Patent Publication Number: US-8975813-B2

Title: Light emitting device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device having an element in which a luminous material is placed between electrodes (hereinafter referred to as light emitting element) (the device will hereafter be called a light emitting device). Specifically, the invention relates to a light emitting device having a light emitting element that employs as the luminous material an organic compound capable of providing EL (electro luminescence) (hereinafter referred to as EL element). 
     2. Description of the Related Art 
     In recent years, researches have been advanced on an EL element having a structure in which a thin film formed of an organic compound capable of providing EL (EL layer) is placed between an anode and a cathode, and light emitting devices utilizing the luminous characteristic of the EL element have been developed. 
     An EL layer usually has a laminate structure typical example of which is one proposed by Tang et al. of Eastman Kodak Company and composed of a hole transporting layer, a light emitting layer, and an electron transporting layer. This structure has so high a light emitting efficiency that it is employed in almost all of EL displays that are under development at present. 
     Other examples of the laminate structure of the EL layer include a structure in which a hole injection layer, a hole transporting layer, a light emitting layer, and an electron transporting layer are layered on an anode in this order, and a structure in which a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injection layer are layered on an anode in this order. The light emitting layer may be doped with a fluorescent pigment or the like. 
     In this specification, all the layers that are placed between an anode and a cathode are collectively called an EL layer. Therefore the hole injection layer, the hole transporting layer, the light emitting layer, the electron transporting layer, and the electron injection layer mentioned above are all included in the EL layer. 
     When a given voltage is applied to the EL layer structured as above by a pair of electrodes, recombination of carriers takes place in the light emitting layer to emit light. A light emitting element composed of an anode, an EL layer, and a cathode is called herein an EL element. 
     In an EL element, degradation of its EL layer is accelerated when a driving voltage is high. Therefore an organic compound emitting light by a triplet exciton (hereinafter referred to as triplet compound) is sometimes used instead of the usual luminous material, namely, a singlet compound (an organic compound that emits light by singlet exciton), because the triplet compound can emit light of high luminance with a low driving voltage. 
     The term singlet compound herein refers to a compound that emits light solely through singlet excitation and the term triplet compound herein refers to a compound that emits light through triplet excitation. 
     The luminance of light emitted from an EL element is controlled by the voltage applied to its EL layer. However, the luminance of emitted light in relation to the applied voltage varies between luminous materials used to form the light emitting layer in the EL layer. To elaborate, a luminous material that emits low luminance light requires application of high voltage if a higher luminance is aimed. Unfortunately, application of high voltage leads to degradation of the luminous material. Furthermore, if EL elements formed on the same substrate receive the same voltage but emit light of varying luminance, different voltages have to be applied in order to make the EL elements to emit light of the same luminance. This results in another problem of varying EL element lifetime. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems, and an object of the present invention is to provide a long-living EL element that can emit light of desired luminance with a low voltage. 
     According to the present invention, a plurality of EL elements formed in a pixel portion on the same substrate include EL elements whose EL layers contain luminous materials emitting low luminance light (singlet compound) and EL elements whose EL layers contain triplet compounds capable of emitting high luminance light with a low voltage. By using the two types of EL elements in a strategically planned combination, the present invention makes it possible to control and equalize the luminance of light emitted from the plural EL elements as well as reduce the power consumption of the EL elements. 
       FIG. 1A  shows a circuit structure of a pixel portion usable in the present invention. Reference symbol  101  denotes a gate wiring line,  102   a  to  102   c ), source wiring lines, and  103   a  to  103   c , current supplying lines. These wiring lines define three regions in which a pixel a ( 104   a ), a pixel b ( 104   b ), and a pixel c ( 104   c ) are respectively formed. 
