driver circuit. 
   When the switching TFT  305  is turned on, the monitor video signal input to the source line  301  is input to the gate electrode of the driving TFT  306  through the switching TFT  301 . Then, when the driving TFT  306  is turned on in accordance with the monitor video signal, the OLED driving voltage is applied between the pixel electrode and the opposing electrode of the monitor OLED  307  by the monitor variable power supply  311 . Thus, the monitor OLED  307  emits light. 
   Then, when the erasing TFT  309  is turned on, the potential difference between the source region and the gate electrode of the driving TFT  306  becomes close to zero, and the driving TFT  306  is turned off. Thus, the monitor OLED  307  does not emit light. 
   In the present invention, while the monitor OLED  307  is emitting light, a current is measured in the ammeter  310 . The measured value as data is sent to the correction circuit  312 . 
   In the correction circuit  312 , the measured value of the current and a fixed current value (reference value) are compared. Then, in the case where there is some difference between the measured value and the reference value, the monitor variable power supply  311  is controlled to correct the voltage between the power supply line  304  and the opposing electrode. Thus, the OLED driving voltage is corrected in the monitor OLED  307  of the monitor pixel  300 , and an OLED driving current with a desired size flows. 
   Note that the OLED driving voltage may be corrected by controlling the potential at the power supply line side, or may be corrected by controlling the potential at the opposing electrode side. Also, the OLED driving voltage may be corrected by controlling both the potential at the power supply line side and the potential at the opposing electrode side. 
   Further, an image for monitor is preferably an image in which as m any monitor OLEDs of the pixels as possible emit light in the pixel portion. Even if there is an error in the current value measured with the ammeter, the ratio of the error in the measured current value to the entire measured value becomes smaller as both the measured value and the reference value become larger. In the monitor image, the gradation at the same level as the average of the pixels is made in order to make the progress of deterioration uniform. 
   Note that, although the structure of the monitor pixel is described in this embodiment, a display pixel also has the same structure. However, in case of the display pixel, the power supply line is not connected to the ammeter, and an opposing electrode of a display OLED is connected to not the monitor variable power supply but a display variable power supply. 
   The structure of the pixel shown in this embodiment is just one example, and the present invention is not limited to this structure. Note that this embodiment can be implemented by freely being combined with Embodiment 1. 
   Embodiment 3 
   In this embodiment, a monitor image displayed in the monitor pixel portion in performing correction of a current in the light emitting device of the present invention is described. 
   In the present invention, the correction of the current may always be conducted, or may be conducted at the time predetermined in advance by setting. A user may arbitrarily conduct the correction of the current. 
   The display pixel portion and the monitor pixel portion are separately provided in the light emitting device of the present invention. Thus, a display is not restricted. 
   A reference value of the current at the time when the monitor image is displayed is stored in the correction circuit. Thus, the correction can be performed without obstruction to and influence on the image display on a screen. 
   Further, monitor images having different reference current values may be used. In this case, a video signal is also input to the correction circuit, and the reference value is calculated in a calculation circuit or the like. In the case where the monitor image is not used, it is not necessary that a monitor video signal is used, and of course, the image to be displayed is not changed against intention of a user. 
   The monitor image during a current monitor is made to satisfy the following condition. 
   
     
       
         
           
             
               
                 
                   
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                     k 
                     · 
                     
                       m 
                       k 
                     
                   
                 
