Patent Publication Number: US-8111215-B2

Title: Display device and electronic device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display device and to a television device having a light emitting element. 
     2. Description of the Related Art 
     In recent years, a display device having a light emitting element represented by an EL (Electro Luminescence) element has been developed and expected to be widely used by taking advantages as a self-luminous type device, such as high image quality, wide viewing angle, thin design, and lightweight. A light emitting element has a property that the luminance thereof is in proportion to a current value. Therefore, there is a display device which employs a constant current drive in which a constant current is supplied to the light emitting element in order to obtain an accurate gray scale (for example, see Patent Document 1). 
     [Patent Document 1]
     Japanese Patent Laid-Open No. 2003-323159   

     SUMMARY OF THE INVENTION 
     A light emitting element has a property that a resistance value (internal resistance) changes in accordance with the ambient temperature (hereinafter referred to as the environment temperature). In specific, with a room temperature set as a normal temperature, when the temperature becomes higher than the normal temperature, the resistance value decreases while the resistance value increases when the temperature becomes lower than the normal temperature. Accordingly, when the temperature rises, a luminance higher than desired is obtained as a current value increases. Thus, in the case of applying the same voltage at a lower temperature, a luminance lower than desired is obtained as a current value decreases. Such a property of a light emitting element is shown in a graph of a relationship between voltage-current (hereinafter also referred to as V-I) characteristics of a light emitting element and temperature (see  FIG. 17A ). Further, a light emitting element has a property that a current value thereof decreases with time. In specific, a resistance value increases in accordance with the degradation of light emitting element when light emission and non-light emission time is accumulated. Accordingly, in the case of applying the same voltage after the light emission and non-light emission time is accumulated, a luminance lower than desired is obtained as a current value decreases. Such a property of a light emitting element is shown in a graph of a relationship between V-I characteristics of a light emitting element and time (see  FIG. 17B ). 
     Due to the aforementioned properties of a light emitting element, luminance thereof varies when the environment temperature changes and changes time occur. In view of the aforementioned, the invention suppresses the influence of changes in current value of a light emitting element due to changes in environment temperature and changes with time. 
     The invention provides a display device provided with a compensation function for the changes in environment temperature and a compensation function for the changes with time (hereinafter also collectively referred to as a compensation function). 
     The invention provides a display device having a first transistor and a second transistor. A drain terminal of the first transistor and a drain terminal of the second transistor are electrically connected. A source terminal of the first transistor and a first electrode for supplying a current to a first light emitting element are electrically connected. A source terminal of the second transistor and a first electrode for supplying a current to a second light emitting element are electrically connected. The other electrode for supplying a current to the second light emitting element and an input terminal of an amplifier circuit are electrically connected. The second electrode for supplying a current to the second light emitting element and a current source circuit are electrically connected. A second electrode for supplying a current to the first light emitting element and an output terminal of the amplifier circuit are electrically connected. 
     The invention provides a display device having a first transistor and a second transistor. A source terminal of the first transistor and a first electrode for supplying a current to the first light emitting element are electrically connected. A source terminal of the second transistor and a first electrode for supplying a current to the second light emitting element are electrically connected. A gate terminal of the second transistor and a drain terminal of the second transistor are electrically connected. The drain terminal of the second transistor and an input terminal of an amplifier circuit are electrically connected. The drain terminal of the second transistor and a current source circuit are electrically connected. A second electrode for supplying a current to the first light emitting element and a second electrode for supplying a current to the second light emitting element are electrically connected. A gate terminal of the first transistor and an output terminal of a video signal generating circuit are electrically connected. An output terminal of the amplifier circuit and an input terminal of the video signal generating circuit are electrically connected. 
     The invention provides a display device having a first transistor and a second transistor. A source terminal of the first transistor and a first electrode for supplying a current to a first light emitting element are electrically connected. A source terminal of the second transistor and a first electrode for supplying a current to the second light emitting element are electrically connected. A drain terminal of the second transistor and an input terminal of an amplifier circuit are electrically connected. The drain terminal of the second transistor and a current source circuit are electrically connected. A second electrode for supplying a current to the first light emitting element and a second electrode for supplying a current to the second light emitting element are electrically connected. An output terminal of the amplifier circuit and a drain terminal of the first transistor are electrically connected. 
     The invention provides a display device having a first transistor, a second transistor, a first light emitting element, and a second light emitting element. A drain terminal of the first transistor and a drain terminal of the second transistor are electrically connected. A source terminal of the first transistor and one electrode of the first light emitting element are electrically connected. A source terminal of the second transistor and one electrode of the second light emitting element are electrically connected. The other electrode of the second light emitting element and an input terminal of a voltage follower circuit are electrically connected. The other electrode of the second light emitting element and a current source circuit are electrically connected. The other electrode of the first light emitting element and an output terminal of the voltage follower circuit are electrically connected. 
     The invention provides a display device having a first transistor, a second transistor, a first light emitting element, and a second light emitting element. A source terminal of the first transistor and one electrode of the first light emitting element are electrically connected. A source terminal of the second transistor and one electrode of the second light emitting element are electrically connected. A gate terminal of the second transistor and a drain terminal of the second transistor are electrically connected. The drain terminal of the second transistor and an input terminal of a voltage follower circuit are electrically connected. The drain terminal of the second transistor and a current source circuit are electrically connected. The other electrode of the first light emitting element and the other electrode of the second light emitting element are electrically connected. A gate terminal of the first transistor and an output terminal of a video signal generating circuit are electrically connected. An output terminal of the voltage follower circuit and an input terminal of the video signal generating circuit are electrically connected. 
     The invention provides a display device having a first transistor, a second transistor, a first light emitting element, and a second light emitting element. A source terminal of the first transistor and one electrode of the first light emitting element are electrically connected. A source terminal of the second transistor and one electrode of the second light emitting element are electrically connected. A drain terminal of the second transistor and an input terminal of a voltage follower circuit are electrically connected. The drain terminal of the second transistor and a current source circuit are electrically connected. The other electrode of the first light emitting element and the other electrode of the second light emitting element are electrically connected. An output terminal of the voltage follower circuit and a drain terminal of the first transistor are electrically connected. 
     In the aforementioned configurations, a channel forming region of each transistor can be formed of an amorphous semiconductor or a semi-amorphous semiconductor. That is, a thin film transistor (hereinafter also referred to as a TFT) formed of an amorphous semiconductor film or a semi-amorphous semiconductor film can be used. 
     The invention provides a television device provided with any one of the aforementioned configurations. The television device is a thin device of which pixels are formed using an electroluminescence material. 
     The invention can provide a display device which suppresses the effect of variations in current values of light emitting elements caused by the change of environment temperature and the change with time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are circuit diagrams showing a configuration of a display device according to one embodiment mode. 
         FIGS. 2A and 2B  are circuit diagrams showing a configuration of a display device according to one embodiment mode. 
         FIG. 3  is a circuit diagram showing a configuration of a display device according to one embodiment mode. 
         FIGS. 4A and 4B  are circuit diagrams showing a configuration of a display device according to one embodiment mode. 
         FIGS. 5A and 5B  are circuit diagrams showing a configuration of a display device according to one embodiment mode. 
         FIG. 6  is a diagram showing an example of correcting a video signal inputted to a signal driver circuit which drives a pixel portion. 
         FIG. 7  is a diagram showing an example of correcting a video signal inputted to a signal driver circuit which drives a pixel portion. 
         FIGS. 8A to 8E  are sectional diagrams showing manufacturing steps of an EL display panel according to one embodiment mode. 
         FIGS. 9A to 9D  are sectional diagrams showing manufacturing steps of an EL display panel according to one embodiment mode. 
         FIG. 10  is a top view of an EL display panel according to one embodiment mode. 
         FIG. 11  is a diagram showing an EL display module according to one embodiment mode. 
         FIG. 12  is a diagram showing an EL display module according to one embodiment mode. 
         FIG. 13  is a diagram showing a structure of a laser beam drawing apparatus 
         FIGS. 14A to 14C  are sectional diagrams showing a structure of an EL display panel according to one embodiment mode. 
         FIG. 15  is a perspective view of a droplet discharge apparatus. 
         FIGS. 16A to 16D  are views of examples of electronic devices. 
         FIGS. 17A and 17B  are graphs each showing a relationship between V-I characteristics of a light emitting element and temperature. 
         FIG. 18  is a circuit diagram showing a configuration of a display device according to one embodiment mode. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the invention will be described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the invention, they should be construed as being included therein. Therefore, the embodiment modes and embodiments are not intended as a definition of the limits of the invention. 
     Embodiment Mode 1 
       FIG. 1A  shows a circuit configuration. A pixel includes a selecting transistor  3001 , a driving transistor  3002 , and a light emitting element  3006 . A source signal line  3003  to which a video signal is inputted and a gate terminal of the driving transistor  3002  are connected through the selecting transistor  3001 . A gate terminal of the selecting transistor  3001  is connected to a gate signal line  3007 . The driving transistor  3002  and the light emitting element  3006  are connected between a first power supply line  3004  and a second power supply line  3005 . A current flows from the first power supply line  3004  to the second power supply line  3005 . The light emitting element  3006  emits light in accordance with the size of current supplied thereto. 
     An analog switch  3009  provided between a video line  3010  to which a video signal is inputted and the source signal line  3003  is controlled using a shift register  3008 . A video signal supplied to the source signal line  3003  is inputted to the gate electrode of the driving transistor  3002 . A current flows to the driving transistor  3002  and the light emitting element  3006  in accordance with the video signal. 
     It is to be noted that a capacitor may be provided for holding a video signal inputted to the gate terminal of the driving transistor  3002 . In that case, a capacitor may be provided between the gate terminal of the driving transistor  3002  and the drain terminal of the driving transistor  3002 . Alternatively, a capacitor may be provided between the gate terminal of the driving transistor  3002  and a source terminal of the driving transistor  3002 . Otherwise, a capacitor may be provided between the gate terminal of the driving transistor  3002  and another wiring (a dedicated wiring, a gate signal line of a pixel of preceding stage and the like). A capacitor is not necessarily provided when gate capacitance of the driving transistor  3002  is large enough. It is to be noted that the driving transistor  3002  and the selecting transistor  3001  are N-channel transistors, however, the invention is not limited to this. 
     In such a pixel configuration, when potentials of the first power supply line  3004  and the second power supply line  3005  are fixed, current keeps flowing to the light emitting element  3006  and the driving transistor  3002 , thereby characteristics thereof degrade. The light emitting element  3006  and the driving transistor  3002  change their characteristics according to the temperature. In specific, V-I characteristics shift when current keeps flowing to the light emitting element  3006 . That is to say, a resistance value of the light emitting element  3006  increases, thus a current value supplied thereto becomes small even with the same voltage applied. Moreover, light emission efficiency decreases and the luminance decreases even with the same current supplied. As a temperature behavior, V-I characteristics of the light emitting element  3006  shift when the temperature falls, thereby a resistance value of the light emitting element  3006  becomes high. 
     Similarly, when current keeps flowing to the driving transistor  3002 , a threshold voltage thereof becomes high. Therefore, a current becomes small even with the same gate voltage applied. A current value flowing changes therethrough according to the temperature as well. 
     In view of this, a monitoring circuit is used for correcting the effect of the aforementioned degradation and changes. In this embodiment mode, by controlling the potential of the second power supply line  3005 , degradation of the light emitting element  3006  and temperature change and the changes in current value of the driving transistor  3002  due to degradation are corrected 
     A configuration of a monitoring circuit is described. A monitoring driving transistor  3014 , a monitoring light emitting element  3011 , and a monitoring current source  3013  are connected between the first power supply line  3004  and a third power supply line  3012 . An input terminal of the voltage follower circuit  3015  is connected at a connection of the monitoring light emitting element  3011  and the monitoring current source  3013 . An output terminal of the voltage follower circuit  3015  is connected to the second power supply line  3005 . Therefore, the potential of the second power supply line  3005  is controlled by an output of the voltage follower circuit  3015 . 
     Next, an operation of the monitoring circuit is described. First, the monitoring current source  3013  supplies the light emitting element  3006  with a current required for the light emitting element  3006  to emit light at the highest gray scale level. A current value at this time is referred to as Imax. A potential Vb which is the same as that of the video signal inputted to the pixel (a gate terminal of the driving transistor  3002 ) when the light emitting element  3006  emits light at the highest gray scale level is applied to a gate terminal of the monitoring driving transistor  3014 . 
     Then, a voltage high enough to supply a current having the size of Imax is applied as a voltage between a gate and a source (hereinafter also referred to as a gate-source voltage) of the monitoring driving transistor  3014 . That is to say, a source potential of the monitoring driving transistor  3014  becomes high enough to supply a current having the size of Imax. Even if a threshold voltage of the monitoring driving transistor  3014  changes due to degradation, temperature, and the like, the gate-source voltage (source potential) changes accordingly, thereby becomes an optimum level. Accordingly, the effect of variation in a threshold voltage (degradation, temperature change and the like) can be corrected. 
     Similarly, a voltage high enough to supply a current having the size of Imax is applied to both terminals of the monitoring light emitting element  3011 . Even if V-I characteristics of the monitoring light emitting element  3011  change due to degradation, temperature, and the like, voltages of the both terminals of the monitoring light emitting element  3011  change accordingly, thereby become an optimum level. Accordingly, the effect of variation in the monitoring light emitting element  3011  (degradation, temperature change and the like) can be corrected. 
     A sum of a voltage applied to the monitoring driving transistor  3014  and a voltage applied to the monitoring light emitting element  3011  is inputted to the input terminal of the voltage follower circuit  3015 . Therefore, a potential of the output terminal of the voltage follower circuit  3015 , that is the second power supply line  3005  is corrected by the monitoring circuit. Therefore, the changes of the light emitting element  3006  and the driving transistor  3002  due to degradation and temperature are also corrected. 