     Denoted by  105  is a switching transistor, which is formed in each of the three pixels. The structure shown here as an example has two channel formation regions between a source region and a drain region. However, the number of channel formation regions may be more than two or only one. 
     A current controlling transistor is denoted by  106  and is provided in each pixel. The current controlling transistor has a gate connected to one switching transistor, a source connected to one current supplying line, and a drain connected to one EL element. Reference symbol  107  denotes a condenser, which holds a voltage applied to the gate of the current controlling transistor  106 . However, the condenser  107  may be omitted. 
     The pixel a ( 104   a ), the pixel b ( 104   b ), and the pixel c ( 104   c ) have an EL element a ( 108   a ), an EL element b ( 108   b ), and an EL element c ( 108   c ), respectively. 
     These EL elements have an element structure shown in  FIG. 1B . An EL element  111  is composed of a cathode  112 , an anode  113 , and an EL layer  114 . The EL layer  114  emits tight when a voltage is applied to the cathode  112  or the anode  113 . 
     The EL layer  114  consists of a plurality of layers including: a light emitting layer  115  formed of a luminous material; an electron injection layer  116  for improved injection of electrons from the cathode; and an electron transporting layer  117  for transporting the injected electrons to the light emitting layer  115 . The layers  116  and  117  are sandwiched between the cathode  112  and the light emitting layer  115 . 
     The EL layer also includes a hole injection layer  118  for improved injection of holes from the anode, and a hole transporting layer  119  for transporting the injected holes to the light emitting layer  115 . The layers  118  and  119  are sandwiched between the anode  113  and the light emitting layer  115 . 
     Usually, light is emitted through recombination between the electrons injected from the cathode  112  and the holes injected from the anode  113  taking place in the light emitting layer  115 . However, the present invention employs a hole transporting layer in order to enhance the luminance of the emitted light. In other words, the invention needs the cathode  112 , the anode  113 , the light emitting layer  115 , and the hole transporting layer but other layers except for the hole transporting layer are provided only when necessary. 
     The present invention uses two types of EL elements; one has a triplet compound in the light emitting layer  115  of the EL layer  114  shown in  FIG. 1B , and the other has a singlet compound in its light emitting layer. The two types of EL elements are combined and formed in each of the pixels a to c ( 104   a  to  104   c ) shown in  FIG. 1A , so that the luminance of light emitted from the plural EL elements is equalized and a lopsided degradation in which some EL elements degrade faster than other EL elements is prevented. 
     When three color pixel display is intended, for example, if the luminance of light emitted from a luminous material for lighting the pixel a ( 104   a ) in one color is lower than the luminance of light of other two colors for respectively lighting the pixel b ( 104   b ) and the pixel c ( 104   c ), a triplet compound is used in the light emitting layer of the EL element a ( 108   a ) while singlet compounds are used in the light emitting layers of the EL elements b and c ( 108   b  and  108   c ). 
     If the luminance of light of two colors for respectively lighting the pixel a ( 104   a ) and the pixel b ( 104   b ) is lower than the luminance of light of one color for lighting the pixel c ( 104   c ), triplet compounds are used in the light emitting layers of the EL element a ( 108   a ) and the EL element b ( 108   b ) while a singlet compound is used in the light emitting layer of the EL element c ( 108   c ). 
     If the luminance of emitted light is low in all of three pixels a, b, and c ( 104   a ,  104   b , and  104   c ) and higher luminance is wanted to be obtained with a lower voltage, a triplet compound is used in every light emitting layer of the three EL elements a, b, and c ( 108   a ,  108   b , and  108   c ). 
     Materials given as typical triplet compounds are organic compounds described in the following articles:
         (1) T. Tsutsui, C. Adachi, S. Saito, Photochemical Processes in Organized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437.   (2) M. A. Baldo, D. F. O&#39;Brien, Y You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1998), p. 151.   (3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75 (1999) p. 4.   (4) T. Tsutsui, M. J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502.       