                 = 
                 
                   const 
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                 ( 
                 
                   Formula 
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   In Formula 1, symbol n indicates the total number of gradations of a video signal. Symbol k indicates the number of gradations, and takes a value of 0 to n. Symbol m k  indicates the number of pixels with the number of gradations of k in the monitor pixel portion. Note that, in case of the light emitting device with the color display Formula 1 is applied to every pixels corresponding to each color. 
   This embodiment can be implemented by being freely combined with Embodiment 1 or 2. 
   Embodiment 4 
   In this embodiment, a driving method of the light emitting device of the present invention in  FIG. 1  and  FIG. 4  is described with reference to  FIG. 9 . Note that, in  FIG. 9 , a horizontal axis indicates time and a vertical axis indicates the position of a display pixel connected to a gate line. In this embodiment, a driving method of the display pixel portion is described. However, a display of the monitor pixel portion can be performed by using the same driving method. 
   First, when a writing period Ta is started, the power supply potential of the power supply lines V 1  to Vx is kept at the same level as the potential of the opposing electrode of the display OLED  107 . Then, the switching TFT  120  of each of all the display pixels connected to the gate line G 1  (display pixels of the first line) is turned on in accordance with a selection signal output from the gate line driver circuit  106 . 
   Then, a video signal of digital (hereinafter referred to as digital video signal) of the first bit input to each of the source lines (S 1  to Sx) by the source line driver circuit  105  is input to the gate electrode of the driving TFT  121  through the switching TFT  120 . 
   Next, the switching TFT  120  of each display pixel of the first line is turned off. Similarly to the display pixels of the first line, the switching TFT  120  of each of the display pixels of the second line which are connected to the gate line G 2  is turned on by the selection signal. Next, the digital video signal of the first bit from each of the source lines (S 1  to Sx) is input to the gate electrode of the driving TFT  121  through the switching TFT  120  of each display pixel of the second line. 
   Then, the digital video signals of the first bit are input to the display pixels of all the lines in order. The period during which the digital video signals of the first bit are input to the display pixels of all the lines is a writing period Ta 1 . Note that, in this embodiment, that the digital video signal is input to the pixel means that the digital video signal is input to the gate electrode of the driving TFT  121  through the switching TFT  120 . 
   The writing period, Ta 1  is completed, and then, a display period Tr 1  is started. In the display period Tr 1 , the power supply potential of the power supply line becomes the potential having a potential difference with the opposing electrode with an extent such that the OLED emits light when the power supply potential is given to the pixel electrode of the OLED. 
   In this embodiment, in the case where the digital video signal has information of “0”, the driving TFT  121  is in an off state. Thus, the power supply potential is not given to the pixel electrode of the display OLED  107 . As a result, the display OLED  107  of the display pixel input with the digital video signal having the information of “0” does not emit light. 
   On the contrary, in the case where the digital video signal has information of “1”, the driving TFT  121  is in an on state. Thus, the power supply potential is given to the pixel electrode of the display OLED  107 . As a result, the display OLED  107  of the display pixel input with the digital video signal having the information of “1” emits light. 
   As described above, the display OLED  107  is in an emission state or a non-emission state in the display period Tr 1 , and all the display pixels perform the display. The period during which the display pixels perform the display is called a display period Tr. Particularly, the display period which starts by the digital video signals of the first bit being input to the display pixels is called the display period Tr 1 . 
   The display period Tr 1  is completed, and then, a writing period Ta 2  is started. The power supply potential of the power supply line again becomes the potential of the opposing electrode of the OLED. Similarly to the case of the writing period Ta 1 , all the gate lines are selected in order, and the digital video signals of the second bit are input to all the display pixels. The period during which the digital video signals of the second bit are input to the display pixels of all the lines is called the writing period Ta 2 . 
   The writing period Ta 2  is completed, and then, a display period Tr 2  is started. The power supply potential of the power supply line becomes the potential having the potential difference with the opposing electrode with an extent such that the OLED emits light when the power supply potential is given to the pixel electrode of the OLED. Then, all the display pixels perform the display. 
   The above-described operation is repeatedly performed until the digital video signals of n-th bit are input to the display pixels, and the writing period Ta and the display period Tr alternately appears. When all the display periods (Tr 1  to Trn) are completed, one image can be displayed. In this specification, a period for displaying one image is called one frame period (F). The one frame period is completed, and then, the next frame period is started. Then, the writing period Ta 1  appears again, and the above-described operation is repeated. 
   In the general light emitting device, it is preferable that 60 or more frame periods are provided for one second. If the number of images displayed for one second is less than 60, a flicker of an image may become visually conspicuous. 
   In this embodiment, it is necessary that the sum of lengths of all the writing periods is shorter than the one frame period, and also that the ratio of the lengths of the display periods is Tr 1 :Tr 2 :Tr 3 : . . . :Tr(n−1):Trn=2 0 :2 1 :2 2 : . . . :2 (n−2) :2 (n−1) . The combination of the above display periods enables the display of a desired gradation among 2 n  gradations. 
   The total sum of the lengths of the display periods during which the display OLED emits light in the one frame period is found, whereby the gradation displayed by the display pixel in the frame period concerned is determined. For example, in case of n=8, it is assumed that the luminance in the case where the display pixel emits light in all the display periods is 100%. When the display pixel emits light in Tr 1  and Tr 2 , a luminance of 1% can be exhibited. When Tr 3 , Tr 5  and Tr 8  are selected, a luminance of 60% can be exhibited. 
   Further, the display periods Tr 1  to Trn may be appeared in any order. For example, the display periods may be appeared in the order of Tr 1 , Tr 3 , Tr 5 , Tr 2 , . . . in the one frame period. 
   Note that, although the height of the power supply potential of the power supply line is changed between the writing periods and the display periods, the present invention is not limited to this. Both the power supply potential and the potential of the opposing electrode may always have the potential difference with an extent such that the display OLED emits light when the power supply potential is given to the pixel electrode of the display OLED. In this case, the display OLED can be made to emit light also in the writing periods. Thus, the gradation displayed by the display pixel in the frame period concerned is determined based on the total sum of the lengths of the writing periods and the display periods during which the display OLED emits light in the one frame period. Note that, in this case, the ratio of the sums of the lengths of the writing periods and the display periods corresponding to the digital video signals of respective bits needs to be (Ta 1 +Tr 1 ):(Ta 2 +Tr 2 ):(Ta 3 +Tr 3 ): . . . :(Ta(n−1)+Tr(n−1)): (Tan+Trn)=2 0 :2 1 :2 2 : . . . :2 (n−2) :2 (n−1) . 
   Note that the driving method shown in this embodiment is just one example, and the driving method of the light emitting device of the present invention in  FIG. 1  and  FIG. 4  is not limited to the driving method in this embodiment. The light emitting device of the present invention shown in  FIG. 1  and  FIG. 4  can perform the display with analogue video signals. 
   Note that this embodiment can be implemented by being freely combined with Embodiment 1 or 3. 
   Embodiment 5 
   In this embodiment, a detailed structure of a source line driving circuit, a gate line driving circuit, which are used for driving a pixel portion of a light emitting device of the present invention are explained. 
   The block figure of a light emitting device of this embodiment is shown in  FIGS. 10A and 10B .  FIG. 10A  shows the source signal line driving  601 , which has a shift register  602 , a latch (A)  603 , and a latch (B)  604 . 
   A clock signal CLK and a start pulse SP are input to the shift register  602  in the source signal line driving circuit  601 . The shift register  602  generates timing signals in order based upon the clock signal CLK and the start pulse SP, and supplies the timing signals one after another to the subsequent stage circuit through the buffer (not illustrated) and the like. 
   Note that, the timing signals output from the shift resister circuit  602  may he buffer amplified by a buffer and the like. The load capacitance (parasitic capacitance) of a wiring to which the timing signals are supplied is large because m any of the circuits or elements are connected to the wiring. The buffer is formed in order to prevent bluntness in the rise and fall of the timing signal, generated due to the large load capacitance. In addition, the buffer is not always necessary provided. 
   The timing signal amplified by a buffer is inputted to the latch (A)  603 . The latch (A)  603  has a plurality of latch stages for processing digital video signals. The latch (A)  603  writes in and maintains the digital video signal input from external of the source signal line driving circuit  601 , when the timing signal is input. 
   Note that the digital video signal may also be input in order to the plurality of latch stages of the latch (A)  603  in writing in the digital video signal to the latch (A)  603 . However, the present invention is not limited to this structure. The plurality of latch stages of the latch (A)  603  may be divided into a certain number of groups, and the digital video signal may be input to the respective groups at the same time in parallel, performing partitioned driving. For example, when the latches are divided into groups every four stages, it is referred to as partitioned driving with 4 divisions. 
   The period during which the digital video signal is completely written into all of the latch stages of the latch (A)  603  is referred to as a line period. In practice, there are cases in which the line period includes the addition of a horizontal return period to the above line period. 
   One line period is completed, the latch signal is inputted to the latch (B)  604 . At the moment, the digital video signal written into and stored in the latch (A)  603  is send all together to be written into and stored in all stages of the latch (B)  604 . 
   In the latch (A)  603  after completing sending the digital video signal to the latch (B)  604 , it is performed to write into the digital video signal in accordance with the timing signal from the shift register  602 . 
   In the second ordered one line period, the digital video signal which is written into and stored in the latch (B)  604  is inputted to the source signal line. 
     FIG. 10B  is a block figure showing the structure of gate line driving circuit. 
   The gate line driving circuit  605  has the shift register  606  and the buffer  607 . According to circumstances, the level shift is provided. 
   In the address gate line driving circuit  605 , the timing signal from the shift register  606  is inputted to the buffer  607 , and then to a corresponding gate line. The gate electrodes of the TFTs for one line of pixels are connected to of the TFTs of the one line of pixels must be placed in an ON state simultaneously. A circuit which is capable of handling the flow of a large electric current is therefore used for the buffer. 