     It is to be noted that the voltage follower circuit is not limited to this. That is, any circuit can be applied as long as it outputs a voltage according to an input current. The voltage follower circuit is one of amplifier circuits, however, the invention is not limited to this. A circuit may be configured by using any one or a plurality of an operational amplifier, a bipolar transistor, and a MOS transistor in combination. 
     It is preferable that the monitoring light emitting element  3011  and the monitoring driving transistor  3014  be formed over the same substrate at the same time as the light emitting element  3006  and the driving transistor  3002  by the same manufacturing method. This is because the same correction cannot be performed if characteristics differ between the monitoring element and the transistor provided in a pixel. 
     The description has been made on the case where a potential as high as a video signal inputted to a pixel (the gate terminal of the driving transistor  3002 ) when the light emitting element  3006  emits light at the highest gray scale level is applied to a gate terminal of the monitoring driving transistor  3014  and a current required for the light emitting element  3006  to emit light at the highest gray scale level is supplied to the monitoring current source  3013  when emits. However, the invention is not limited to this. 
     The monitoring light emitting element  3011  and the monitoring driving transistor  3014  degrade more than the light emitting element  3006  and the driving transistor  3002  which are provided in a pixel if the potential based on the highest gray scale level is applied. Therefore, a potential outputted from the voltage follower circuit  3015  is more corrected. Therefore, the monitoring circuit may be set so as to degrade at the same rate as an actual pixel. For example, when the light emission efficiency of the entire screen is 30%, the monitoring circuit may operate at a gray scale level corresponding to a luminance of 30%. 
     In specific, a potential as high as a video signal inputted to a pixel (the gate terminal of the driving transistor  3002 ) when the light emitting element  3006  emits light at a gray scale level corresponding to a luminance of 30% may be applied to the gate terminal of the monitoring driving transistor  3014 . A current having the size to be supplied to the light emitting element  3006  when the light emitting element  3006  emits light at a gray scale level corresponding to the luminance of 30% may be supplied to the monitoring current source  3013 . 
     It is to be noted that a voltage of a video signal is to be increased as shown in  FIG. 1B  for increasing the gray scale level of the light emitting element when the light emitting element drives in the saturation region. In this embodiment mode, a potential of the second power supply line  3005  is corrected which is connected to one electrode of the light emitting element  3006 . Therefore, a voltage of a video signal (video voltage) for increasing the gray scale level of the light emitting element is not required to be corrected. 
     It is to be noted that a potential which is more corrected is outputted when the monitoring circuit operates in accordance with the highest gray scale level, however, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level since image persistence (luminance variation due to the variations in degradation rates among pixels) becomes less noticeable. Therefore, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level. 
     It is to be noted that the driving transistor  3002  may operate only in the saturation region, both in the saturation region and the linear region, or only in the linear region. 
     In the case where the driving transistor  3002  operates only in the linear region, the driving transistor  3002  operates mostly as a switch. Accordingly, variations in characteristics due to degradation, temperature change and the like of the driving transistor  3002  do not affect much. However, the effect of variations in characteristics due to degradation, temperature change and the like of the light emitting element  3006  is corrected. In the case where the driving transistor  3002  operates only in the linear region, whether a current is supplied to the light emitting element  3006  is often controlled digitally. In that case, a time gray scale method, an area gray scale method and the like are often used in combination for performing a multi-gray scale display. 
     Embodiment Mode 2 
     In this embodiment mode, description is made on the case of performing correction using a video signal. 
       FIG. 2A  shows a circuit configuration. A pixel includes the selecting transistor  3001 , the driving transistor  3002 , and the light emitting element  3006 . The source signal line  3003  to which a video signal is inputted and the gate terminal of the driving transistor  3002  are connected through the selecting transistor  3001 . The gate terminal of the selecting transistor  3001  is connected to the gate signal line  3007 . The driving transistor  3002  and the light emitting element  3006  are connected between the first power supply line  3004  and a second power supply line  4005 . A current flows from the first power supply line  3004  to a second power supply line  4005 . The light emitting element  3006  emits light in accordance with the current supplied thereto. 
     The analog switch  3009  provided between the video line  3010  to which a video signal is inputted and the source signal line  3003  is controlled using the shift register  3008 . A video signal supplied to the source signal line  3003  is inputted to the gate electrode of the driving transistor  3002 . A current flows to the driving transistor  3002  and to the light emitting element  3006  in accordance with the video signal. 
     A video signal generating circuit  4031  is connected as a circuit for supplying a video signal to the video line  3010 . The video signal generating circuit  4031  has a function to process a video signal for correcting variations of the driving transistor  3002  and the light emitting element  3006  due to degradation, temperature change and the like. 
     In such a pixel configuration, when potentials of the first power supply line  3004  and the second power supply line  4005  are fixed, current keeps flowing to the light emitting element  3006  and the driving transistor  3002 , thereby characteristics thereof degrade. The light emitting element  3006  and the driving transistor  3002  change their characteristics according to the temperature. 
     In specific, V-I characteristics shift when current keeps flowing to the light emitting element  3006 . That is to say, a resistance value of the light emitting element  3006  increases, thus a current value supplied thereto becomes small even with the same voltage applied. Moreover, light emission efficiency decreases and the luminance decreases even with the same current supplied. As temperature characteristics, V-I characteristics of the light emitting element  3006  shift when the temperature falls, thereby a resistance value of the light emitting element  3006  becomes high. 
     Similarly, when current keeps flowing to the driving transistor  3002 , a threshold voltage thereof becomes high. Therefore, a current becomes small even with the same gate voltage applied. A current value flowing therethrough changes according to the temperature as well. 
     In view of this, a monitoring circuit is used for correcting the aforementioned effect of degradation and variation. In this embodiment mode, by controlling a voltage of the video signal, variations of the light emitting element  3006  and the driving transistor  3002  due to degradation and temperature are corrected. 
     First, a configuration of a monitoring circuit is described. A monitoring current source  4013 , monitoring driving transistor  4014 , and a monitoring light emitting element  4011  are connected between the first power supply line  4012  and the second power supply line  4005 . An input terminal of a voltage follower circuit  4015  is connected at a connection of the monitoring current source  4013  and the monitoring light emitting element  4011 . An output terminal of the voltage follower circuit  4015  is connected to the video signal generating circuit  4031 . Therefore, the voltage of the video signal is controlled by an output of the voltage follower circuit  4015 . 
     Next, an operation of the monitoring circuit is described. First, the monitoring current source  4013  supplies to the light emitting element  3006  a current required for the light emitting element  3006  to emit light at the highest gray scale level. A current value at this time is referred to as Imax. A gate terminal of the monitoring driving transistor  4014  is connected to a drain terminal of the monitoring driving transistor  4014 . 
     Then, a voltage high enough to supply a current having the size of Imax is applied as a gate-source voltage of the monitoring driving transistor  4014 . That is to say, a source potential of the monitoring driving transistor  4014  becomes high enough to supply a current having the size of Imax. As the drain terminal is connected to the gate terminal, the drain potential becomes high enough to supply a current having the size of Imax. Even if a threshold voltage of the monitoring driving transistor  4014  changes due to degradation, temperature, and the like, the gate-source voltage (source potential and drain potential) changes accordingly, thereby becomes an optimum level. Accordingly, the effect of variation of a threshold voltage (degradation, temperature change and the like) can be corrected. 
     Similarly, a voltage high enough to supply a current having the size of Imax is applied to both terminals of the monitoring light emitting element  4011 . Even if V-I characteristics of the monitoring light emitting element  4011  change due to degradation, temperature, and the like, voltages of the both terminals of the monitoring light emitting element  4011  change accordingly, thereby become an optimum level. Accordingly, an effect of variation of the monitoring light emitting element  4011  (degradation, temperature change and the like) can be corrected. 
     A sum of a voltage applied to the monitoring driving transistor  4014  and a voltage applied to the monitoring light emitting element  4011  is inputted to the input terminal of the voltage follower circuit  4015 . Therefore, a potential of the output terminal of the voltage follower circuit  4015 , that is a potential of a video signal outputted from the video signal generating circuit  4031  is corrected by the monitoring circuit. Therefore, the variations of the light emitting element  3006  and the driving transistor  3002  due to degradation and temperature-change are corrected. 
     It is to be noted that the voltage follower circuit is not limited to this. That is, any circuit can be applied as long as it outputs a voltage according to an input current. The voltage follower circuit is one of amplifier circuits, however, the invention is not limited to this. A circuit may be configured by using any one or a plurality of an operational amplifier, a bipolar transistor, and a MOS transistor in combination. 
     It is preferable that the monitoring light emitting element  4011  and the monitoring driving transistor  4014  be formed over the same substrate at the same time as the light emitting element and the driving transistor  3002  by the same manufacturing method. This is because the same correction cannot be performed if characteristics differ between the monitoring element and the transistor provided in a pixel. 
     The description has been made on the case where a potential as high as a video signal inputted to a pixel (the gate terminal of the driving transistor  3002 ) when the light emitting element  3006  emits light at the highest gray scale level is applied to the gate terminal of the monitoring driving transistor  4014  and a current required for the light emitting element  3006  to emit light at the highest gray scale level is supplied to the monitoring current source  4013 . However, the invention is not limited to this. 
     The monitoring light emitting element  4011  and the monitoring driving transistor  4014  degrade more than the light emitting element  3006  and the driving transistor  3002  that are provided in a pixel if the potential based on the highest gray scale level is applied. Therefore, a potential outputted from the voltage follower circuit  4015  is more corrected. Therefore, the monitoring circuit may be set so as to degrade at the same rate as an actual pixel. For example, when the light emission efficiency of the entire screen is 30%, the monitoring circuit may operate at a gray scale level corresponding to a luminance of 30%. 
     It is to be noted that a voltage of a video signal is to be increased as shown in  FIG. 2B  for increasing the gray scale level of the light emitting element when the light emitting element operates in the saturation region. In this embodiment mode, a potential of the gate terminal of the driving transistor  3002  is corrected. Therefore, a desired luminance of a light emitting element can be displayed by correcting a voltage of a video signal (a video voltage) to be as shown in  FIG. 2B  in accordance with the change in characteristics of the light emitting element  3006 . 
     In specific, a current of a desired size to be supplied to the light emitting element  3006  when the light emitting element  3006  emits light at a gray scale level corresponding to a luminance of 30% may be supplied to the monitoring current source  4013 . The video signal generating circuit  4031  may output a video signal accordingly. 
     It is to be noted that a potential which is more corrected is outputted when the monitoring circuit operates in accordance with the highest gray scale level, however, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level since image persistence (luminance variation due to the variation of degradation among pixels) becomes less noticeable. Therefore, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level. 
     It is to be noted that the driving transistor  3002  may operate only in the saturation region, or both in the saturation region and the linear region. 
     Embodiment Mode 3 
     In this embodiment mode, description is made on the case of performing correction using a potential of the first power supply line. 
       FIG. 3  shows a circuit configuration. A pixel includes the selecting transistor  3001 , the driving transistor  3002 , and the light emitting element  3006 . The source signal line  3003  to which a video signal is inputted and the gate terminal of the driving transistor  3002  are connected through the selecting transistor  3001 . The gate terminal of the selecting transistor  3001  is connected to the gate signal line  3007 . The driving transistor  3002  and the light emitting element  3006  are connected between a first power supply line  5004  and a second power supply line  5005 . A current flows from the first power supply line  5004  to the second power supply line  5005 . The light emitting element  3006  emits light in accordance with the size of current supplied thereto. 
     The analog switch  3009  provided between the video line  3010  to which a video signal is inputted and the source signal line  3003  is controlled using the shift register  3008 . A video signal supplied to the source signal line  3003  is inputted to the gate electrode of the driving transistor  3002 . A current flows to the driving transistor  3002  and the light emitting element  3006  in accordance with the video signal size. 
     In such a pixel configuration, when potentials of the first power supply line  5004  and the second power supply line  5005  are fixed, characteristics of the light emitting element  3006  and the driving transistor  3002  degrade when current keeps flowing therethrough. The light emitting element  3006  and the driving transistor  3002  change their characteristics according to the temperature. 
     In specific, V-I characteristics shift when current keeps flowing to the light emitting element  3006 . That is to say, a resistance value of the light emitting element  3006  increases, thus a current value supplied thereto becomes small even with the same voltage applied. Moreover, light emission efficiency decreases and the luminance decreases even with the same current supplied. As temperature characteristics, V-I characteristics of the light emitting element  3006  shift when the temperature falls, thereby a resistance value of the light emitting element  3006  becomes high. 
     Similarly, when current keeps flowing to the driving transistor  3002 , a threshold voltage thereof becomes high. Therefore, a current becomes small even with the same gate voltage applied. A current value flowing therethrough changes according to the temperature as well. 
     In view of this, a monitoring circuit is used for correcting the aforementioned degradation effect of and variation. In this embodiment mode, by controlling the potential of the first power supply line  5004 , variations of the light emitting element  3006  and the driving transistor  3002  due to degradation and temperature are corrected. 
     A configuration of a monitoring circuit is described. A monitoring current source  5013 , a monitoring driving transistor  5014 , and a monitoring light emitting element  5011  are connected between a first power supply line  5012  and the second power supply line  5005 . An input terminal of a voltage follower circuit  5015  is connected at a connection of the monitoring current source  5013  and the monitoring light emitting element  5011 . An output terminal of the voltage follower circuit  5015  is connected to the first power supply line  5004 . Therefore, the potential of the first power supply line  5004  is controlled by an output of the voltage follower circuit  5015 . 
     Next, an operation of the monitoring circuit is described. First, the monitoring current source  5013  supplies to the light emitting element  3006  a current required for the light emitting element  3006  to emit light at the highest gray scale level. A current value at this time is referred to as Imax. A potential Vc as high as a video signal inputted to a pixel (the gate terminal of the driving transistor  3002 ) when the light emitting element  3006  emits light at the highest gray scale level is applied to the gate terminal of the monitoring driving transistor  5014 . 
     Then, a voltage high enough to supply a current having the size of Imax is applied as a gate-source voltage or between the drain and the source (hereinafter referred to as drain-source) of the monitoring driving transistor  5014 . That is to say, a source potential and a drain potential of the monitoring driving transistor  5014  become high enough to supply a current having the size of Imax. Even if a threshold voltage of the monitoring driving transistor  5014  changes due to degradation, temperature, and the like, the gate-source voltage (source potential) and the drain-source voltage (drain potential) change accordingly, thereby becomes an optimum level. Accordingly, the effect of variation of a threshold voltage (degradation, temperature change and the like) can be corrected. 
     Similarly, a voltage high enough to supply a current having the size of Imax is applied to both terminals of the monitoring light emitting element  5011 . Even if V-I characteristics of the monitoring light emitting element  5011  change due to degradation, temperature, and the like, voltages of the both terminals of the monitoring light emitting element  5011  change accordingly, thereby become an optimum level. Accordingly, the effect of variation of the monitoring light emitting element  5011  (degradation, temperature change and the like) can be corrected. 
     A sum of a voltage applied to the monitoring driving transistor  5014  and a voltage applied to the monitoring light emitting element  5011  is inputted to the input terminal of the voltage follower circuit  5015 . Therefore, a potential of the output terminal of the voltage follower circuit  5015 , that is a potential of the first power supply line  5004  is corrected by the monitoring circuit. Therefore, the variations of the light emitting element  3006  and the driving transistor  3002  due to degradation and temperature change are corrected. 
     It is to be noted that the voltage follower circuit is not limited to this. That is, any circuit can be applied as long as it outputs a voltage according to an input current. The voltage follower circuit is one of amplifier circuits, however, the invention is not limited to this. A circuit may be configured by using any one or a plurality of an operational amplifier, a bipolar transistor, and a MOS transistor in combination. 
     It is preferable that the monitoring light emitting element  5011  and the monitoring driving transistor  5014  be formed over the same substrate at the same time as the light emitting element  3006  and the driving transistor  3002  by the same manufacturing method. This is because the same correction cannot be performed if characteristics differ between the monitoring element and the transistor provided in a pixel. 
     There is often a period when a current is not supplied to the light emitting element  3006  and the driving transistor  3002  which are disposed in a pixel. Therefore, when a current keeps flowing to the monitoring light emitting element  5011  and the monitoring driving transistor  5014 , they degrade more than the light emitting element  3006  and the driving transistor  3002 . Accordingly, a potential outputted from the voltage follower circuit  5015  is more corrected. Therefore, the monitoring circuit may be set so as to degrade at the same rate as an actual pixel. For example, when the light emission ratio of the entire display is 30%, a current may be set to flow to the monitoring light emitting element  5011  and the monitoring driving transistor  5014  only in a period corresponding to a luminance of 30%. At that time, there is a period when a current is not supplied to the monitoring light emitting element  5011  and the monitoring driving transistor  5014 , however, it is required that a voltage be applied from the output terminal of the voltage follower circuit  5015  without change. In order to realize this, a capacitor is provided at the input terminal of the voltage follower circuit  5015  for holding a potential of the time when a current is supplied to the monitoring light emitting element  5011  and the monitoring driving transistor  5014 . 
     It is to be noted that a potential which is more corrected is outputted when the monitoring circuit operates in accordance with the highest gray scale level, however, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level since image persistence (luminance variation due to the variation in degradation among pixels) becomes less noticeable. Therefore, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level. 
     It is preferable that the driving transistor  3002  operate in the linear region. This is because a drain potential of the driving transistor  3002  changes for correcting the potential of the first power supply line  5004  in this embodiment mode. When the driving transistor  3002  operates in the saturation region, a current flowing through the driving transistor  3002  does not change much even if the drain potential thereof is changed. On the other hand, when the driving transistor  3002  operates in the linear region, a current value changes when the drain potential changes, thus a correction has a major effect. Therefore, it is preferable that the driving transistor  3002  operate in the linear region. 
     When the driving transistor  3002  operates only in the saturation region, it operates mostly as a switch. Accordingly, variations in characteristics of the driving transistor  3002  due to degradation, temperature and the like do not affect much. However, the effect of the variations in characteristics of the light emitting element  3006  due to degradation, temperature and the like are corrected. When the driving transistor  3002  operates only in the linear region, whether a current is supplied to the light emitting element  3006  is often controlled digitally. In that case, a time gray scale method, an area gray scale method and the like are often used in combination for performing a multi-gray scale display. 
     Embodiment Mode 4 
       FIG. 4A  shows a circuit configuration. A pixel includes a selecting transistor  6001 , a driving transistor  6002 , a holding transistor  6009 , a capacitor  6010 , and a light emitting element  6006 . A source signal line  6003  to which a video signal is inputted and a source terminal of the driving transistor  6002  are connected through the selecting transistor  6001 . A gate terminal of the selecting transistor  6001  is connected to a gate signal line  6007 . The driving transistor  6002  and the light emitting element  6006  are connected between a first power supply line  6004  and a second power supply line  6005 . A current flows from the first power supply line  6004  to the second power supply line  6005 . The light emitting element  6006  emits light in accordance with the size of current supplied thereto. The capacitor  6010  is provided between the gate and the source of the driving transistor  6002  while the holding transistor  6009  is connected between the drain and the source of the driving transistor  6002 . A gate terminal of the holding transistor  6009  is connected to the gate signal line  6007 . 
     A signal driver circuit includes a video current source circuit  6008 . The video current source circuit  6008  supplies to a pixel a current of the size corresponding to a video signal. When the gate signal line  6007  is selected, a video signal is supplied to the source signal line  6003  and inputted to the driving transistor  6002 . At this time, as the potential of the first power supply line  6004  is changed, a current does not flow to the light emitting element  6006  because of the potential of the second power supply line  6005 . In accordance with the magnitude of the video signal, a gate-source voltage of a desired level of the driving transistor  6002  is accumulated in the capacitor  6010 . After that, the gate signal line  6007  becomes a non-selected state, thereby the charge accumulated in the capacitor  6010  is held. Therefore, even when a drain potential and a source potential of the driving transistor  6002  changes, the gate-source voltage of the driving transistor  6002  does not change. Then, the potential of the first power supply line  6004  turns back and a current of the size corresponding to the video signal flows to the driving transistor  6002 , and then to the light emitting element  6006 . 
       FIG. 4B  shows a timing chart of potentials of the gate signal line  6007  and the first power supply line  6004 . First, a signal for turning on the selecting transistor  6001  and the holding transistor  6009  is inputted from an i-th gate signal line Vp (i). At the same time, a signal of which potential is an inversion of that of the gate signal line Vp (i) is inputted to an i-th first power supply line Vg (i). Accordingly, a gate-source voltage high enough to flow to the driving transistor  6002  a current corresponding to the magnitude of the video signal is accumulated in the capacitor  6010 . At the same time, a current supplied by the driving transistor  6002  being on can be controlled not to be supplied to the light emitting element  6006  by a relationship with the potential of the second power supply line  6005 . At this time, by making the potential of the second power supply line  6005  higher, a current can be controlled not to be supplied to the light emitting element  6006 . In that case, a gate-source voltage for flowing to the driving transistor  6002  a current corresponding to the magnitude of the video signal of the video current source circuit  6008  is accumulated in the capacitor  6010  of all the pixels in an address period (writing period), thereby all pixels may emit light at once in a sustain period (light emission period). Similar operations are performed in an (i+1)-th gate signal line Vp (i+1), (i+1)th first power supply line Vg (i+1), and an (i+2)-th gate signal line Vp (i+2), and an (i+2)-th first power supply line Vg (i+2). 
     It is to be noted that the driving transistor  6002  and the selecting transistor  6001  are N-channel transistors. However, the invention is not limited to this. 
     In such a pixel configuration, when a current keeps flowing to the light emitting element  6006 , characteristics thereof degrade. Moreover, the characteristics of the light emitting element  6006  change due to temperature of the light emitting element or around the light emitting element. 
     In specific, when a current keeps flowing to the light emitting element  6006 , a light emission efficiency decreases and the luminance decreases even with the same current supplied. 
     In view of this, the aforementioned effect of degradation and variation is corrected by using a monitoring circuit. In this embodiment mode, by controlling the size of current of a video signal, variations of the light emitting element  6006  due to degradation and temperature are corrected. 
     A configuration of the monitoring circuit is described. A monitoring current source  6013 , a monitoring driving transistor  6014 , and a monitoring light emitting element  6011  are connected between a first power supply line  6012  and the second power supply line  6005 . An input terminal of a voltage follower circuit  6015  is connected at a connection of the monitoring current source  6013  and the monitoring driving transistor  6014 . An output terminal of the voltage follower circuit  6015  is connected to an input terminal of a video signal generating circuit  6031  which controls the size of current outputted by the video current source circuit  6008 . Accordingly, the size of current outputted by the video current source circuit  6008  is controlled by the output of the voltage follower circuit  6015 . 
     Next, an operation of the monitoring circuit is described. First, the monitoring current source  6013  supplies to the light emitting element  6006  a current required for the light emitting element  6006  to emit light at the highest gray scale level. A current value at this time is referred to as Imax. 
     Then, a voltage high enough to supply a current having the size of Imax is applied as a gate-source voltage of the monitoring driving transistor  6014  of which gate terminal and drain terminal are connected. That is to say, a source potential and a drain potential of the monitoring driving transistor  6014  become high enough to supply a current having the size of Imax. 
     Similarly, a voltage high enough to supply a current having the size of Imax is applied to both terminals of the monitoring light emitting element  6011 . Even if V-I characteristics of the monitoring light emitting element  6011  change due to degradation, temperature, and the like, voltages of the both terminals of the monitoring light emitting element  6011  change accordingly, thereby become an optimum level. Accordingly, an effect of variation of the monitoring light emitting element  6011  (degradation, temperature change and the like) can be corrected. 
     A sum of a voltage applied to the monitoring driving transistor  6014  and a voltage applied to the monitoring light emitting element  6011  is inputted to the input terminal of the voltage follower circuit  6015 . Therefore, the size of the current outputted from the output terminal of the voltage follower circuit  6015 , that is the video current source circuit  6008  is corrected by the monitoring circuit. Therefore, the variations of the light emitting element  6006  due to degradation and temperature change are also corrected. 
     It is to be noted that the voltage follower circuit is not limited to this. That is, any circuit can be applied as long as it outputs a voltage according to an input current. The voltage follower circuit is one of amplifier circuits, however, the invention is not limited to this. A circuit may be configured by using any one or a plurality of an operational amplifier, a bipolar transistor, and a MOS transistor in combination. 
     It is preferable that the monitoring light emitting element  6011  and the monitoring driving transistor  6014  be formed over the same substrate at the same time as the light emitting element  6006  and the driving transistor  6002  by the same manufacturing method. This is because the same correction cannot be performed if characteristics differ between the monitoring element and the transistor provided in a pixel. 
     The description has been made on the case where the monitoring current source  6013  is supplied with a current required for the light emitting element  6006  to emit light at the highest gray scale level, however, the invention is not limited to this. 
     In accordance with the highest gray scale level, the monitoring light emitting element  6011  degrade more than the light emitting element  6006  which is disposed in a poxel. Accordingly, a potential outputted from the voltage follower circuit  6015  is more corrected. Therefore, the monitoring circuit may be set so as to degrade at the same rate as an actual pixel. For example, when an average light emission ratio of the entire display is 30%, the monitoring circuit may operate according to the gray scale level corresponding to the luminance of 30%. In specific, a current of a desired size to be supplied to the light emitting element  6006  may be supplied to the monitoring current source  6013  when the light emitting element  6006  emits light at a gray scale level corresponding to the luminance of 30%. The video signal generating circuit  6031  may output a video signal accordingly. 
     It is to be noted that a potential which is more corrected is outputted when the monitoring circuit operates in accordance with the highest gray scale level, however, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level since image persistence (luminance variation due to the variation in degradation among pixels) becomes less noticeable. Therefore, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level. 
     It is to be noted that the driving transistor  6002  may operate only in the saturation region, both in the saturation region and the linear region, or only in the linear region. 
     It is to be noted that the pixel configuration is not limited to  FIG. 4 . In  FIG. 4 , a current having a size according to a video signal is supplied to the pixel. Even when current characteristics of the driving transistor  6002  vary, a current having a size according to the video signal can be supplied to the light emitting element  6006 . That is, the variations in current characteristics of the driving transistor  6002  are corrected.  FIG. 18  shows another pixel configuration as an example in which variations in current characteristics of a driving transistor are corrected by supplying to a pixel a current having a size according to a video signal. 
     A pixel includes a selecting transistor  1801 , a driving transistor  1802 , a conversion transistor  1811 , a holding transistor  1809 , a capacitor  1810 , and a light emitting element  1806 . A source signal line  1803  to which a video signal is inputted and a gate terminal of the driving transistor  1802  are connected through the selecting transistor  1801  and the holding transistor  1809 . The selecting transistor  1801  is provided between the source signal line  1803  and a drain terminal of the conversion transistor  1811 . Gate terminals of the selecting transistor  1801  and the holding transistor  1809  are connected to a gate signal line  1807 . The driving transistor  1802  and the light emitting element  1806  are connected between a first power supply line  1804  and a second power supply line  1805 . A current flows from the first power supply line  1804  to the second power supply line  1805 . The light emitting element  1806  emits light according to the current flowing between the first power supply line  1804  and the second power supply line  1805 . A capacitor  1810  is connected to a gate terminal of the driving transistor  1802  and holds a gate potential thereof. The capacitor  1810  is connected between the gate terminal of the driving transistor  1802  and a wiring  1812 , however, the invention is not limited to this. The capacitor  1810  may be connected between the gate and source of the driving transistor  1802 . The holding transistor  1809  is connected between a drain and gate of the conversion transistor  1811 . The driving transistor  1802  and the conversion transistor  1811  form a current mirror in which the gate terminals thereof are connected to each other and source terminals thereof are connected to each other. 