   Further, the source signal driving circuit can be provided specially by the pixel portion for display and the pixel portion for monitor. 
   The driving circuit shown in this embodiment is mere an example. Note that it is possible to implement Embodiment 5 in combination with Embodiments 1 to 4. 
   Embodiment 6 
   In this embodiment, an appearance of the light emitting device of the present invention is described with reference to  FIGS. 11A to 11C . 
     FIG. 11A  is a top view of the light emitting device,  FIG. 11B  is a cross sectional view taken along with a line A–A′ of  FIG. 11A , and  FIG. 11C  is a cross sectional view taken along with a line B–B′ of  FIG. 11A . 
   A seal member  4009  is provided so as to surround a display pixel portion  400 , a monitor pixel portion  4070 , a source line driver circuit  4003  and a gate line driver circuit  4004 , which are provided on a substrate  4001 . Further, a sealing material  4008  is provided on the display pixel portion  4002 , the monitor pixel portion  4070 , the source line driver circuit  4003  and the gate line driver circuit  4004 . Thus, the display pixel portion  4002 , the monitor pixel portion  4070 , the source line driver circuit  4003  and the gate line driver circuit  4004  are sealed by the substrate  4001 , the seal member  4009  and the sealing material  4008  together with a filler  4210 . 
   Further, the display pixel portion  4002 , the monitor pixel portion  4070 , the source line driver circuit  4003  and the gate line driver circuit  4004 , which are provided on the substrate  4001 , have a plurality of TFTs. In  FIG. 11B , a driver circuit TFT (Here, an n-channel TFT and a p-channel TFT are shown in the figure.)  4201  included in the source line driver circuit  4003  and a driving TFT (TFT for controlling the current to the OLED)  4202  included in the display pixel portion  4002 , which are formed on a base film  4010 , are typically shown. 
   In this embodiment, the p-channel TFT or the n-channel TFT manufactured by a known method is used as the driver circuit TFT  4201 , and the p-channel TFT manufactured by a known method is used as the driving TFT  4202 . Further, the display pixel portion  4002  is provided with a storage capacitor (not shown) connected to a gate electrode of the driving TFT  4202 . 
   An interlayer insulating film (leveling film)  4201  is formed on the driver circuit TFT  4201  and the driving TFT  4202 , and a pixel electrode (anode)  4203  electrically connected to a drain of the driving TFT  4202  is formed thereon. A transparent conductive film having a large work function is used for the pixel electrode  4203 . A compound of indium oxide and tin oxide, a compound of indium oxide and zinc oxide, zinc oxide, tin oxide or indium oxide can be used for the transparent conductive film. The above transparent conductive film added with gallium may also be used. 
   Then, an insulating film  4302  is formed on the pixel electrode  4203 , and the insulating film  4302  is formed with an opening portion on the pixel electrode  4203 . In this opening portion, an organic light emitting layer  4204  is formed on the pixel electrode  4203 . A known organic light emitting material or inorganic light emitting material may be used for the organic light emitting layer  4204 . Further, there exist a low molecular weight (monomer) material and a high molecular weight (polymer) material as the organic light emitting materials, and both the materials may be used. 
   A known evaporation technique or application technique may be used as a method of forming the organic light emitting layer  4204 . Further, the structure of the organic light emitting layer may take a lamination structure or a single layer structure by freely combining a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer and an electron injecting layer. 
   A cathode  4205  made of a conductive film having light shielding property (typically, conductive film containing aluminum, copper or silver as its main constituent or lamination film of the above conductive film and another conductive film) is formed on the organic light emitting layer  4204 . Further, it is desirable that moisture and oxygen that exist on an interface of the cathode  4205  and the organic light emitting layer  4204  are removed as much as possible. Therefore, such a device is necessary that the organic light emitting layer  4204  is formed in a nitrogen or rare gas atmosphere, and then, the cathode  4205  is formed without exposure to oxygen and moisture. In this embodiment, the above-described film deposition is enabled by using a multi-chamber type (cluster tool type) film forming device. In addition, a predetermined voltage is given to the cathode  4205 . 
   As described above, a display OLED  4303  constituted of the pixel electrode (anode)  4203 , the organic light emitting layer  4204  and the cathode  4205  is formed. Further, a protective film  4209  is formed on the insulating film  4302  so as to cover the display OLED  4303 . The protective film  4209  is effective in preventing oxygen, moisture and the like from permeating the display OLED  4303 . 
   Reference numeral  4005   a  denotes a wiring drawn to be connected to the power supply line, and the wiring  4005   a  is electrically connected to a source region of the driving TFT  4202 . The drawn wiring  4005   a  passes between the seal member  4009  and the substrate  4001 , and is electrically connected to an FPC wiring  4301  of an FPC  4006  through an anisotropic conductive film  4300 . 
   A glass material, a metal material (typically, stainless material), a ceramics material or a plastic material (including a plastic film) can be used for the sealing material  4008 . As the plastic material, an FRP (fiberglass-reinforced plastics) plate, a PVF (polyvinyl fluoride) film, a Mylar film, a polyester film or an acrylic resin film may be used. Further, a sheet with a structure in which an aluminum foil is sandwiched with the PVF film or the Mylar film can also be used. 
   However, in the case where the light from the display OLED is emitted toward the cover member side, the cover member needs to be transparent. In this case, a transparent substance such as a glass plate, a plastic plate, a polyester film or an acrylic film is used. 
   Further, in addition to an inert gas such as nitrogen or argon, an ultraviolet curable resin or a thermosetting resin may be used as the filler  4210 , so that PVC (polyvinyl chloride), acrylic, polyimide, epoxy resin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylene vinyl acetate) can be used. In this embodiment, nitrogen is used for the filler. 
   Moreover, a concave portion  4007  is provided on the surface of the sealing material  4008  on the substrate  4001  side, and a hygroscopic substance or a substance that can absorb oxygen  4207  is arranged therein in order that the filler  4210  is made to be exposed to the hygroscopic substance (preferably, barium oxide) or the substance that can absorb oxygen. Then, the hygroscopic substance or the substance that can absorb oxygen  4207  is held in the concave portion  4007  by a concave portion cover member  4208  such that the hygroscopic substance or the substance that can absorb oxygen  4207  is not scattered. Note that the concave portion cover member  4208  has a fine mesh form, and has a structure in which air and moisture are penetrated while the hygroscopic substance or the substance that can absorb oxygen  4207  is not penetrated. The deterioration of the display OLED  4303  can be suppressed by providing the hygroscopic substance or the substance that can absorb oxygen  4207 . 
   As shown in  FIG. 11C , the pixel electrode  4203  is formed, and at the same time, a conductive film  4203   a  is formed so as to contact the drawn wiring  4005   a.    
   Further, the anisotropic conductive film  4300  has conductive filler  4300   a . The conductive film  4203   a  on the substrate  4001  and the FPC wiring  4301  on the FPC  4006  are electrically connected to each other by the conductive filler  4300   a  by heat-pressing the substrate  4001  and the FPC  4006 . 
   Incidentally, the light emitted from the monitor pixel portion may penetrate the substrate  4001  or the cover member  4208  or not. In the case where the light penetrates the substrate  4001  or the cover member  4208 , the image displayed in the monitor pixel portion can be effectively utilized for displaying something. 
   The ammeter, the variable power supply and the correction circuit of the light emitting device of the present invention are formed on a substrate (not shown) different from the substrate  4001 , and are electrically connected to the power supply line and the cathode  4205 , which are formed on the substrate  4001 , through the FPC  4006 . 
   Note that this embodiment can be implemented by being freely combined with Embodiments 1 to 5. 
   Embodiment 7 
   In this embodiment, an example is described in which the ammeter, the variable power supply and the correction circuit of the light emitting device of the present invention are formed on a substrate different from the substrate on which the display pixel portion is formed, and are connected to the wirings on the substrate on which the display pixel portion is formed by a means such as a wire bonding method or a COG (chip-on-glass) method. 
     FIG. 12  is a diagram of an appearance of a light emitting device of this embodiment. A seal member  5009  is provided so as to surround a display pixel portion  5002 , a monitor pixel portion  5070 , a source line driver circuit  5003  and a gate line driver circuit  5004  which are provided on a substrate  5001 . Further, a sealing material  5008  is provided on the display pixel portion  5002 , the monitor pixel portion  5070 , the source line driver circuit  5003  and the gate line driver circuit  5004 . Thus, the display pixel portion  5002 , the monitor pixel portion  5070 , the source line driver circuit  5003  and the gate line driver circuit  5004  are sealed by the substrate  5001 , the seal member  5009  and the sealing member  5008  together with a filler (not shown). 
   A concave portion  5007  is provided on the surface of the sealing material  5008  on the substrate  5001  side, and a hygroscopic substance or a substance that can absorb oxygen is arranged therein. 
   A wiring (drawn wiring) drawn onto the substrate  5001  passes between the seal member  5009  and the substrate  5001 , and is connected to an external circuit or element of the light emitting device through an FPC  5006 . 
   The ammeter, the variable power supply and the correction circuit of the light emitting device of the present invention are formed on a substrate (hereinafter referred to as chip)  5020  different from the substrate  5001 . The chip  5020  is attached onto the substrate  5001  by the means such as the COG (chip-on-glass) method, and is electrically connected to the power supply line and a cathode (not shown) which are formed on the substrate  5001 . 
   In this embodiment, the chip  5020  on which the ammeter, the variable power supply and the correction circuit are formed is attached onto the substrate  5001  by the wire bonding method, the COG method or the like. Thus, the light emitting device can be structured based on one substrate, and therefore, the device itself is made compact and also the mechanical strength is improved. 
   Note that a known method can be applied with regard to a method of connecting the chip onto the substrate. Further, circuits and elements other than the ammeter, the variable power supply and the correction circuit may be attached onto the substrate  5001 . 
   This embodiment can be implemented by beings freely combined with Embodiments 1 to 6. 
   Embodiment 8 
   In the present invention, an external light emitting quantum efficiency can he remarkably improved by using an organic material by which phosphorescence from a triplet exciton can be employed for emitting a light. As a result, the power consumption of the OLED can be reduced, the lifetime of the OLED can be elongated and the weight of the OLED can be lightened. 
   The following is a report where the external light emitting quantum efficiency is improved by using the triplet exciton (T. Tsutsui, C. Adachi, S. Saito, Photochemical processes in Organized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo. 1991) p. 437). 
   The molecular formula of an organic light emitting material (coumarin pigment) reported by the above article is represented as follows. 
                       
(M. A. Baldo, D. F. O Brien, Y. You, A. Shoustikov, S. Sible, M. E. Thompson, S. R. Forrest, Nature 395 (1998) p. 151)
 
   The molecular formula of an organic light emitting material (Pt complex) reported by the above article is represented as follows. 
                       
(M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75 (1999) p. 4.)
 
(T. Tsutsui, M. -J. Yang, M. Yahiro, K. Nakamura, T. Watanzibe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999)) L1502)
 
   The molecular formula of an organic light emitting material (Ir complex) reported by the above article is represented as follows. 
   
     
       
       
           
           
       
     
   
   As described above, if phosphorescence from a triplet exciton can be put to practical use, it can realize the external light emitting quantum efficiency three to four times as high as that in the case of using fluorescence from a singlet exciton in principle. 
   The structure according to this embodiment can be freely implemented in combination of any structures of the Embodiments 1 to 7. 
   Embodiment 9 
   Next, described with reference to  FIGS. 13 to 16  is a method of forming the light emitting device of the present invention. Here, the method of simultaneously forming, on the same substrate, the switching TFT and the driving TFT of the pixel portion, and the TFTs of a driving portion provided surrounding the pixel portion is described in detail according to steps. 
   This embodiment uses a substrate  900  of a glass such as barium borosilicate glass or aluminoborosilicate glass as represented by the glass #7059 or the glass #1737 of Corning Co. There is no limitation on the substrate  900  provided it has a property of transmitting light, and there may be used a quartz substrate. There may be further used a plastic substrate having heat resistance capable of withstanding the treatment temperature of this embodiment. 
   Referring next to  FIG. 13(A) , an underlying film  901  comprising an insulating film such as silicon oxide film, silicon nitride film or silicon oxynitride film is formed on the substrate  900 . In this embodiment, the underlying film  901  has a two-layer structure. There, however, may be employed a structure in which a single layer or two or more layers are laminated on the insulating film. The first layer of the underlying film  901  is a silicon oxynitride film  901   a  formed maintaining a thickness of from 10 to 200 nm (preferably, from 50 to 100 nm) relying upon a plasma CVD method by using SiH 4 , NH 3  and N 2 O as reaction gases. In this embodiment, the silicon oxynitride film  901   a  (having a composition ratio of Si=32%, O=27%, N=24%. H=17%) is formed maintaining a thickness of 50 nm. The second layer of the underlying film  901  is a silicon oxynitride film  901   b  formed maintaining a thickness of from 50 to 200 nm (preferably, from 100 to 150 nm) relying upon the plasma CVD method by using SiH 4  and N 2 O as reaction gases. In this embodiment, the silicon oxynitride film  901   b  (having a composition ratio of Si=32%, O=59%, N=7%, H=2%) is formed maintaining a thickness of 100 nm. 
   Then, semiconductor layers  902  to  905  are formed on the underlying film  901 . The semiconductor layers  902  to  905  are formed by forming a semiconductor film having an amorphous structure by a known means (sputtering method, LPCVD method or plasma CVD method) followed by a known crystallization processing (laser crystallization method, heat crystallization method or heat crystallization method using a catalyst such as nickel), and patterning the crystalline semiconductor film thus obtained into a desired shape. The semiconductor layers  902  to  905  are formed in a thickness of from 25 to 80 nm (preferably, from 30 to 60 nm). Though there is no limitation on the material of the crystalline semiconductor film, there is preferably used silicon or a silicon-germanium (Si x Ge 1−x (X=0.0001 to 0.02)) alloy. In this embodiment, the amorphous silicon film is formed maintaining a thickness of 55 nm relying on the plasma CVD method and, then, a solution containing nickel is held on the amorphous silicon film. The amorphous silicon film is dehydrogenated (500° C. one hour), heat-crystallized (550° C., 4 hours) and is, further, subjected to the laser annealing to improve the crystallization, thereby to form a crystalline silicon film. The crystalline silicon film is patterned by the photolithographic method to form semiconductor layers  902  to  905 . 
   The semiconductor layers  902  to  905  that have been formed may further be doped with trace amounts of an impurity element (boron or phosphorus) to control the threshold value of the TFT. 
   In forming the crystalline semiconductor film by the laser crystallization method, further, there may be employed an excimer laser of the pulse oscillation type or of the continuously light-emitting type, a YAG laser or a YVO 4  laser. When these lasers are to be used, it is desired that a laser beam emitted from a laser oscillator is focused into a line through an optical system so as to fall on the semiconductor film. The conditions for crystallization are suitably selected by a person who carries out the process. When the excimer laser is used, the pulse oscillation frequency is set to be 300 Hz and the laser energy density to be from 100 to 400 mJ/cm 2  (typically, from 200 to 300 mJ/cm 2 ). When the YAG laser is used, the pulse oscillation frequency is set to be from 30 to 300 kHz by utilizing the second harmonics and the laser energy density to be from 300 to 600 mJ/cm 2  (typically, from 350 to 500 mJ/cm 2 ). The whole surface of the substrate is irradiated with the laser beam focused into a line of a width of 100 to 1000 μm, for example, 400 μm, and the overlapping ratio of the linear beam at this moment is set to be 50 to 90%. 
   Then, a gate insulating film  906  is formed to cover the semiconductor layers  902  to  905 . The gate insulating film  906  is formed of an insulating film containing silicon maintaining a thickness of from 40 to 150 nm by the plasma CVD method or the sputtering method. In this embodiment, the gate insulating film is formed of a silicon oxynitride film (composition ratio of Si=32%, O=59%, N=7%, H=2%) maintaining a thickness of 110 nm by the plasma CVD method. The gate insulating film is not limited to the silicon oxynitride film but may have a structure on which is laminated a single layer or plural layers of an insulating film containing silicon. 
   When the silicon oxide film is to be formed, TEOS (tetraethyl orthosilicate) and O 2  are mixed together by the plasma CVD method, and are reacted together under a reaction pressure of 40 Pa, at a substrate temperature of from 300 to 400° C., at a frequency of 13.56 MHz and a discharge electric power density of from 0.5 to 0.8 W/cm 2 . The thus formed silicon oxide film is, then, heat annealed at 400 to 500° C. thereby to obtain the gate insulating film having good properties. 
   Then, a heat resistant conductive layer  907  is formed on the gate insulating film  906  maintaining a thickness of from 200 to 400 nm (preferably, from 250 to 350 nm) to form the gate electrode. The heat-resistant conductive layer  907  may be, formed as a single layer or may, as required, be formed in a structure of laminated layers of plural layers such as two layers or three layers. The heat resistant conductive layer contains an element selected from Ta, Ti and W, or contains an alloy of-the above element, or an alloy of a combination of the above elements. The heat-resistant conductive layer is formed by the sputtering method or the CVD method, and should contain impurities at a decreased concentration to decrease the resistance and should, particularly, contain oxygen at a concentration of not higher than 30 ppm. In this embodiment, the W film is formed maintaining a thickness of 300 nm. The W film may be formed by the sputtering method by using W as a target, or may be formed by the hot CVD method by using tungsten hexafluoride (WF 6 ). In either case, it is necessary to decrease the resistance so that it can be used as the gate electrode. It is, therefore, desired that the W film has a resistivity of not larger than 20 μΩcm. The resistance of the W film can be decreased by coarsening the crystalline particles. When W contains much impurity elements such as oxygen, the crystallization is impaired and the resistance increases. When the sputtering method is employed, therefore, a W target having a purity of 99.9999% is used, and the W film is formed while giving a sufficient degree of attention so that the impurities will not be infiltrated from the gaseous phase during the formation of the film, to realize the resistivity or from 9 to 20 μΩcm. 