     A signal line driver circuit is provided with a video current source circuit  1808 . The video current source circuit  1808  supplies to a pixel a current having a size according to a video signal. A video signal supplied to the source signal line  6003  when the gate signal line  1807  is selected is inputted to the conversion transistor  1811 . A gate potential of the conversion transistor  1811  having a required level is accumulated in the capacitor  1810 . After that, the gate signal line  1807  becomes a non-selected state, thus a charge accumulated in the capacitor  1810  is stored. As the driving transistor  1802  and the conversion transistor  1811  form a current mirror, a current having a size according to the current supplied to the conversion transistor  1811  flows to the driving transistor  1802 . As a result, a current having a size according to the video signal flows to the driving transistor  1802  and then to the light emitting element  1806 . Here, by designing current capacity (a ratio W/L of channel width W to a channel length L) of the driving transistor  1802  smaller than that of the conversion transistor  1811 , a larger current can be supplied to the conversion transistor  1811 . As a result, a larger current can be supplied from the video current source circuit  1808  to the pixel. As a result, a write speed of a signal to the pixel can be increased. 
     Embodiment Mode 5 
       FIG. 5A  shows a circuit configuration. A pixel includes a selecting transistor  7001 , a driving transistor  7002 , a holding transistor  7009 , a capacitor  7010 , and a light emitting element  7006 . A source signal line  7003  to which a video signal is inputted and a gate terminal of the driving transistor  7002  are connected through the selecting transistor  7001 . A gate terminal of the selecting transistor  7001  is connected to a gate signal line  7007 . The driving transistor  7002  and the light emitting element  7006  are connected between a first power supply line  7004  and a second power supply line  7005 . A current flows from the first power supply line  7004  to the second power supply line  7005 . The light emitting element  7006  emits light in accordance with the size of current supplied thereto. The capacitor  7010  is provided between the gate and the source of the driving transistor  7002  while the holding transistor  7009  is connected between the drain and source of the driving transistor  7002 . A gate terminal of the holding transistor  7009  is connected to a second gate signal line  7016 . 
     In the circuit configuration shown in  FIG. 5A , the holding transistor  7009  is turned on according to a signal inputted from the second gate signal line  7016 . A gate-source voltage of the driving transistor  7002  according to a threshold voltage thereof is accumulated in the capacitor  7010 . Accordingly, variations in threshold voltage of each driving transistor can be corrected in advance. It is to be noted that a charge higher than the threshold voltage may be accumulated in the capacitor in advance by making the potential of the second power supply line higher only for a moment. 
     By using a shift register  7008 , the analog switch  3009  provided between a video line  7040  to which a video signal is inputted and the source signal line  7003  is controlled. The video signal inputted to the source signal line  7003  is inputted to the gate electrode of the driving transistor  7002 . A current flows to the driving transistor  7002  in accordance with the magnitude of the video signal and is supplied to the light emitting element  7006 . 
     It is to be noted that the driving transistor  7002  and the selecting transistor  7001  are N-channel transistors. However, the invention is not limited to this. 
     A video signal generating circuit  7031  is connected as a circuit for supplying a video signal to the video line  7040 . The video signal generating circuit  7031  has a function to process the video signal for correcting variations of the driving transistor  7002  and the light emitting element  7006  due to degradation, temperature and the like. 
     In such a pixel configuration, when potentials of the first power supply line  7004  and the second power supply line  7005  are fixed in the case where the light emitting element  7006  emits light, current keeps flowing to the light emitting element  7006  and the driving transistor  7002 , thereby characteristics thereof degrade. The light emitting element  7006  and the driving transistor  7002  change their characteristics according to the temperature. 
     In specific, V-I characteristics shift when current keeps flowing to the light emitting element  7006 . That is to say, a resistance value of the light emitting element  7006  increases, thus a current value supplied thereto becomes small even with the same voltage applied. Moreover, light emission efficiency decreases and the luminance decreases even with the same current supplied. As temperature characteristics, V-I characteristics of the light emitting element  7006  shift when the temperature falls, thereby a resistance value of the light emitting element  7006  becomes high. 
     Similarly, when current keeps flowing to the driving transistor  7002 , a threshold voltage thereof becomes high. Therefore, a current flowing therethrough becomes small even with the same gate voltage applied. A current value flowing therethrough changes according to the temperature as well. 
     In view of this, a monitoring circuit is used for correcting the aforementioned effect of degradation and variation. In this embodiment mode, by controlling the potential of the video signal, variations of the light emitting element  7006  and the driving transistor  7002  due to degradation and temperature are corrected. 
     A configuration of a monitoring circuit is described. A monitoring current source  7013 , a monitoring driving transistor  7014 , and a monitoring light emitting element  7011  are connected between the first power supply line  7004  and a second power supply line  7012 . An input terminal of a voltage follower circuit  7015  is connected at a connection of the monitoring current source  7013  and the monitoring driving transistor  7014 . An output terminal of the voltage follower circuit  7015  is connected to the video signal generating circuit  7031 . Therefore, the voltage of the video signal is controlled by the output of the voltage follower circuit  7015 . 
     Next, an operation of the monitoring circuit is described. First, the monitoring current source  7013  supplies to the light emitting element  7006  a current required for the light emitting element  7006  to emit light at the highest gray scale level. A current value at this time is referred to as Imax. 
     Then, a voltage high enough to supply a current having the size of Imax is applied as a gate-source voltage of the monitoring driving transistor  7014  of which gate terminal and drain terminal are connected. That is to say, a source potential and a drain potential of the monitoring driving transistor  7014  become high enough to supply a current having the size of Imax. Even if a threshold voltage of the monitoring driving transistor  7014  changes due to degradation, temperature, and the like, the gate-source voltage (source potential and drain potential) changes accordingly, thereby becomes an optimum level. Accordingly, the effect of variation of a threshold voltage (degradation, temperature change and the like) can be corrected. 
     Similarly, a voltage high enough to supply a current having the size of Imax is applied to both terminals of the monitoring light emitting element  7011 . Even if V-I characteristics of the monitoring light emitting element  7011  change due to degradation, temperature, and the like, voltages of the both terminals of the monitoring light emitting element  7011  change accordingly, thereby become an optimum level. Accordingly, the effect of variation of the monitoring light emitting element  7011  (degradation, temperature change and the like) can be corrected. 
     A sum of a voltage applied to the monitoring driving transistor  7014  and a voltage applied to the monitoring light emitting element  7011  is inputted to the input terminal of the voltage follower circuit  7015 . Therefore, a potential of the output terminal of the voltage follower circuit  7015 , that is a potential of a video signal is corrected by the monitoring circuit. Therefore, the variations of the light emitting element  7006  and the driving transistor  7002  due to degradation and temperature change are corrected. 
     It is to be noted that the voltage follower circuit is not limited to this. That is, any circuit can be applied as long as it outputs a voltage according to an input current. The voltage follower circuit is one of amplifier circuits, however, the invention is not limited to this. A circuit may be configured by using any one or a plurality of an operational amplifier, a bipolar transistor, and a MOS transistor in combination. 
     It is preferable that the monitoring light emitting element  7011  and the monitoring driving transistor  7014  be formed over the same substrate at the same time as the light emitting element  7006  and the driving transistor  7002  by the same manufacturing method. This is because the same correction cannot be performed if characteristics differ between the monitoring element and the transistor provided in a pixel. 
     The description has been made on the case where the monitoring current source  7013  supplies to the light emitting element  7006  a current required for the light emitting element  7006  to emit light at the highest gray scale level, however, the invention is not limited to this. 
     In accordance with the highest gray scale level, the monitoring light emitting element  7011  and the monitoring driving transistor  7014  degrade more than the light emitting element  7006  and the driving transistor  7002  provided in a pixel. Accordingly, a potential outputted from the voltage follower circuit  7015  is more corrected. Therefore, the monitoring circuit may be set so as to degrade at the same rate as an actual pixel. For example, when an average light emission ratio of the entire display is 30%, the monitoring circuit may operate according to the gray scale level corresponding to the luminance of 30%. 
     In specific, a current of a desired size to be supplied to the light emitting element  7006  may be supplied to the monitoring current source  7013  when the light emitting element  7006  emits light at a gray scale level corresponding to the luminance of 30%. The video signal generating circuit  7031  may output a video signal accordingly. 
     In order to increase the gray scale level of a light emitting element, a voltage of a video signal is to be increased as shown in  FIG. 5B  when the light emitting element operates in the saturation region. In this embodiment mode, a potential of a gate terminal of the driving transistor  7002  is corrected. Accordingly, by correcting the voltage of the video signal (video voltage) in accordance with change in characteristics of the light emitting element  7006 , a desired luminance can be obtained. 
     It is to be noted that a potential which is more corrected is outputted when the monitoring circuit operates in accordance with the highest gray scale level, however, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level since image persistence (luminance variation due to the variation in degradation among pixels) becomes less noticeable. Therefore, it is preferable that the monitoring circuit operate in accordance with the highest gray scale level. 
     It is to be noted that the driving transistor  7002  may operate only in the saturation region, both in the saturation region and the linear region, or only in the linear region. 
     When the driving transistor  7002  operates only in the saturation region, it operates mostly as a switch. Accordingly, it is not likely to be affected by the variations in characteristics of the driving transistor  7002  due to degradation, temperature and the like do not affect much. However, the effect of the variations of characteristics of the light emitting element  7006  due to degradation, temperature and the like are corrected. When the driving transistor  7002  operates only in the linear region, whether a current is supplied to the light emitting element  7006  is often controlled digitally. In that case, a time gray scale method, an area gray scale method and the like are often used in combination for performing a multi-gray scale display. 
     Embodiment Mode 6 
       FIG. 6  shows an example of correcting a video signal inputted to a signal driver circuit which drives a pixel portion. The example shown in  FIG. 6  includes a source signal driver circuit  9901 , a gate signal driver circuit  9902 , a pixel portion  9903 , an adder circuit  9904 , a video input terminal  9905 , a differential amplifier  9906 , a reference power source  9907 , a buffer amplifier  9908 , a current source  9909 , a monitoring TFT  9910 , a monitoring light emitting element  9911 , and an electrode  9912 . 
     Hereinafter described is an operation thereof. A current is supplied from the current source  9909  to the monitoring TFT  9910  and the monitoring light emitting element  9911 . Accordingly, a voltage according to the current is generated in the monitoring light emitting element  9911  and the monitoring TFT  9910 . The voltage is inputted to a first input terminal of the differential amplifier  9906  through the buffer amplifier  9908  while a voltage of the reference power source  9907  is inputted to a second input terminal thereof. A different voltage between an output voltage of the buffer amplifier  9908  and an output voltage of the reference power source  9907  is inputted to the adder circuit  9904  after being amplified by the differential amplifier  9906 . An output voltage of the differential amplifier  9906  and a video signal inputted from the video signal input terminal  9905  are added in the adder circuit  9904  and then inputted to the source signal driver circuit  9901 . According to the video signal after the addition, the source signal driver circuit  9901  and the gate signal driver circuit  9902  can write a video signal into the pixel portion  9903 . 
     In the initial stage, an output voltage of the buffer amplifier  9908  and an output voltage of the reference power source  9907  are set almost equal to each other. Accordingly, in the initial stage, a video signal inputted from the video signal input terminal  9905  is written to the pixel portion  9903  as it is. When the monitoring TFT  9910  and the monitoring light emitting element  9911  degrade with time, the voltages thereof change. When the voltage is inputted to the differential amplifier  9907  through the buffer amplifier  9908 , a different voltage between the output voltages of the buffer amplifier  9908  and the reference power source  9907  is amplified by the differential amplifier  9906  and inputted to the adder circuit  9904 . In the adder circuit  9904 , the output voltage of the differential amplifier  9906  and a video signal are added, thereby an output voltage of the adder circuit  9904  becomes to a voltage after the correction of the degradation. By writing the output voltage of the adder circuit  9904  into the pixel portion  9903  by the source signal driver circuit  9901 , data to be displayed is corrected. In this manner, degradation of TFT and light emitting element can be corrected. 
       FIG. 7  shows an example of correcting a video signal inputted to a signal driver circuit which drives a pixel portion. The example shown in  FIG. 7  includes a source signal driver circuit  9801 , a gate signal driver circuit  9802 , a pixel portion  9803 , an adder circuit  9804 , a video input terminal  9805 , a differential amplifier  9806 , buffer amplifiers  9807  and  9808 , current sources  9809  and  9813 , monitoring TFTs  9810  and  9814 , monitoring light emitting elements  9811  and  9815 , and an electrode  9812 . 
     Hereinafter described is an operation thereof. A current is supplied from the current source  9809  to the monitoring TFT  9810  and the monitoring light emitting element  9811 . Accordingly, a voltage according to the current is generated in the monitoring light emitting element  9811  and the monitoring TFT  9810 . The voltage is inputted to a first input terminal of the differential amplifier  9806  through the buffer amplifier  9808 . A current is supplied from the current source  9813  to the monitoring TFT  9814  and the light emitting element  9815 . Accordingly, a voltage according to the current is generated in the monitoring TFT  9814  and the monitoring light emitting element  9815 . The voltage is inputted to a second input terminal of the differential amplifier  9806  through the buffer amplifier  9807 . At this time, a current of the current source  9809  is set larger than that of the current source  9813 . Because of the difference of current, a voltage of the first input terminal of the differential amplifier  9806  is different than that of the second input terminal thereof. This potential difference is compensated in the differential amplifier  9806  to make the voltage of the first and second terminals of the differential amplifier  9806  equal to each other. 
     An output voltage of the differential amplifier  9806  is inputted to the adder circuit  9804 . In the adder circuit  9804 , the output voltage of the differential amplifier  9806  and a video signal inputted from the video signal input terminal  9805  are added and inputted to the source signal driver circuit. According to the video signal after the addition, the source signal driver circuit and the gate signal driver circuit can write a video signal into the pixel portion  9803 . 
     In the initial stage, an output voltage of the buffer amplifier  9808  and an output voltage of the buffer amplifier  9807  are different, however, the differential amplifier  9808  outputs a signal of zero because of the compensation by the differential amplifier  9806  as mentioned above. Accordingly, a video signal inputted from the video signal input terminal  9805  is written to the pixel portion  9803  as it is. 
     When the monitoring TFTs  9810  and  9814 , and the monitoring light emitting elements  9811  and  9815  degrade with time, the voltages thereof change. The monitoring TFT  9810  and the monitoring light emitting element  9811  to which more current degrade is supplied more while the monitoring TFT  9814  and the monitoring light emitting element  9815  to which less current is supplied degrade less. Accordingly, although an output voltage of the buffer amplifier  9808  does not change much from the initial stage, an output voltage of the buffer amplifier  9807  considerably changes. The differential amplifier  9806  can output a voltage for the degradation of the monitoring TFT  9810  and the monitoring light emitting element  9811  according to a difference therebetween. The voltage for degradation is amplified by the differential amplifier  9806  and inputted to the adder circuit  9804 . In the adder circuit  9804 , an output voltage of the differential amplifier  9806  and the video signal are added, thereby an output voltage of the adder circuit  9804  corresponds to the one after the correction of degradation. By writing the output voltage of the adder circuit  9804  into the pixel portion  9803  by the source signal driver circuit, data for display is corrected. In this manner, degradation of the TFTs and the light emitting element can be corrected. 
     Embodiment Mode 7 
     In this embodiment mode, an example of manufacture of an active matrix display device having a channel etch type TFT as a switching element is described with reference to the drawings. 
     As shown in  FIG. 8A , a base layer  111  is formed for improving adhesion of a substrate  110  and a material layer which is formed later thereover by droplet discharge method. The base layer  111  is formed quite thin, therefore, it does not necessarily have a stacked-layer structure. The base layer  111  is formed by forming over the entire surface a photocatalytic substance (titanium oxide (TiO x ), strontium titanate (SrTiO 3 ), cadmium selenide (CdSe), potassium tantalate (KTaO 3 ), cadmium sulfide (CdS), zirconium oxide (ZrO 2 ), niobium oxide (Nb 2 O 5 ), zinc oxide (ZnO), iron oxide (Fe 2 O 3 ), tungsten oxide (WO 3 )) by spraying or sputtering method. Alternatively, an organic material (an coating insulating film formed by using a material having a skeleton structure of polyimide, acrylic, or a Si—O bond having at least one of hydrogen, fluoride, alkyl group, or aromatic carbon hydride as a substituent) may be selectively formed by ink-jetting or sol-gel method. This can be regarded as a pretreatment of the base layer as well. 
     Here, an example of providing the pretreatment of the base layer for improving adhesion between a discharged conductive material and a substrate has been described. In the case of forming a material layer (for example, an organic layer, an inorganic layer, or a metal layer), or further a material layer (for example, an organic layer, an inorganic layer, or a metal layer) on the discharged conductive layer by droplet discharge method, a TiOx deposition treatment may be performed for improving adhesion between the material layers. That is to say, when drawing by discharging a conductive material by a droplet discharge method, it is preferable to provide a pretreatment of the base layer for the top and bottom interfaces of the conductive material layer for improving adhesion thereof. 
     The base layer  111  is not limited to be formed of a photocatalytic material but can be formed of a 3d transition metal (Sc, Ti, Cr, Ni, V, Mn, Fe, Co, Cu, Zn, and the like), or an oxide, a nitride, or an oxynitride thereof. 
     It is to be noted that a substrate  100  may be a non-alkaline glass substrate formed by a fusing method or a floating method, such as a barium borosilicate glass, an aluminoborosilicate glass, and an aluminosilicate glass as well as a plastic substrate and the like having a heat resistance against processing temperature of this manufacturing step. 
     Next, a conductive layer pattern  112  is formed by discharging a liquid conductive material by a droplet discharge method represented by an ink-jetting method (see  FIG. 8A ). As a conductive material contained in the liquid conductive material, gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), tungsten (W), nickel (Ni), tantalum (Ta), bismuth (Bi), lead (Pb), indium (In), tin (Sn), zinc (Zn), titanium (Ti), or aluminum (Al), or an alloy thereof, dispersive nanoparticles of these, or micro particles of halogenated silver are used. In particular, it is preferable that a gate wiring have low resistance, therefore, gold, silver, or copper dissolved or dispersed in solvent is preferably used in consideration of specific resistance value. More preferably, low resistant silver or copper is used. In the case of using silver or copper, however, a barrier film is preferably provided in combination for preventing impurities from dispersing. For the solvent, esters such as butyl acetate, alcohols such as isopropyl alcohol, organic solvent such as acetone and the like are used. The surface tension and viscosity are arbitrarily controlled by controlling the concentration of solvent, adding surfactant, or the like. 
       FIG. 15  shows an example of a droplet discharge apparatus. In  FIG. 15 , reference numeral  1500  denotes a large substrate,  1504  denotes an image pick-up means,  1507  denotes a stage,  1511  denotes a marker,  1503  denotes a region for one panel. Heads  1505   a ,  1505   b , and  1505   c  each of which has the same width as the width of one panel are provided, which scan in zigzag or back and forth to form a pattern of a material layer appropriately while moving the stage. A head having the same width as that of the large substrate can be used as well, however, the head of a panel size as shown in  FIG. 15  is easy to operate. In order to improve throughput, it is preferable to discharge a material while the stage is moving. 
     Moreover, it is preferable that the heads  1505   a ,  1505   b , and  1505   c  and the stage  1507  each have a temperature control function. Note that the distance between the head (tip of a nozzle) and the large substrate is about 1 mm. The shorter this distance is, the higher the discharge accuracy is. 
     In  FIG. 15 , each of the heads  1505   a ,  1505   b , and  1505   c  arranged in three columns to a scan direction may be capable of forming different material layers respectively, or may discharge the same material. By patterning an interlayer insulating film  128  by discharging the same material using the three heads, throughput is improved. When scanning by the apparatus shown in  FIG. 15 , a substrate  1500  can be moved with a head portion fixed or the head portion can be moved with the substrate  1500  fixed. 
     Each of the heads  1505   a ,  1505   b , and  1505   c  of the droplet discharge apparatus is connected to a control means which enables to draw a programmed pattern in advance by using a computer. The amount of discharge is controlled by a pulse voltage to be applied. The timing to draw is, for example, based on the marker formed on the substrate. Alternatively, a reference point may be determined based on the frame of the substrate. This is detected by an image pick-up means such as a CCD, then a digital signal converted by an image processing means is processed by a computer to generate a control signal to be transmitted to the control means. It is needless to say that data on pattern to be formed over the substrate is stored in a memory medium. Based on this data, a control signal is transmitted to the control means to control each of the heads of the droplet discharge apparatus independently. 
     Next, a portion of the conductive film pattern is exposed by selective laser light irradiation (see  FIG. 8B ). A photosensitive material is contained in advance in the liquid conductive film material to be discharged so that it chemically reacts with the laser light. The photosensitive material here is a negative type that a portion which chemically reacts with the laser light remains. By laser irradiation, an accurate pattern can be formed, in particular a wiring of thin width can be obtained. 
     Here, description is made on a laser beam drawing apparatus with reference to  FIG. 13 . A laser beam drawing apparatus  401  includes a personal computer (hereinafter also referred to as a PC)  402  which executes various controls in laser beam irradiation, a laser oscillator  403  which outputs a laser beam, a power source  404  of the laser oscillator  403 , an optical system (ND filter)  405  for attenuating a laser beam, an acoustic-optic modulator (AOM)  406  for modulating the intensity of a laser beam, a lens for enlarging or narrowing the laser beam cross-section, an optical system  407  formed by a mirror and the like for changing light path, a substrate moving assembly  409  having an X stage and a Y stage, a D/A converter  410  for converting control data outputted from the PC between digital and analog, a driver  411  for controlling the acoustic-optic modulator  406  in accordance with an analog voltage outputted from the D/A converter  410 , and a driver  412  for outputting a driving signal for driving the substrate moving assembly  409 . 
     For the laser oscillator  403 , a laser oscillator capable of oscillating ultraviolet light, visible light, or infrared light can be used. As such a laser oscillator, an excimer laser oscillator such as KrF, ArF, XeCl, and Xe, a gas laser oscillator such as He, He—Cd, Ar, He—Ne, and HF, a solid state laser oscillator using a crystal obtained by doping Cr, Nd, Er, Ho, Ce, Co, Ti, or Tm to YAG GdVO 4 , YVO 4 , YLF, and YAlO 3 , a semiconductor laser oscillator such as GaN, GaAs, GaAlAs, and InGaAsP can be used. It is to be noted that first to fifth harmonic waves of the fundamental wave are preferably used for the solid state laser oscillator. 
     Hereinafter described is an exposing method of a photosensitive material using a laser beam direct drawing apparatus. It is to be noted that the photosensitive material here is a conductive film material (including a photosensitive material) to be a conductive film pattern 
     After a substrate  408  is mounted on the substrate moving assembly  409 , the PC  402  detects the position of the marker on the substrate by a camera which is not shown in the drawing. Then, the PC  402  generates moving data for moving the substrate moving assembly  409  based on the detected position data of the marker and drawing pattern data which is inputted in advance. After that, the PC  402  controls the amount of output light from the acoustic-optic modulator  406  through the driver  411 , thereby laser beam outputted from the laser oscillator  403  is attenuated by the optical system  405  and controlled so as to be a predetermined amount by the acoustic-optic modulator  406 . On the other hand, the laser beam outputted from the acoustic-optic modulator  406  are changed in light path and beam shape by the optical system  407  and condensed by the lens. Then, the photosensitive material formed over the substrate is irradiated with the laser beam to be exposed. At this time, the substrate moving assembly  409  is controlled to move in X and Y directions based on the moving data generated by the PC  402 . As a result, a predetermined place is irradiated with laser beam, thereby the photosensitive material is exposed. 
     It is to be noted that a portion of the energy of laser light irradiated to the photosensitive material is converted into heat, which makes a portion of the photosensitive material react. Therefore, a pattern width becomes slightly wider than that of the laser beam. It is preferable to use laser beam of shorter wavelength to form a pattern of fine width since a beam diameter can be condensed to be small. 
     A spot shape of the laser beam on the surface of the photosensitive material is processed by the optical system to be a dot shape, a circular shape, an oval shape, a rectangular shape, or a linear shape (specifically shape of an elongated rectangle). It is to be noted that the spot shape may be a circular shape, however, a linear shape is more preferable to obtain a pattern of even width. 
     According to the apparatus shown in  FIG. 13 , the surface of the substrate is exposed by laser light irradiation, however, the back side of the substrate may be exposed by the laser light by appropriately changing the optical system and the substrate moving assembly. It is to be noted that the laser beam is selectively irradiated by moving the substrate, however, the invention is not limited to this. The laser beam can be scanned in X-Y directions to be irradiated. In this case, it is preferable to use a polygon mirror or a galvanometer mirror for the optical system  407 . 
     Next, development is performed by using etchant (or developer) to remove unnecessary portions, then main baking is performed to form a metal wiring  115  to be a gate electrode or a gate wiring (see  FIG. 8C ). 
     A wiring  140  which extends to a terminal is formed similarly to the metal wiring  115 . Although not shown here, a power source line for supplying a current to a light emitting element may be formed as well. Moreover, a capacitor electrode or a capacitor wiring for forming a capacitor is formed as required. When using a positive type photosensitive material, laser irradiation is performed to a portion to be removed to achieve chemical reaction therein. Then, that portion is dissolved by etchant. Moreover, laser light may be irradiated after performing room temperature drying or selective baking after discharging liquid conductive film material. 
     Next, a gate insulating film  118 , a semiconductor film, and an N-type semiconductor film are sequentially deposited by a plasma CVD method or a sputtering method. For the gate insulating film  118 , a material containing silicon oxide, silicon nitride, or silicon nitride oxide as a main component, which is obtained by a PCVD method is used. Moreover, a SiO x  film containing an alkyl group may be used for the gate insulating film  118  after discharging by a droplet discharge method using siloxane-based polymer and baking. 
     A semiconductor film is formed of an amorphous semiconductor film or a semi-amorphous film formed by a vapor phase epitaxy method, a sputtering method, a thermal CVD method using a semiconductor material gas represented by silane and germane, or a semi-amorphous semiconductor film. For amorphous semiconductor film, an amorphous silicon film formed by the PCVD method using SiH 4  or a mixed gas of SiH 4  and H 2  can be used. Moreover, for the semi-amorphous (also referred to as microcrystal) semiconductor film, a semi-amorphous silicon film obtained by the PCVD method using a mixed gas obtained by diluting SiH 4  with 3 to 1000 times of H 2 , a mixed gas obtained by diluting Si 2 H 6  with GeF 4  at a gas flow rate of 20 to 40:0.9 (Si 2 H 6 :G 3 F 4 ), a mixed gas of Si 2 H 6  and F 2 , or a mixed gas of SiH 4  and F 2 . Note that the semi-amorphous silicon film is favorably used since more crystallinity can be given to the interface with the base layer. 