   On the other hand, the Ta film that is used as the heat-resistant conductive layer  907  can similarly be formed by the sputtering method. The Ta film is formed by using Ar as a sputtering gas. Further, the addition of suitable amounts of Xe and Kr into the gas during the sputtering makes it possible to relax the internal stress of the film that is formed and to prevent the film from being peeled off. The Ta film of α-phase has a resistivity of about 20 μΩcm and can be used as the gate electrode but the Ta film of β-phase has a resistivity of about 180 μΩcm and is not suited for use as the gate electrode. The TaN film has a crystalline structure close to the α-phase. Therefore, if the TaN film is formed under the Ta film, there is easily formed the Ta film of α-phase. Further, though not diagramed, formation of the silicon film doped with phosphorus (P) maintaining a thickness of about 2 to about 20 nm under the heat resistant conductive layer  907  is effective in fabricating the device. This helps improve the intimate adhesion of the conductive film formed thereon, prevent the oxidation, and prevent trace amounts of alkali metal elements contained in the heat resistant conductive layer  907  from being diffused into the gate insulating film  906  of the first shape. In any way, it is desired that the heat-resistant conductive layer  907  has a resistivity over a range of from 10 to 50 μΩcm. 
   Next, a mask  908  is formed by a resist relying upon the photolithographic technology. Then, a first etching is executed. This embodiment uses an ICP etching device, uses Cl 2  and CF 4  as etching gases, and forms a plasma with RF (13.56 MHz) electric power of 3.2 W/cm 2  under a pressure of 1 Pa. The RF (13.56 MHz) electric power of 224 mW/cm 2  is supplied to the side of the substrate (sample stage), too, whereby a substantially negative self bias voltage is applied. Under this condition, the W film is etched at a rate of about 100 nm/min. The first etching treatment is effected by estimating the time by which the W film is just etched relying upon this etching rate, and is conducted for a period of time which is 20% longer than the estimated etching time. 
   The conductive layers  909  to  912  having a first tapered shape are formed by the first etching treatment. The conductive layers  909  to  912  are tapered at an angle of from 15 to 30°. To execute the etching without leaving residue, over-etching is conducted by increasing the etching time by about 10 to 20%. The selection ratio of the silicon oxynitride film (gate insulating film  906 ) to the W film is 2 to 4 (typically, 3). Due to the over etching, therefore, the surface where the silicon oxynitride film is exposed is etched by about 20 to about 50 nm ( FIG. 13(B) ). 
   Then, a first doping treatment is effected to add an impurity element of a first type of electric conduction to the semiconductor layer. Here, a step is conducted to add an impurity element for imparting the n-type. A mask  908  forming the conductive layer of a first shape is left, and an impurity element is added by the ion-doping method to impart the n-type in a self-aligned manner with the conductive layers  909  to  912  having a first tapered shape as masks. The dosage is set to be from 1×10 13  to 5×10 14  atoms/cm 2  so that the impurity element for imparting the n-type reaches the underlying semiconductor layer penetrating through the tapered portion and the date insulating film  906  at the ends of the gate electrode, and the acceleration voltage is selected to be from 80 to 160 keV. As the impurity element for imparting the n-type, there is used an element belonging to the Group 15 and, typically, phosphorus (P) or arsenic (As). Phosphorus (P) is used, here. Due to the ion-doping method, an impurity element for imparting the n-type is added to the first impurity regions  914  to  917  over a concentration range of from 1×10 20  to 1×10 21  atoms/cm 3  ( FIG. 13(C) ). 
   In this step, the impurities turn down to the lower side of the conductive layers  909  to  912  of the first shape depending upon the doping conditions, and it often happens that the first impurity regions  914  to  917  are overlapped on the conductive layers  909  to  912  of the first shape. 
   Next, the second etching treatment is conducted as shown in  FIG. 13(D) . The etching treatment, too, is conducted by using the ICP etching device, using a mixed gas of CF 4  and Cl 2  as an etching gas, using an RF electric power of 3.2 W/cm 2  (13.56 MHz), a bias power of 45 mW/cm 2  (13.56 MHz) under a pressure of 1.0 Pa. Under this condition, there are formed the conductive layers  918  to  921  of a second shape. The end portions thereof are tapered, and the thicknesses gradually increase from the ends toward the inside. The rate of isotropic etching increases in proportion to a decrease in the bias voltage applied to the side of the substrate as compared to the first etching treatment, and the angle of the tapered portions becomes 30 to 60°. The mask  908  is ground at the edge by etching to form a mask  922 . In the step of  FIG. 13(D) , the surface of the gate insulating film  906  is etched by about 40 nm. 
   Then, the doping is effected with an impurity element for imparting the n-type under the condition of an increased acceleration voltage by decreasing the dosage to be smaller than that of the first doping treatment. For example, the acceleration voltage is set to be from 70 to 120 keV, the dosage is set to be 1×10 13 /cm 2  thereby to form first impurity regions  924  to  927  having an increased impurity concentration, and second impurity regions  928  to  931  that are in contact with the first impurity regions  924  to  927 . In this step, the impurity may turn down to the lower side of the conductive layers  918  to  921  of the second shape, and the second impurity regions  928  to  931  may be overlapped on the conductive layers  918  to  921  of the second shape. The impurity concentration in the second impurity regions is from 1×10 16  to 1×10 18  atoms/cm 3  ( FIG. 14(A) ). 
   Referring to  FIG. 14(B) , impurity regions  933   933   a ,  933   b ),and  934  ( 934   a .  934   b ) of the conduction type opposite to the one conduction type are formed in the semiconductor layers  902 ,  905  that form the p-channel TFTs. In this case, too, an impurity element for imparting the p-type is added using the conductive layers  918 ,  921  of the second shape as masks to form impurity regions in a self-aligned manner. At this moment, the semiconductor layers  903  and  904  forming the n-channel TFTs are entirely covered for their surfaces by forming a mask  932  of a resist. Here, the impurity regions  933  and  934  are formed by the ion-doping method by using diborane (B 2 H 6 ). The impurity element for imparting the p-type is added to the impurity regions  933  and  934  at a concentration of from 2×10 20  to 2×10 21  atoms/cm 3 . 
   If closely considered, however, the impurity, regions  933 ,  934  can be divided into two regions containing an impurity element that imparts the n-type. Third impurity regions  933   a  and  934   a  contain the impurity element that imparts the n-type at a concentration of from 1×10 20  to 1×10 21  atoms/cm 3  and fourth impurity regions  933   b  and  934   b  contain the impurity element that imparts the n-type at a concentration of from 1×10 17 to  1×10 20  atoms/cm 3 . In the impurity regions  933   b  and  934   b , however, the impurity element for imparting the p-type is contained at a concentration of not smaller than 1×10 19  atom/cm 3  and in the third impurity regions  933   a  and  934   a , the impurity element for imparting the p-type is contained at a concentration which is 1.5 to 3 times as high as the concentration of the impurity element for imparting the n-type. Therefore, the third impurity regions work as source regions and drain regions of the p-channel TFTs without arousing any problem. 
   Referring next to  FIG. 14(C) , a first interlayer insulating film  937  is formed on the conductive layers  918  to  921  of the second shape and on the gate insulating film  906 . The first interlayer insulating film  937  may be formed of a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or a laminated layer film of a combination thereof. In any case, the first interlayer insulating film  937  is formed of an inorganic insulating material. The first interlayer insulating film  937  has a thickness of 100 to 200 nm. When the silicon oxide film is used as the first interlayer insulating film  937 , TEOS and O 2  are mixed together by the plasma CVD method, and are reacted together under a pressure of 40 Pa at a substrate temperature of 300 to 400° C. while discharging the electric power at a high frequency (13.56 MHz) and at a power density of 0.5 to 0.8 W/cm 2 . When the silicon oxynitride film is used as the first interlayer insulating film  937 , this silicon oxynitride film may be formed from SiH 4 , N 2 O and NH 3 , or from SiH 4  and N 2 O by the plasma CVD method. The conditions of formation in this case are a reaction pressure of from 20 to 200 Pa, a substrate temperature of from  300  to 400° C. and a high-frequency (60 MHz) power density of from 0.1 to 1.0 W/cm 2 . As the first interlayer insulating film  937 , further, there may be used a hydrogenated silicon oxynitride film formed by using SiH 4 , N 2 O and H 2 . The silicon nitride film, too, can similarly be formed by using SiH 4  and NH 3  by the plasma CVD method. 