     Further, the crystallinity may be improved by irradiating the semi-amorphous silicon film obtained by a PCVD method using a mixed gas of SiH 4  and F 2  with laser light. 
     The N-type semiconductor film may be an amorphous semiconductor film or a semi-amorphous semiconductor film formed by a PCVD method using a silane gas and phosphine gas. When an N-type semiconductor film  120  is provided, it is preferable that contact resistance between the semiconductor film and an electrode (electrode formed later) is required to be low. 
     Next, a mask  121  is provided and the semiconductor film and the N-type semiconductor film are selectively etched to obtain an island shape semiconductor film  119  and an N-type semiconductor film  120  (see  FIG. 8D ). The mask  121  is formed by a droplet discharge method and a printing method (relief printing plate, flat plate, copperplate printing, screen and the like). A desired mask pattern may be formed directly by a droplet discharge method or a printing method, however, a rough resist pattern may be formed by a droplet discharge method and a printing method and then selectively exposed by laser light to obtain a fine resist pattern with accuracy. 
     By using the laser beam drawing apparatus shown in  FIG. 13 , exposure of resist can be performed. In that case, the resist mask  121  is to be formed by exposing by laser light with a photosensitive material as a resist. 
     Next, after removing the mask  121 , a mask (not shown) is provided to etch a gate insulating film selectively, thereby a contact hole is formed. The gate insulating film is removed in the terminal. The mask may be formed by a typical photolithography technique, by forming a resist pattern by droplet discharge method, or by forming a resist pattern by applying a positive resist over the entire surface and performing exposure with laser light and development. In an active matrix light emitting device, a plurality of TFTs are formed in one pixel, which are connected to a wiring of the upper layer through the gate electrode and the gate insulating film. 
     Next, a composition containing a conductive material (Ag (silver), Au (gold), Cu (copper), W (tungsten), Al (aluminum) and the like) is selectively discharged by droplet discharge method to form source or drain (referred to as source/drain) wirings  122  and  123 , or a leading electrode  117 . Similarly, a power source line for supplying a current to the light emitting element and a connecting wiring (not shown) at the terminal are formed (see  FIG. 8E ). 
     Next, the N-type semiconductor film and a top layer of the semiconductor film are etched using the source/drain wirings  122  and  123  as masks to obtain a state of  FIG. 9A . At this stage, a channel etch type TFT provided with a channel forming region  124  to be an active layer, a source region  126 , and a drain region  125  is completed. 
     Next, a protective film  127  is formed for protecting the channel forming region  124  from being contaminated by impurities (see  FIG. 9B ). For the protective film  127 , a material mainly containing silicon nitride or silicon nitride oxide obtained by sputtering method or, PCVD method is used. Here, the protective film  127  is formed as an example, however, it is not necessarily formed. 
     Next, an interlayer insulating film  128  is selectively formed by droplet discharge method. The interlayer insulating film  128  is formed of a resin material such as an epoxy resin, an acrylic resin, a phenol resin, a novolac resin, a melamine resin, and an urethane resin is used. In addition, the interlayer insulating film  128  is formed by a droplet discharge method using an organic material such as benzocyclobutene, parylene, flare, or light-transmissive polyimide; a compound material made from polymerization of such as siloxane polymer; a composition material containing water-soluble homopolymer and water-soluble copolymer; or the like. The interlayer insulating film  128  is not limited to be formed by a droplet discharge method, and it can be formed over the entire surface by a coating method, a PCVD method and the like. 
     Next, the protective film  127  is etched using the interlayer insulating film  128  as a mask to form a projecting portion (pillar)  129  formed of a conductive material over portions of the source/drain wirings  122  and  123 . The projecting portion (pillar)  129  may be formed by a stacked-layer by repeating discharging and baking a composition containing a conductive material (Ag (silver), Au (gold), Cu (copper), W (tungsten), Al (aluminum) and the like). 
     A first electrode  130  in contact with the projecting portion (pillar)  129  is formed over the interlayer insulating film  128  (see  FIG. 9C ). It is to be noted that a terminal electrode  141  in contact with a wiring  140  is formed similarly. Here, it is an example that a driving TFT is an N-channel TFT, therefore, it is preferable that the first electrode  130  function as a cathode. In the case of a light-transmissive type, the first electrode  130  is formed by a droplet discharge method or a printing method using a predetermined pattern is formed of a composition containing indium tin oxide (ITO), indium tin oxide containing silicon oxide (ITSO), zinc oxide (ZnO), tin oxide (SnO 2 ) and the like, and then baked to form the first electrode  130  and the terminal electrode  141 . Moreover, in the case of reflecting light on the first electrode  130 , a predetermined pattern is formed by a droplet discharge method using a composition containing mainly metal particles such as Ag (silver), Au (gold), Cu (copper), W (tungsten), and Al (aluminum), and then baked to form the electrode  130  and the terminal electrode  141 . Alternatively, the first electrode  130  may be formed by forming a light-transmissive conductive film or a light-reflective conductive film by a sputtering method, forming a mask pattern by droplet discharge method, and performing etching in combination. 
       FIG. 10  is an example of a top view of the pixel of the  FIG. 9C . A sectional view of the right side of the pixel portion of  FIG. 9C  corresponds to a sectional view taken along a chain line A-A′ in  FIG. 10 , while the left side thereof corresponds to a sectional view taken along a chain line B-B′. In  FIG. 10 , the same reference numerals are used for the identical portions to  FIGS. 8A to 9D . In  FIG. 10 , an edge of a partition wall  134  formed later is shown by a dotted line. 
     Although the interlayer insulating film  128  and the projecting portion (pillar)  129  are formed separately as the protective film  127  is provided here, they can be formed by the same apparatus by droplet discharge method when the protective film  127  is not provided. 
     Next, a partition wall  134  for covering a peripheral portion of the first electrode  130  is formed. The partition wall (also referred to as a bank)  134  is formed of a material containing silicon, an organic material, and a compound material. Moreover, a porous film may be used as well. By using a photosensitive or non-photosensitive material such as acrylic and polyimide, it is preferable that a side thereof has a curvature radius which continuously changes, thus an upper thin film can be formed without breaking. 
     In above-mentioned manner, a TFT substrate for a light emitting display panel in which a bottom gate (also referred to as an inverted staggered type) TFT and the first electrode  130  are formed over the substrate  100  is completed. 
     Next, a layer which functions as an electroluminescent layer (also referred to as an EL layer), that is a layer  136  containing an organic compound is formed. The layer  136  containing an organic compound has a stacked-layer structure each of which is formed by a vapor deposition method or a coating method. For example, an electron transporting layer (electron injection layer), a light emitting layer, a hole transporting layer, and a hole injection layer are sequentially stacked on a cathode. 
     The electron transporting layer contains a charge injecting-transporting substance. As a charge injecting-transporting material having a high electron transporting property, a metal complex or the like having a quinoline skeleton or a benzoquinoline skeleton such as tris(8-quinolinolate) aluminum (Alq 3 ), tris(5-methyl-8-quinolinolate) aluminum (Almq 3 ), bis(10-hydroxybenzo[h]-quinolinato) beryllium (BeBq 2 ), and bis(2-methyl-8-quinolinolate)-4-phenylphenolato-aluminum (BAlq) can be nominated. As a material having a high hole transporting property, an aromatic amine-based compound (that is, the one having a benzene ring-nitrogen bond) such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (a-NPD), 4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenyl amine (TDATA), and 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenyl amine (MTDATA) can be used. 
     Among the charge injecting-transporting material, as a material especially having a high electron injection property, a compound of an alkali metal or an alkali earth metal such as lithium fluoride (LiF), cesium fluoride (CsF), and calcium fluoride (CaF 2 ) can be used. Besides, mixture of a material having a high electron transportation property such as Alq 3  and an alkali earth metal such as magnesium (Mg) can be used. 
     A light-emitting layer is formed by a charge injecting-transporting material and a light-emitting material, each of which contains an organic compound or an inorganic compound. The light emitting layer may include a layer formed of one or a plurality of layers selected based on its number of molecules from a low molecular weight organic compound, an intermediate molecular weight organic compound (which can be defined as an organic compound which does not have subliming property, and has the number of molecules of 20 or less, or a molecular chain length of 10 μm or less), and a high molecular weight (also referred to as a polymer) organic compound. An inorganic compound having an electron injecting-transporting property or a hole injecting-transporting property may be used in combination 
     As a material for the light-emitting layer, various materials can be used. As a low molecular weight organic light emitting material, 4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-julolidylyl-9)ethenyl]-4H-pyran (DCJT), 4-dicyanomethylene-2-t-butyl-6-[2-(1,1,7,7-tetramethyl julolidine-9-yl)ethenyl]-4H-pyran (DCJTB), periflanthene, 2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl)ethenyl]benzene, N,N′-dimethyl quinacridon (DMQd), coumarin 6, coumarin 545T, tris(8-quinolinolate)aluminum (Alq 3 ), 9,9-bianthryl, 9,10-diphenylanthracene (DPA), 9,10-bis(2-naphthyl)anthracene (DNA), or the like can be used. Other materials may be used as well. 
     A high molecular weight organic light emitting material is physically stronger than a low molecular weight organic light emitting material. A light emitting element formed by the high molecular weight organic light emitting material is superior in durability. A light emitting element using the high molecular weight organic light emitting material can be manufactured rather easily since a light emitting layer can be formed by coating. The structure of the light emitting element using the high molecular weight organic light emitting material is basically the same as that using the low molecular weight organic light emitting material, that is, a cathode, an organic light emitting layer, and an anode are stacked sequentially. However, in the case where the light emitting layer is formed of the high molecular weight organic light emitting material, it is difficult to form a stacked-layer structure like the case of using the low molecular weight organic light emitting material. Therefore, the light emitting element using the high molecular weight organic light emitting material is formed to have a two-layer structure in many cases. Specifically, a cathode, a light emitting layer, a hole transporting layer, and an anode are stacked sequentially. 
     Emission color is determined by a material of the light emitting layer. Accordingly, a light emitting element which exhibits desired emission color can be formed by selecting the material for the light emitting layer. As a high molecular weight-based electroluminescent material for forming the light emitting layer, a polyparaphenylene vinylene-based material, a polyparaphenylene-based material, a polythiophene-based material, a polyfluorene-based material can be used. 
     As the polyparaphenylene vinylene-based material, a derivative of poly(paraphenylene vinylene) (PPV), poly(2,5-dialkoxy-1,4-phenylen vinylene) (RO-PPV), poly(2-(2′-ethyl-hexoxy)-5-methoxy-1,4-phenylene vinylene) (MEH-PPV), poly(2-dialkoxyphenyl)-1,4-phenylenevinylene) (ROPh-PPV), and the like can be used. As the polyparaphenylene-based material, a derivative of polyparaphenylene (PPP), poly(2,5-dialkoxy-1,4-phenylene) (RO-PPP), poly(2,5-dihexoxy-1,4-phenylene), and the like can be used. As the polythiophene-based material, a derivative of polythiophene (PT), poly(3-alkylthiophene) (PAT), poly(3-hexylthiophene) (PHT), poly(3-cyclohexylthiophene) (PCHT), poly(3-cyclohexyl-4-methylthiophene) (PCHMT), poly(3,4-dicyclohexylthiophene) (PDCHT), poly[3-(4-octylphenyl)-thiophene] (POPT), poly[3-(4-octylphenyl)-2,2-bithiophene] (PTOPT), and the like can be used. As the polyfluorene-based material, a derivative of polyfluorene (PF), poly(9,9-dialkylfluorene) (PDAF), poly(9,9-dioctylfluorene) (PDOF), and the like can be used. 
     A hole injection property from the anode can be improved by interposing a high molecular weight-based organic light emitting material having a hole transporting property between the anode and a high molecular weight organic light emitting material having a light emitting property. Generally, the high molecular weight-based organic light emitting material having a hole transporting property and an acceptor material dissolved in water together are coated by spin coating. The high molecular weight-based organic light emitting material having a hole transporting property is insoluble in organic solvent, thus, the organic light emitting material having a light emitting property can be stacked over the material. As the high molecular weight-based organic light emitting material having a hole transporting property, mixture of PEDOT and camphoric sulfonic acid (CSA) as an acceptor material, mixture of polyaniline (PANI) and polystyrene sulfonic acid (PSS) as an acceptor material, and the like can be used. 
     Besides a singlet excited light emitting material, a triplet excited material containing a metal complex or the like can be used for the light emitting layer. For example, among a red light emitting pixel, a green light emitting pixel, and a blue light emitting pixel; a red light emitting pixel whose luminance is reduced by half luminance in a relatively short time is formed by a triplet excited light emitting material and the others are formed by singlet excited light emitting materials. The triplet excited light emitting material has a characteristic that it consumes less power than the singlet excited light emitting material to obtain a certain luminance since the triplet excited light emitting material has high luminous efficiency. In the case where the triplet excited light emitting material is used for forming the red light emitting pixel, the reliability can be improved since the light emitting element requires a small amount of current. To reduce power consumption, the red light emitting pixel and the green light emitting pixel may be formed by the triplet excited light emitting material, and the blue light emitting pixel may be formed by a singlet excited light emitting material. The power consumption of a green light emitting element which is highly visible to human eyes can be reduced by using the triplet excited light emitting material. 
     As an example for the triplet excited light emitting material, a material using a metal complex as a dopant including platinum which is the third transition element as a central metal or a metal complex including iridium as a central metal is well known. The triplet excited light emitting material is not limited to these compounds. A compound that has the aforementioned structure and has an element belonging to groups 8 to 10 of the periodic table as a central metal can be used. 
     The hole transporting layer contains a charge injecting-transporting substance. As a material having a high hole injection property, for example, metal oxide such as molybdenum oxide (MoOx), vanadium oxide (VOx), ruthenium oxide (RuOx), tungsten oxide (WOx), and manganese oxide (MnOx), can be nominated. Besides, a phthalocyanine-based compound such as phthalocyanine (H 2 Pc) or copper phthalocyanine (CuPc) can be used. 
     Before forming the layer  136  containing an organic compound, plasma treatment in the oxygen atmosphere or heat treatment in vacuum atmosphere is preferably performed. In the case of employing vapor deposition, an organic compound is vaporized by resistance heating in advance and scattered toward a substrate by opening a shutter in depositing the organic compound. The vaporized organic compound is scattered upward and deposited over the substrate through an opening portion provided in a metal mask. To realize a full color display, a mask may be aligned per emission color (R, G, and B). 
     A light emitting layer may have the structure in which light emitting layers having different emission wavelength bands respectively are provided to each pixel for realizing a full color display. Typically, light emitting layers corresponding to the colors of R (red), G (green), and B (blue) are formed. In this case, color purity can be improved and a pixel portion can be prevented from being a mirror surface (glare) by providing a filter (colored layer) which transmits light of each emission wavelength band to the light emission side of the pixel. By providing the filter (colored layer), a circular polarizer or the like which is conventionally required is not required any longer. Further, light can be emitted from the light emitting layer without any loss. Moreover, change of tone occurring when the pixel portion (display screen) is seen obliquely can further be reduced. 
     Alternatively, a full color display can be realized by using a material exhibiting a monochromatic emission as the layer  136  containing an organic compound, and combining a color filter or a color conversion layer without separate deposition. For example, in the case where an electroluminescent layer exhibiting white or orange emission is formed, a full color display can be realized by separately providing a color filter, a color conversion layer, or a combination of the color filter and the color conversion layer on the light emission side of the pixel. The color filter or the color conversion layer may be formed, for example, over a second substrate (sealing substrate) and attached to the substrate  100 . Further, as described above, all of the material exhibiting monochromatic emission, the color filter, and the color conversion layer can be formed by droplet discharge method. 
     To form a light emitting layer which exhibits white emission, for example, Alq 3 , Alq 3  partly doped with Nile red, Alq 3 , p-EtTAZ, TPD (aromatic diamine) are deposited sequentially by vapor deposition. In the case where the EL layer is formed by spin coating, the coated layer is preferably baked by vacuum heating after being coated. For example, poly(ethylene dioxythiophene)/poly(styrene sulfonate) solution (PEDOT/PSS) which acts as the hole injecting layer may be coated over the whole surface and baked, and then polyvinylcarbazole (PVK) doped with emission center pigments (1,1,4,4-tetraphenyl-1,3-butadiene (TPB), 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran (DCM1), Nile red, coumarin 6, or the like) acts as the light emitting layer may be coated over the whole surface and baked. 
     The light emitting layer may be formed of a single layer as well. In this case, the light emitting layer may be formed of polyvinylcarbazole (PVK) with the hole transporting property dispersed with a 1,3,4-oxadiazole derivative (PBD) with the electron transporting property. Further, white emission can be obtained by dispersing PBD by 30 wt % as the electron transporting material and dispersing an appropriate amount of four kinds of pigments (TPB, coumarin 6, DCM1, and Nile red). 
     The aforementioned materials for forming the layer containing an organic compound are only examples. The light emitting element can be formed by accordingly stacking functional layers such as a hole injecting-transporting layer, a hole transporting layer, an electron injecting-transporting layer, an electron transporting layer, a light emitting layer, an electron blocking layer, and a hole blocking layer. A mixed layer or mixed junction of mixing the aforementioned layers may also be formed. The layer structure of the light emitting layer may vary. Instead of providing a specific electron injecting region or light emitting region, modifications of the structure such as providing an electrode in order to be used for the electron injecting region or the light emitting region, or providing a dispersed light emitting material can be allowed unless such modifications depart from the scope of the invention. 
     It is needless to say that a monochromatic light emission display can be performed. For example, an area color type light emitting display device can be formed by utilizing monochromatic light emission. A passive matrix type display portion is suitable for the area color type display device. The display device can display mainly texts or symbols. 
     Then, a second electrode  137  is formed. The second electrode  137  serving as an anode of the light emitting element is formed of a light transmissive conductive film, for example, a light-transmissive conductive film such as ITO, ITSO, or a film obtained by mixing indium oxide with zinc oxide (ZnO) by 2 to 20%. The light emitting element has a structure in which the layer  136  containing an organic compound is interposed between the first electrode  130  and the second electrode  137 . A material for the first electrode  130  and the second electrode  137  is required to be selected in consideration of a work function. Either of the first electrode  130  of the second electrode  137  can be an anode or a cathode depending on a pixel structure. 
     The light emitting element formed by the foregoing materials emits light under forward bias. A pixel of a display device formed by using the light emitting element can drive by either a passive matrix (also referred to as a simple matrix) driving method or an active matrix driving method. At any rate, each pixel emits light by applying forward bias at a certain timing. Further, the respective pixels are in non-light emission state for a certain period. The reliability of the light emitting element can be improved by applying reverse bias in the non-light emission state. The light emitting element may be in a deterioration mode in which emission intensity decreases under a certain driving condition or may be in a deterioration mode in which apparent luminance decreases due to the expansion of a non-light emission region within the pixel. The deterioration can be delayed by alternating current (AC) driving to apply forward bias and reverse bias, which leads to improve the reliability of the light emitting device. 
     In order to lower the resistance of the second electrode  137 , an auxiliary electrode may be provided over the second electrode  137  which does not serve as a light emitting region. A protective film for protecting the second electrode  137  may be formed as well. For example, a protective film composed of a silicon nitride can be formed by using a disc-form target formed of silicon in a deposition chamber of nitrogen atmosphere or atmosphere including nitrogen and argon. Further, a thin film containing carbon as a main component (a DLC film, a CN film, or an amorphous carbon film) can be formed as the protective film and other deposition chamber using chemical vapor deposition (hereinafter referred to as CVD) method may be provided additionally. A diamond like carbon film (also referred to as a DLC film) can be formed by plasma CVD method (typically, RF plasma CVD method, microwave CVD method, electron cyclotron resonance (ECR) CVD method, heat filament CVD method, or the like), a combustion-flame method, a sputtering method, an ion beam deposition method, a laser deposition method, or the like. A hydrogen gas and a hydrocarbon gas (CH 4 , C 2 H 2 , C 6 H 6 , or the like) are used as a reaction gas for deposition. The reaction gases are ionized by glow discharge, and the ions are accelerated to collide with a cathode applied with negative self-bias, then, the DLC film is deposited. Further, the carbon nitride film (also referred to as a CN film) may be formed by using a C 2 H 4  gas and a N 2  gas as reaction gases. In addition, the DLC film and the CN film are insulating films transparent or semitransparent to visible light. The term “transparent to visible light” means having a transmittance of 80 to 100% for visible light. The term “semitransparent to visible light” means having a transmittance of 50 to 80% for visible light. The protective film is not necessarily provided. 
     Then, the sealing substrate  135  is attached by sealant (not shown) to seal the light emitting element. The space surrounded by the sealant is filled with a light-transmissive filler  138 . The filler  138  is not particularly limited as long as it transmits light. Representatively, an ultraviolet ray curable or heat curable epoxy resin may be used. Here, a high heat-resistant UV epoxy resin (manufactured by Electrolyte Cooperation: 2500 Clear) having refractivity of 1.50, viscosity of 500 cps, shore D hardness of 90, tensile intensity of 3000 psi, Tg point of 150° C., volume resistance of 1×10 15  O·cm, withstand voltage of 450 V/mil is used. By filling the filler  138  between a pair of substrates, the transmittance can be improved as a whole. 
     At last, an FPC  146  is attached to the terminal electrode  141  by an anisotropic conductive film  145  by a known method (see  FIG. 9D ). In this manner, an active matrix light emitting device can be manufactured. 
       FIG. 11  is a top view showing an example of a structure of an EL display panel.  FIG. 11  shows a structure of a light emitting display panel which controls a signal to be inputted to a scan line and a signal line by an external driver circuit. A pixel portion  201  in which a pixel  202  is arranged in matrix, a scan line input terminal  203 , and a signal line input terminal  204  are formed over a substrate  200  having an insulating surface. The number of pixels may be set according to various specifications, for example, 1024×768×3 (RGB) for XGA, 1600×1200×3 (RGB) for UXGA, or 1920×1080×3 (RGB) in the case of a full spec high vision. 
     The pixels  202  are arranged in matrix with scan lines extending from the scan line input terminal  203  and signal lines extending from the signal line input terminal  204  crossing each other. Each of the pixels  202  is provided with a switching element and a pixel electrode connected thereto. A typical example of the switching element is a TFT. Each of the pixels can be independently controlled by signals inputted externally as a gate electrode of the TFT is connected to the scan line and a source or drain electrode is connected to the signal line. 
     In the case of forming the first electrode  130  shown in  FIGS. 9A to 9D  by a light transmissive material and forming the second electrode  137  by a metal material, a structure of emitting light through the substrate  100 , that is a bottom emission type is formed. Alternatively, in case of forming the first electrode  130  by a metal material and forming the second electrode  137  by a light transmissive material, a structure of emitting light through the sealing substrate  135 , that is a top emission type is formed. Moreover, in the case of forming the first electrode  130  and the second electrodes  137  by light transmissive materials, a structure of emitting light through both of the substrate  100  and the sealing substrate  135  can be formed. The invention may appropriately adopt any one of the aforementioned structures. Further, a driver circuit may be mounted in the EL display panel. One mode thereof is described with reference to  FIG. 12 . 
     First, a display device employing a COG method is described with reference to  FIG. 12 . A pixel portion  301  for displaying data such as texts and images and a scan driver circuit  302  are provided over a substrate  300 . A substrate provided with a plurality of driver circuits is divided into rectangles, and the divided driver circuits (hereinafter referred to as driver ICs)  305   a  and  305   b  are mounted on the substrate  300 .  FIG. 12  shows a mode of mounting a plurality of driver ICs  305   a  and  305   b  and tapes  304   a  and  304   b  at the end of the driver ICs  305   a  and  305   b . In addition, a divided size may be almost the same as the length of a side of the pixel portion on a signal line side, and a tape may be mounted at the end of a single driver IC. 
     A TAB method may be adopted, in which case a plurality of tapes may be attached, on which a driver IC may be mounted. Similarly to the case of the COG method, a single driver IC may be mounted on a single tape, in which case a metal piece or the like for fixing the driver IC may be attached by strength problems. 
     A plurality of the driver ICs to be mounted on an EL display panel are preferably formed over a rectangular substrate having a side of 300 to 1000 mm or longer in view of improving productivity. In other words, a plurality of circuit patterns including a driver circuit portion and an input/output terminal as a unit are formed over the substrate, and may be finally divided and taken out. In consideration of the side length of the pixel portion and the pixel pitch, the driver IC may be formed to be a rectangle having a long side of 15 to 80 mm and a short side of 1 to 6 mm. Alternatively, the driver IC may be formed to have a side length that the side length of the pixel region or the pixel portion is added to the side length of each driver circuit. 
     In terms of external dimension, a driver IC is more advantageous than an IC chip in length of a long side. When a driver IC having a long side of 15 to 80 mm is used, the less number thereof is required to be mounted in accordance with the pixel portion than the case of using an IC chip. Therefore, a manufacturing yield can be improved. When a driver IC is formed over a glass substrate, productivity does not fall as a mother substrate is not limited in shape. This is a great advantage as compared with the case of taking IC chips out of a circular silicon wafer. 
     In  FIG. 12 , the driver ICs  305   a  and  305   b  each provided with a driver circuit are mounted in a region outside the pixel portion  301 . The driver ICs  305   a  and  305   b  are driver circuits of signal line sides. In order to form a pixel portion corresponding to RGB full color, 3072 signal lines are required for XGA and 4800 signal lines are required for UXGA. The signal lines formed in such numbers are divided into several blocks on an edge of the pixel portion  301  and are provided with leading lines. The leading lines are gathered in relation to pitches of output terminals of the driver ICs  305   a  and  305   b.    
     The driver IC is preferably formed of a crystalline semiconductor formed over a substrate. The crystalline semiconductor is preferably formed by being irradiated with continuous wave laser light. Therefore, a continuous wave solid laser or gas laser is used as an oscillator for generating the laser light. There are few crystal defects when a continuous wave laser is used, and as a result, a transistor can be formed by using a polycrystalline semiconductor layer with a large grain size. In addition, high-speed driving is possible since mobility and response are favorable, and it is possible to further improve an operating frequency of an element than that of the conventional element. Therefore, high reliability can be obtained since there are few characteristics variations. Note that a channel-length direction of a transistor and a scanning direction of laser light may be preferably the same to further improve an operating frequency. This is because the highest mobility can be obtained when a channel length direction of a transistor and a scanning direction of laser light with respect to a substrate are almost parallel (preferably, from −30° to 30°) in a laser crystallization step by a continuous wave laser. The channel length direction coincides with a flowing direction of a current, in other words, a direction in which a charge moves in a channel formation region. The transistor manufactured in this manner has an active layer including a polycrystalline semiconductor layer in which a crystal grain extends in a channel direction, and this means that a crystal grain boundary is formed almost along a channel direction. 
     In order to perform laser crystallization, it is preferable to largely narrow down the laser light, and a beam spot thereof preferably has the same width as that of a short side of the driver IC, which is approximately from 1 to 3 mm. In addition, in order to obtain an enough and effective energy density for an object to be irradiated, an irradiation region of the laser light is preferably in a linear shape. A linear shape here does not refer to a line in a strict sense but refers to a rectangle or an oblong shape with a large aspect ratio, for example, an aspect ratio of 2 or higher (preferably from 10 to 10000). Thus, it is possible to provide a manufacturing method of a display device in which productivity is improved by making a beam spot width of the laser light the same as that of a short side length of the driver IC. 