   Then, a step is conducted for activating the impurity elements that impart the n-type and the p-type added at their respective concentrations. This step is conducted by thermal annealing method using an annealing furnace. There can be further employed a laser annealing method or a rapid thermal annealing method (RTA method). The thermal annealing method is conducted in a nitrogen atmosphere containing oxygen at a concentration of not higher than 1 ppm and, preferably, not higher than 0.1 ppm at from 400 to 700° C. and, typically, at from 500 to 600° C. In this embodiment, the heat treatment is conducted at 550° C. for 4 hours. When a plastic substrate having a low heat resistance temperature is used as the substrate  501 , it is desired to employ the laser annealing method. 
   Following the step of activation, the atmospheric gas is changed, and the heat treatment is conducted in an atmosphere containing 3 to 100% of hydrogen at from 300 to 450° C. for from 1 to 12 hours to hydrogenate the semiconductor layer. This step is to terminate the dangling bonds of 10 16  to 10 18 /cm 3  in the semiconductor layer with hydrogen that is thermally excited. As another means of hydrogenation, the plasma hydrogenation may be executed (using hydrogen excited with plasma). In any way, it is desired that the defect density in the semiconductor layers  902  to  905  is suppressed to be not larger than 10 16 /cm 3 . For this purpose, hydrogen may be added in an amount of from 0.01 to 0.1 atomic %. 
   Then, a second interlayer insulating film  939  of an organic insulating material is formed maintaining an average thickness of from 1.0 to 2.0 μm. As the organic resin material, there can be used polyimide, acrylic resin, polyamide, polyimideamide, BCB (benzocyclobutene). When there is used, for example, a polyimide of the type that is heat polymerized after being applied onto the substrate, the second interlayer insulating film is formed being fired in a clean oven at 300° C. When there is used an acrylic resin, there is used the one of the two-can type. Namely, the main material and a curing agent are mixed together, applied onto the whole surface of the substrate by using a spinner, pre-heated by using a hot plate at 80° C. for 60 seconds, and are fired at 250° C. for 60 minutes in a clean oven to form the second interlayer insulating film. 
   Thus, the second interlayer insulating film  939  is formed by using, an organic insulating material featuring good and flattened surface. Further, the organic resin material, in general, has a small dielectric constant and lowers the parasitic capacitance. The organic resin material, however, is hygroscopic and is not suited as a protection film. It is, therefore, desired that the second interlayer insulating film is used in combination with the silicon oxide film, silicon oxynitride film or silicon nitride film formed as the first interlayer insulating film  937 . 
   Thereafter, the resist mask of a predetermined pattern is formed, and contact holes are formed in the semiconductor layers to reach the impurity regions serving as source regions or drain regions. The contact holes are formed by dry etching. In this case, a mixed gas of CF 4 , O 2  and He is used as the etching gas to, first, etch the second interlayer insulating film  939  of the organic resin material. Thereafter, CF 4  and O 2  are used as the etching gas to etch the first interlayer insulating film  937 . In order to further enhance the selection ratio relative to the semiconductor layer, CHF 3  is used as the etching gas to etch the gate insulating film  570  of the third shape, thereby to form the contact holes. 
   Here, the conductive metal film is formed by sputtering and vacuum vaporization and is patterned by using a mask and is, then, etched to form source wirings  940  to  943 , drain wirings  944  to  946 . Further, though not diagramed in this embodiment, the wiring is formed by a laminate of a 50 nm thick Ti film and a 500 nm thick alloy film (alloy film of Al and Ti). 
   Then, a transparent conductive film is formed thereon maintaining a thickness of 80 to 120 nm, and is patterned to form a pixel electrode  947  ( FIG. 15(A) ). Therefore, the pixel electrode  947  is formed by using an indium oxide-tin (ITO) film as a transparent electrode or a transparent conductive film obtained by mixing 2 to 20% of a zinc oxide (ZnO) into indium oxide. 
   Further, the pixel electrode  947  is formed being in contact with, and overlapped on, the drain wiring  946  that is electrically connected to the drain region of the driving TFT. 
   Next, a third interlayer insulating film  949  having an opening at the position that coincides with the pixel electrode  947  is formed as shown in  FIG. 15(B) . The third interlayer insulating film  949  is capable of insulating, and functions as a bank to separate organic light emitting layers of adjacent pixels from one another. In this embodiment, a resist is used to form the third interlayer insulating film  949 . 
   In this embodiment, the third interlayer-insulating film  949  is about 1 μm in thickness and the aperture is shaped to have a so-called reverse tapered shape in which the width is increased toward the pixel electrode  947 . This is obtained by covering the resist film with a mask except the portion where the aperture is to be formed, exposing the film through irradiation of UV light, and then removing the exposed portion using a developer. 
   The third interlayer insulating film  949  reversely tapered as in this embodiment separates organic light emitting layers of adjacent pixels from each other when the organic light emitting layers are formed in a later step. Therefore cracking or peeling of the organic light emitting layers can be prevented even if the organic light emitting layers and the third interlayer insulating film  949  have different thermal expansion coefficient. 
   Although a resist film is used in this embodiment for the third interlayer insulating film, polyimide, polyamide, acrylic, BCB (benzocycrobutene), or silicon oxide film may be used in some cases. The third interlayer insulating film  949  may be organic or inorganic as long as the material is capable of insulating. 
   An organic light emitting layer  950  is formed by evaporation. A cathode (MgAg electrode)  951  and a protective electrode  952  are also formed by evaporation. Desirably, heat treatment is performed on the pixel electrode  947  to remove moisture completely from the electrode before forming the organic light emitting layer  950  and the cathode  951 . Though the cathode of OLED is a MgAg electrode in this embodiment, other known materials may be used instead. 
   The organic light emitting layer  950  can be formed from a known material. In this embodiment, the organic light emitting layer has a two-layer structure consisting of a hole transporting layer and a light emitting layer. The organic light emitting layer may additionally have a hole injection layer, an electron injection layer, or an electron transporting layer. Various combinations of these layers have been reported and any of them can be used. 
   In this embodiment, the hole transporting layer is polyphenylene vinylene deposited by evaporation. The light emitting layer is obtained by evaporation of polyvinyl carbazole with molecular dispersion of 30 to 40% of PBD that is a 1, 3, 4-oxadiazole derivative and by doping the resultant film with about 1% of coumarine 6 as green color luminescent center. 
   The protective electrode  952  alone can protect the organic light emitting layer  950  from moisture and oxygen but adding a protective film  953  is more desirable. The protective film  953  in this embodiment is a silicon nitride film with a thickness of 300 nm. The protective electrode  952  and the protective film may be formed in succession without exposing the substrate to the air. 
   The protective electrode  952  also prevents degradation of the cathode  951 . Typically, a metal film containing aluminum as its main ingredient is used for the protective electrode. Other materials may of course be used. The organic light emitting layer  950  and the cathode  951  are very weak against moisture. Therefore it is desirable to form them and the protective electrode  952  in succession without exposing the substrate to the air to protect them from the outside air. 