       FIG. 12  shows a mode in which the scan line driver circuit is integrated with the pixel portion and the driver IC is mounted as the signal line driver circuit. However, the invention is not limited to this and the driver ICs may be mounted as both the scan line driver circuit and the signal line driver circuit. In that case, it is preferable that specifications of the driver ICs to be used on the scan line side and on the signal line side be different. 
     In the pixel portion  301 , the signal line and the scan line intersect to form a matrix and a transistor is arranged at each intersection. A TFT having an amorphous semiconductor or a semi-amorphous semiconductor as a channel portion is used as the transistor arranged in the pixel portion  301  in the invention. The amorphous semiconductor is formed by a plasma CVD method, a sputtering method, or the like. The semi-amorphous semiconductor can be formed at a temperature of 300° C. or lower by a plasma CVD method. A film thickness necessary to form a transistor is formed in a short time even in the case of a non-alkaline glass substrate of an external size of, for example, 550×650 mm. The feature of such a manufacturing technique is effective in manufacturing a large-area display device. In addition, a semi-amorphous TFT can obtain field effect mobility of 2 to 10 cm 2 /V·sec by forming a channel formation region of an SAS. Therefore, this TFT can be used as a switching element of pixel and as an element constituting the driver circuit of a scan line side. Thus, an EL display panel in which system-on-panel is realized can be manufactured. 
     Note that  FIG. 12  is shown on the premise that the scan line driver circuit is also integrated over the substrate by using a TFT having a semiconductor layer formed of a semi-amorphous semiconductor (SAS). In the case of using a TFT having a semiconductor layer formed of a semi-amorphous semiconductor, a driver IC may be mounted as both the scan line driver circuit and the signal line driver circuit. 
     In that case, it is preferable that specifications of the driver ICs to be used on the scan line side and on the signal line side be different. For example, a transistor constituting the scan line driver IC is required to withstand a voltage of approximately 30 V, however, a driving frequency is 100 kHz or less, thus a high-speed operation is not required much. Therefore, it is preferable to set the channel length (L) of the transistor included in the scan line driver sufficiently long. On the other hand, a transistor of the signal line driver IC is required to withstand a voltage of approximately 12 V, however, a driving frequency is around 65 MHz at 3 V, thus a high speed operation is required. Therefore, it is preferable to set the channel length or the like of the transistor included in a driver based on a micron rule. 
     A method for mounting a driver IC is not particularly limited and a known method such as a COG method, a wire bonding method, or a TAB method can be employed. The height between the driver IC and the opposing substrate can be made almost the same by forming the driver IC to have the same thickness as that of the opposing substrate, which contributes to form a thinner display device as a whole. When both substrates are formed of the same material, thermal stress is not generated and characteristics of a circuit including a TFT are not damaged even when temperature changes in the display device. Furthermore, the number of driver ICs to be mounted on one pixel region can be reduced by mounting a longer driver IC as a driver circuit than an IC chip as described in this embodiment mode. 
     As described above, a fine pattern can be formed by exposing a conductive pattern formed by droplet discharge method with laser light and developing it. Moreover, by forming various patterns directly on a substrate by droplet discharge method, an EL display panel can be easily formed even by using a glass substrate of the fifth generation or later having a side of 1000 mm or longer. 
     Further, in this embodiment mode, a step in which a spin coating is not performed and an exposure step using a photo mask are not performed as much as possible is shown, however, the invention is not limited to this. An exposure step in which a photo mask is used as a part of patterning may be performed as well. 
     Various electronic devices can be formed by using an EL display panel manufactured as described above. Examples of the electronic devices include a television device, a video camera, a digital camera, a goggle type display, a navigation system, an audio reproducing device (a car audio set, an audio component system and the like), a personal computer, a game machine, a portable information terminal (a mobile computer, a portable phone, a portable game machine, an electronic book, or the like), an image reproducing device provided with a recording medium (specifically, a device which reproduces a recording medium such as a Digital Versatile Disc (DVD) and is provided with a display capable of displaying the reproduced image) and the like. In particular, it is preferable to apply the invention to a large television device with a large screen. Specific examples of these electronic devices are shown in  FIGS. 16A to 16  D. 
       FIG. 16A  illustrates a large television device having a large screen of 22 to 50 inches, which includes a housing  2001 , a support base  2002 , a display portion  2003 , a video input terminal  2005  and the like. The display device includes all display devices for displaying information such as for receiving television broadcast, and interactive television. According to the invention, a relatively inexpensive large display device can be realized even by using a glass substrate of the fifth generation or later having a side of 1000 mm or longer. 
       FIG. 16B  illustrates a personal computer including a main body  2201 , a housing  2202 , a display portion  2203 , a keyboard  2204 , an external connecting port  2205 , a pointing mouse  2206  and the like. According to the invention, a relatively inexpensive laptop personal computer can be realized. 
       FIG. 16C  illustrates a portable image reproducing device provided with a recording medium (specifically, a DVD reproducing device), including a main body  2401 , a housing  2402 , a display portion A  2403 , a display portion B  2404 , a recording medium (DVD and the like) reading portion  2405 , an operating key  2406 , a speaker portion  2407  and the like. The display portion A  2403  mainly displays image data while the display portion B  2404  mainly displays text data. It is to be noted that the image reproducing device provided with a recording medium includes a home game machine and the like. According to the invention, a relatively inexpensive image reproducing device can be realized. 
       FIG. 16D  illustrates a television device having a portable and wireless display. A housing  2602  includes a battery and a signal receiver. The battery drives a display portion  2603  and a speaker portion  2607 . The battery is rechargeable by a charger  2600 . Moreover, the charger  2600  can send and receive a video signal and transmit it to the signal receiver of the display. The housing  2602  is controlled by an operating key  2606 . The device shown in  FIG. 16D  can be used for a video/audio interactive communication device since a signal can be transmitted from the housing  2602  to the charger  2600  by operating the operating key  2606 . By operating the operating key  2606 , a signal is transmitted from the housing  2602  to the charger  2600  and then a signal which the charger  2600  can transmit is received by another electronic device, thereby communication of another electronic device can be controlled. Thus, it can also be used as a general remote control device. According to the invention, a relatively large (22 to 50 inches) portable television can be provided by an inexpensive manufacturing process. 
     As described above, the light emitting device according to the invention can be used as a display portion of various electronic devices. It is to be noted that a TFT is formed of amorphous silicon or semi-amorphous silicon in this embodiment mode, however, the invention is not limited to this. Similar operation effects can be obtained by applying a TFT of which channel forming region is formed of a polysilicon material. 
     Embodiment Mode 8 
     In this embodiment mode, a light emitting device having a thin film transistor is described with reference to  FIGS. 14A to 14C . 
     As shown in  FIG. 14A , a top gate N-channel TFT having an active layer formed of a semi-amorphous silicon film is provided in a driver circuit portion  1310  and a pixel portion  1311 . 
     In this embodiment mode, an N-channel TFT connected to a light emitting element formed in the pixel portion  1311  is referred to as a driving TFT  1301 . An insulating film  1302  called a bank or a partition wall is formed so as to cover an end of an electrode (referred to as a first electrode) of the driving TFT  1301 . For the insulating film  1302 , an inorganic material (silicon oxide, silicon nitride, silicon oxynitride and the like), a photosensitive or non-photosensitive organic material (polyimide, acrylic, polyamide, polyimide amide, resist, or benzocyclobutene), a material having a back bone structure of Si—O bond and containing at least hydrogen or at least one of fluoride, an alkyl group, or aromatic carbon hydride as a substituent, that is a so-called siloxane, and a stacked-layer structure of these can be used. As an organic material, a positive type photosensitive organic resin or a negative type photosensitive organic resin can be used. 
     An aperture portion is formed in the insulating film  1302  over the first electrode. An electroluminescent layer  1303  is formed in the aperture portion, and a second electrode  1304  of a light emitting element is provided so as to cover the electroluminescent layer and the insulating film  1302 . Note that a singlet excited state and a triplet excited state can be given as a kind of a molecular exciton generated in the electroluminescent layer. A ground state is normally a singlet state; therefore, luminescence from a singlet excited state is referred to as fluorescence and luminescence from a triplet excited state is referred to as phosphorescence. Luminescence from the electroluminescent layer includes the case where either excited state contributes. In addition, fluorescence and phosphorescence can be used in combination, and can be selected in accordance with a luminescence property (such as luminance or life) of each RGB. 
     The electroluminescent layer  1303  is formed by sequentially stacking an HIL (hole injecting layer), an HTL (hole transporting layer), an EML (emission layer), an ETL (electron transporting layer), and an EIL (electron injecting layer) in this order from the first electrode side. Note that the electroluminescent layer can have a single layer structure or a mixed structure as well as a stacked-layer structure. 
     In the case of a full color display, a material which exhibits light of red (R), green (G), and blue (B) may be selectively formed as the electroluminescent layer  1303  by an ink-jet method, an evaporation method using an evaporation mask for each, or the like. Specifically, CuPc or PEDOT is used as the HIL; a-NPD as the HTL; BCP or Alq 3  as the ETL; and BCP:Li or CaF 2  as the EIL. In addition, Alq 3  doped with a dopant in accordance with the respective colors of R, G, and B (DCM or the like in the case of R, and DMQD or the like in the case of G) may be used as the EML, for example. Note that the electroluminescent layer is not limited to a material having the aforementioned stacked-layer structure. For example, a hole injection property can be enhanced by co-evaporating oxide such as molybdenum oxide (MoO x : x=2 to 3) and a-NPD or rubrene. An organic material (including a low molecular weight material or a high molecular weight material) or a composite material of an organic material and an inorganic material can be used as the material. 
     In the case of forming an electroluminescent layer which emits white light, a full color display may be performed by separately providing a color filter or a color filter and a color conversion layer, and the like. The color filter and the color conversion layer may be formed on a second substrate (sealing substrate) before being attached. The color filter or the color conversion layer can be formed by ink-jet method. It is needless to say that monochrome light emitting device may be formed by forming an electroluminescent layer which exhibits light emission except white. In addition, an area color type display device which can perform monochrome display may be formed. 
     The first electrode and the second electrode  1304  are required to be formed of a material selected in consideration of a work function. However, the first and second electrodes can be either an anode or a cathode depending on a pixel configuration. In this embodiment mode, it is preferable that the first electrode be a cathode and the second electrode be an anode as the driving TFT is an N-channel transistor. In the case where the driving TFT polarity is a P-channel type, the first electrode is preferably an anode and the second electrode is preferably a cathode. 
     In consideration of a moving direction of electrons of the driving TFT as an N-channel transistor, the first electrode as a cathode, an EIL (electron injecting layer), an ETL (electron transporting layer), an EML (light emitting layer), an HTL (hole transporting layer), an HIL (hole injecting layer), and the second electrode as an anode are preferably stacked sequentially. 
     As a passivation film for covering the second electrode, an insulating film is preferably formed of DLC or the like by sputtering or CVD method. As a result, moisture or oxygen can be prevented from penetrating. Further, moisture or oxygen can be prevented from penetrating by covering the side of a display device with the first electrode, the second electrode, or another electrode. Then, the sealing substrates is attached. The space formed by the sealing substrate may be filled with nitrogen or further provided with a drying agent. The space formed by the sealing substrate may be filled with resin having a light-emitting property and a high moisture absorption property. 
     To increase contrast, a polarizer or a circular polarizer may be provided. For example, a polarizer or a circular polarizer can be provided over one surface or both surfaces of the display. 
     In the light emitting device having the structure formed as described above, a material having a light transmissive property (ITO or ITSO) is used for the first electrode and the second electrode. Therefore, light is emitted from the electroluminescent layer to both directions  1305  and  1306  at luminance corresponding to a video signal inputted from a signal line. Further,  FIG. 14B  shows a structure example which is partly different than  FIG. 14A . 
     In the structure of a light emitting device illustrated in  FIG. 14B , a channel etch N-channel TFT is provided in the driver circuit portion  1310  and the pixel portion  1311 . A manufacturing method of this channel etch TFT is described in Embodiment Mode 4, therefore, a detailed description thereon is omitted here. 
     Similarly to  FIG. 14A , an N-channel TFT connected to a light emitting element formed in the pixel portion  1311  is denoted as a driving TFT  1301 . The structure illustrated in  FIG. 14B  is different than  FIG. 14A  in that the first electrode is formed of a conductive film having a non-light transmissive property and preferably a highly reflective film, and the second electrode  1304  is formed of a conductive film having a light transmissive property. Therefore, a light emitting direction  1305  is only to the sealing substrate side.  FIG. 14C  shows a structure example which is partly different than  FIG. 14A . 
     In a structure of a light emitting device shown in  FIG. 14C , a channel stop N-channel TFT is provided for the driver circuit portion  1310  and the pixel portion  1311 . A manufacturing method of the channel stop N-channel TFT is described in Embodiment Mode 5, therefore, a detailed description thereon is omitted here. 
     Similarly to  FIG. 14A , an N-channel TFT connected to a light emitting element formed in the pixel portion  1311  is denoted to as the driving TFT  1301 . The structure shown in  FIG. 14C  is different than  FIG. 14A  in that the first electrode is formed of a conductive film having a light transmissive property, and the second electrode  1304  is formed of a conductive film having a non-light transmissive property and preferably a highly reflective film. Therefore, a light emitting direction  1306  is only to the substrate side. 
     The structure of a light emitting device using each thin film transistor has been described. The structure of the thin film transistor and the structure of the light emitting device can be freely combined with each other. 
     This application is based on Japanese Patent Application serial no. 2004-180306 filed in Japan Patent Office on May 22, 2004, the contents of which are hereby incorporated by reference.