   The organic light emitting layer  950  is 10 to 400 nm in thickness (typically 60 to 150 nm). The cathode  951  is 80 to 200 nm in thickness (typically 100 to 150 nm). 
   Thus completed is a light emitting device structured as shown in  FIG. 15B . A portion  954  where the pixel electrode  947 , the organic light emitting layer  950 , and the cathode  951  overlap corresponds to the OLED. 
   A p-channel TFT  960  and an n-channel TFT  961  are TFTs of the driving circuit and constitute a CMOS. A switching TFT  962  and a driving TFT  963  are TFTs of the pixel portion. The TFTs of the driving circuit and the TFTs of the pixel portion can be formed on the same substrate. 
   In the case of a light emitting device using OLED, its driving circuit can be operated by a power supply having a voltage of 5 to 6V, 10 V, at most. Therefore, degradation of TFTs due to hot electron is not a serious problem. Also, smaller gate capacitance is preferred for the TFTs since the driving circuit needs to operate at high speed. Accordingly, in a driving circuit of a light emitting device using OLED as in this embodiment, the second impurity region  929  and the fourth impurity region  933   b  of the semiconductor layers of the TFTs preferably do not overlap the gate electrode  918  and the gate electrode  919 , respectively. 
   The method of manufacturing the light emitting device of the present invention is not limited to the one described in this embodiment. The light emitting device of the present invention may be manufactured by a known method. 
   This embodiment may be combined freely with Embodiments 1 through 8. 
   Embodiment 10 
   In this embodiment, a method of manufacturing a light emitting device different from that in Embodiment 9 is described. 
   The process through the formation of the second interlayer insulating film  939  is the same as in Embodiment 5. As shown in  FIG. 16A , after the second interlayer insulating film  939  is formed, a passivation film  981  is formed so as to contact the second interlayer insulating film  939 . 
   The passivation film  981  is effective in preventing moisture contained in the second interlayer insulating film  939  from permeating the organic light emitting layer  950  through the pixel electrode  947  or a third interlayer insulating film  982 . In the case where the second interlayer insulating film  939  includes an organic resin material, it is particularly effective to provide the passivation film  981  since the organic resin material contains a large amount of moisture. 
   In this embodiment, a silicon nitride film is used as the passivation film  981   
   Thereafter, a resist mask having a predetermined pattern is formed, and contact holes reaching impurity regions, which are source regions or drain regions, are formed in the respective semiconductor layers. The contact holes are formed by a dry etching method. In this case, the second interlayer insulating film  939  comprised of the organic resin material is first etched by using a gas mixture of CF 4 , O 2  and He as an etching gas. Subsequently, the first interlayer insulating film  937  is etched with CF 4  and O 2  as an etching gas. Further, in order to raise a selection ratio with the semiconductor layer, the etching gas is changed to CHF 3  to etch the third shape gate insulating film  906 , whereby the contact holes can be formed. 
   Then, a conductive metal film is formed by a sputtering method or a vacuum evaporation method, patterning is performed with a mask, and thereafter, etching is performed. Thus, the source wirings  940  to  943  and the drain wirings  944  to  946  are formed. Although not shown, the wirings are formed of a lamination film of a Ti film with a thickness of 50 nm and an alloy film with a thickness of 500 nm (alloy film of Al and Ti) in this embodiment. 
   Subsequently, a transparent conductive film is formed thereon with a thickness of 80 to 120 nm, and the pixel electrode  947  is formed by patterning ( FIG. 16A ). Note that an indium-tin oxide (ITO) film or a transparent conductive film in which indium oxide is mixed with 2 to 20% of zinc oxide (ZnO) is used for a transparent electrode in this embodiment. 
   Further, the pixel electrode  947  is formed so as to contact and overlap the drain wiring  946 . Thus, electrical connection between the pixel electrode  947  and the drain region of the driving TFT is formed. 
   Next, as shown in  FIG. 16B , the third interlayer insulating film  982  having an opening portion at the position corresponding to the pixel electrode  947  is formed. In this embodiment, side walls having a tapered shape are formed by using a wet etching method in forming the opening portion. Differently from the case shown in Embodiment 5, the organic light emitting layer formed on the third interlayer insulating film  982  is not separated. Thus, the deterioration of the organic light emitting layer which derives from a step becomes a conspicuous problem if the side walls of the opening portion are not sufficiently gentle, which requires attention. 
   Note that although a film made of silicon oxide is used as the third interlayer insulating film  982  in this embodiment, an organic resin film such as polyimide, polyamide, acrylic or BCB (benzocyclobutene) may also be used depending on circumstances. 
   Then, it is preferable that, before the organic light emitting layer  950  is formed on the third interlayer insulating film  982 , plasma processing using argon is conducted to the surface of the third interlayer insulating film  982  to make close the surface of the third interlayer insulating film  982 . With the above structure, it is possible to prevent moisture from permeating the organic light emitting layer  950  from the third interlayer insulating film  982 . 
   Next, the organic light emitting layer  950  is formed by an evaporation method, and further, the cathode (MgAg electrode)  951  and the protecting electrode  952  are formed by the evaporation method. At this time, it is desirable that heat treatment is conducted to the pixel electrode  947  to completely remove moisture prior to the formation of the organic light emitting layer  950  and the cathode  951 . Note that, the MgAg electrode is used as the cathode of the OLED in this embodiment, but other known materials may also be used. 
   Note that a known material can be used for the organic light emitting layer  950 . In this embodiment, the organic light emitting layer takes a two-layer structure constituted of a hole transporting layer and a light emitting layer. However, there may be a case where any one of a hole injecting layer, an electron injecting layer and an electron transporting layer is included in the organic light emitting layer. Various examples of combinations have been reported as described above, and any structure among those may be used. 
   In this embodiment, polyphenylene vinylene is formed by the evaporation method for forming the hole transporting layer. Further, polyvinylcarbazole dispersed with PBD of 1, 3, 4-oxadiazole derivative with 30 to 40% molecules is formed by the evaporation method for forming the light emitting layer, and about 1% of coumarin 6 is added thereto as the emission center of green color. 
   Further, it is possible to protect the organic light emitting layer  950  from moisture and oxygen in the protecting electrode  952 , but the protective film  953  may be, more preferably, provided. In this embodiment, a silicon nitride film with a thickness of 300 nm is provided as the protective film  953 . This protective film may be continuously formed without exposure to an atmosphere after the formation of the protecting electrode  952 . 
   Moreover, the protecting electrode  952  is provided for preventing deterioration of the cathode  951  and is typified by a metal film containing aluminum as its main constituent. Of course, other materials may also be used. Further, since the organic light emitting layer  950  and the cathode  951  are extremely easily affected by moisture, it is desirable that the formation is continuously performed through the formation of the protecting electrode  952  without exposure to an atmosphere to thereby protect the organic light emitting layer against an outer atmosphere. 
   Note that the thickness of the organic light emitting layer  950  may be 10 to 400 nm (typically, 60 to 150 nm) and the thickness of the cathode  951  mast be 80 to 200 nm (typically, 100 to 150 nm). 
   Thus, the light emitting device with the structure as shown in  FIG. 16B  is completed. Note that the portion  954 , where the pixel electrode  947 , the organic light emitting layer  950  and the cathode  951  are overlapped one another, corresponds to the OLED. 
   The p-channel TFT  960  and the n-channel TFT  961  are the TFTs of the driver circuit, and form a CMOS. The switching TFT  962  and the driving TFT  963  are the TFTs of the pixel portion. The TFTs of the driver circuit and the TFTs of the pixel portion can be formed-on the same substrate. 
   The method of manufacturing the light emitting device of the present invention is not limited to the manufacturing method described in this embodiment. The light emitting device of the present invention can be manufactured by using a known method. 
   Note that this embodiment can be implemented by freely being combined with Embodiments 1 to 9. 
   Embodiment 11 
   The light emitting device is of the self-emission type, and thus exhibits more excellent recognizability of the displayed image in a light place as compared to the liquid crystal display device. Furthermore, the light emitting device has a wider viewing angle. Accordingly, the light emitting device can be applied to a display portion in various electronic devices. 
   Such electronic devices using a light emitting device of the present invention include a video camera, a digital camera, a goggles-type display (head mount display), a navigation system, a sound reproduction device (a car audio equipment and an audio set), note-size personal computer, a game machine, a portable information terminal (a mobile computer, a portable telephone, a portable game machine, an electronic book, or the like), an image reproduction apparatus including a recording medium (more specifically, an apparatus which can reproduce a recording medium such as a digital versatile disc (DVD) and so forth, and includes a display for displaying the reproduced image), or the like. In particular, in the case of the portable information terminal, use of the light emitting device is preferable, since the portable information terminal that is likely to be viewed from a tilted direction is often required to have a wide viewing angle.  FIGS. 17A to 17H  respectively shows various specific examples of such electronic devices. 
     FIG. 17A  illustrates an organic light emitting display device which includes a casing  2001 , a support table  2002 , a display portion  2003 , a speaker portion  2004 , a video input terminal  2005  or the like. The present invention is applicable to the display portion  2003 . The light emitting device is of the self-emission type and therefore requires no back light. Thus, the display portion thereof can have a thickness thinner than that of the liquid crystal display device. The organic light emitting display device is including all of the display device for displaying information, such as a personal computer, a receiver of TV broadcasting and an advertising display. 
     FIG. 17B  illustrated a digital still camera which includes a main body  2101 , a display portion  2102 , an image receiving portion  2103 , an operation key  2104 , an external connection port  2105 , a shutter  2106 , or the like. The light emitting device in accordance with the present invention can be used as the display portion  2102 . 
     FIG. 17C  illustrates a laptop computer which includes a main body  2201 , a casing  2202 , a display portion  2203 , a keyboard  2204 , an external connection port  2205 , a pointing mouse  2206 , or the like. The light emitting device in accordance with the present invention can be used as the display portion  2203 . 
     FIG. 17D  illustrated a mobile computer which includes a main body  2301 , a display portion  2302 , a switch  2303 , an operation key  2304 , an infrared port  2305 , or the like. The light emitting device in accordance with the present invention can be used as the display portion  2302 . 
     FIG. 17E  illustrates an image reproduction apparatus including a recording medium (more specifically, a DVD reproduction apparatus), which includes a main body  2401 , a casing  2402 , a display portion A  2403 , another display portion B  2404 , a recording medium (DVD or the like) reading portion  2405 , an operation key  2406 , a speaker portion  2407  or the like. The display portion A  2403  is used mainly for displaying image information, while the display portion B  2404  is used mainly for displaying character information. The light emitting device in accordance with the present invention can be used as these display portions A and B. The image reproduction apparatus including a recording medium further includes a game machine or the like. 
     FIG. 17F  illustrates a goggle type display (head mounted display) which includes a main body  2501 , a display portion  2502 , an arm portion  2503 . The light emitting device in accordance with the present invention can be used as the display portion  2502 . 
     FIG. 17G  illustrates a video camera which includes a main body  2601 , a display portion  2602 , a casing  2603 , an external connecting port  2604 , a remote control receiving portion  2605 , an image receiving portion  2606 , a battery  2607 , a sound input portion  2608 , an operation key  2609 , or the like. The light emitting device in accordance with the present invention can be used as the display portion  2602 . 
     FIG. 17H  illustrates a mobile phone which includes a main body  2701 , a casing  2702 , a display portion  2703 , a sound input portion  2704 , a sound output portion  2705 , an operation key  2706 , an external connecting port  2707 , an antenna  2708 , or the like. The light emitting device in accordance with the present invention can be used as the display portion  2703 . Note that the display portion  2703  can reduce power consumption of the portable telephone by displaying white-colored characters on a black-colored background. 
   When the brighter luminance of light emitted from the organic light emitting material becomes available in the future, the light emitting device in accordance with the present invention will be applicable to a front-type or rear-type projector in which light including output image information is enlarged by means of lenses or the like to be projected. 
   The aforementioned electronic devices are more likely to be used for display information distributed through a telecommunication path such as Internet, a CATV (cable television system), and in particular likely to display moving picture information. The light emitting device is suitable for displaying moving pictures since the organic light emitting material can exhibit high response speed. 
   A portion of the light emitting device that is emitting light consumes power, so it is desirable to display information in such a manner that the light emitting portion therein becomes as small as possible. Accordingly, when the light emitting device is applied to a display portion which mainly displays character information, e.g., a display portion of a portable information terminal, and more particular, a portable telephone or a sound reproduction device, it is desirable to drive the light emitting device so that the character information is formed by a light emitting portion while a non-emission portion corresponds to the background. 
   As set forth above, the present invention can be applied variously to a wide range of electronic devices in all fields. The electronic device in this embodiment can be obtained by utilizing a light emitting device having the configuration in which the structures in Embodiments 1 through 10 are freely combined. 
   According to the present invention, the reduction of the luminance of the OLED is suppressed even if the organic light emitting layer is deteriorated with the structure easily used in practical use, as a result of which a clear image can be displayed. Further, in case of the light emitting device with the color display in which the OLEDs corresponding to respective colors are used, the balance of the luminance among the respective colors is prevented from being lost, and a desired color can be kept being displayed even if the organic light emitting layers of the OLEDs deteriorate at different speeds in accordance with the corresponding colors. 
   Further, the change of the luminance of the OLED can be suppressed even if the temperature of the organic light emitting layer is influenced by the outer temperature, the heat generated by the OLED panel itself, or the like. Also, the increase in power consumption with the temperature rise can be prevented. Further, in case of the light emitting device with the color display the change of the luminance of the OLED of each color can be suppressed without being influenced by the temperature change. Thus, the balance of the luminance among the respective colors is prevented from being lost, and a desired color can be displayed.