     Other than the luminous materials described in the articles above, ones (specifically, metal complexes or organic compounds) expressed by the following molecular formula may also be used: 
     [Chemical Formula 1] 
                         
[Chemical Formula 2]
 
     
       
         
         
             
             
         
       
     
     In the above chemical formulae, M represents an element belonging to Groups 8 to 10 in the periodic table and n represents 2 or 3. Platinum or iridium is used in the articles above. Nickel, cobalt, or palladium is preferable because its physical characteristics are similar to those of platinum or iridium. Nickel is particularly preferable as a central metal, for it easily forms a complex. 
     Still another material usable as the triplet compound is a rare earth complex which is formed by an ion of a rare earth element, such as europium, terbium, or cerium, and from a ligand. 
     The triplet compound has a higher light emission efficiency than the singlet compound and hence needs lower operation voltage (a voltage required to cause an EL element to emit light) in emitting light of the same luminance. 
     Furthermore, the present invention improves the mobility of carriers (electrons and holes) injected from an anode by providing a plurality of hole transporting layers between the anode and a light emitting layer  125  as shown in  FIGS. 2B and 2C . Although shown in this specification is a case of making only the transporting layer a laminate, the electron transporting layer may also be a laminate similar to the hole transporting layer. In this case, a layer formed of a compound that can reduce the difference in energy level (LUMO level) is placed between the cathode and the electron transporting layer. 
       FIG. 2A  shows an EL element structure similar to the one shown in  FIG. 1B . The light emitting layer  125  is placed between a cathode  123  and an anode  124 . An electron injection layer  126  and an electron transporting layer  127  are placed between the cathode  123  and the light emitting layer  125 . A hole injection layer  128  and a hole transporting layer  1  ( 129 ) are placed between the anode  124  and the light emitting layer  125 . 
     In contrast to this,  FIG. 2B  shows a laminate structure in which one more layer, namely, a hole transporting layer  2  ( 130 ) is added between the hole transporting layer  1  ( 129 ) and the hole injection layer  128 . 
     The laminate structure is translated into a band structure of  FIG. 2C . Reference symbols used in  FIG. 2C  are identical with those in  FIGS. 2A and 2B . By the laminate structure formed forming the hole transporting layer  2  ( 130 ) between the hole transporting layer  1  ( 129 ) and the hole injection layer  128 , the difference in HOMO level between the hole injection layer and the hole transporting layer can be reduced. This facilitates movement of holes from the hole injection layer to the hole transporting layer, and the EL element can have a high luminance with a low voltage as a result. 
     The case shown here as an example has a laminate structure consisting of the hole transporting layer  1  ( 129 ) and the hole transporting layer  2  ( 130 ). However, the laminate structure of the hole transporting layer may have two or more layers formed of different materials if the difference in HOMO level between the hole injection layer and the hole transporting layer is reduced as mentioned above. Preferably, the laminate structure has two to five layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIGS. 1A and 1B  are diagrams illustrating a light emitting device; 
         FIGS. 2A to 2C  are diagrams illustrating a laminate structure of an EL element; 
         FIGS. 3A to 3E  are diagrams showing a process of manufacturing a light emitting device; 
         FIGS. 4A to 4D  are diagrams showing a process of manufacturing a light emitting device; 
         FIGS. 5A and 5B  are diagrams showing a process of manufacturing a light emitting device; 
         FIGS. 6A and 6B  are diagrams respectively showing a top structure of a light emitting device and a sectional structure thereof; 
         FIGS. 7A to 7D  are diagrams showing laminate structures of EL elements; 
         FIGS. 8A and 8B  are graphs showing element characteristics of EL elements; 
         FIGS. 9A to 9C  are diagrams showing laminate structures of EL elements; 
         FIGS. 10A and 10B  are graphs showing element characteristics of EL elements; 
         FIGS. 11A to 11C  are graphs showing element characteristics of EL elements; 
         FIG. 12  is a diagram showing a sectional structure of a light emitting device; 
         FIGS. 13A and 13B  are diagrams showing the circuit structure of pixels in a light emitting device; 
         FIG. 14  is a diagram showing a sectional structure of a light emitting device; 
         FIG. 15  is a diagram showing the circuit structure of pixels in a light emitting device; 
         FIG. 16  is a diagram showing a sectional structure of a light emitting device; 
         FIGS. 17A to 17C  are diagrams showing a process of manufacturing a light emitting device; 
         FIG. 18  is a diagram showing the circuit structure of pixels in a light emitting device; 
         FIGS. 19A and 19B  are diagrams showing the structure of a light emitting device with external driving circuit; 
         FIGS. 20A and 20B  are diagrams showing the structure of a light emitting device with external controller; 
         FIGS. 21A to 21F  are diagrams showing specific examples of an electric machine; 
         FIGS. 22A to 22F  are diagrams showing specific examples of an electric machine; 
         FIGS. 23A and 23B  are diagrams respectively showing a top structure of a light emitting device and a sectional structure thereof; 
         FIG. 24  is a diagram showing the circuit structure of pixels in a light emitting device; and 
         FIG. 25  is a diagram showing element characteristics of an EL element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Modes for carrying out the present invention will be described in detail through the following embodiments. 
     Embodiment 1 
     In this embodiment, a description will be given of a method of manufacturing a pixel portion and a driving circuit provided at its periphery on the same insulator. However, for simplification of the description, with respect to the driving circuit, a CMOS circuit in which an n-channel transistor and a p-channel transistor are combined will be shown. 
     First, as shown in  FIG. 3A , a glass substrate  201  is prepared as a insulator. In this embodiment, not-shown protection films (carbon films, specifically diamond-like carbon films) are provided on both surfaces (the front surface and the rear surface) of the glass substrate  201 . As long as it is transparent to visible light, a material other than glass (for example, plastic) may be used. 
     Next, an base film  202  having a thickness of 300 nm is formed on the glass substrate  201 . In this embodiment, as the base film  202 , silicon oxynitride films are laminated and are used. At this time, it is appropriate that the concentration of nitrogen of a layer adjacent to the glass substrate  201  is made 10 to 25 wt %, and nitrogen is made to be contained at the concentration rather higher than that of another layer. 
     Next, an amorphous silicon film (not shown) having a thickness of 50 nm is formed on the base film  202  by a sputtering method. Note that, it is not necessary to limit the film to the amorphous silicon film, but any semiconductor films (including a microcrystalline semiconductor film) containing amorphous structure may be used. As the amorphous semiconductor film, an amorphous silicon film or an amorphous silicon germanium film (a silicon film containing germanium at a concentration of 1×10 18  to 1×10 21  atoms/cm 3 ) may be used. The film thickness may be 20 to 100 nm. 
     Then, crystallization of the amorphous silicon film is performed by using a well-known laser crystallizing, method, and a crystalline silicon film  203  is formed. In this embodiment, although a solid laser (specifically, second harmonic of Nd:YAG laser) is used, an excimer laser may also be used. As the crystallizing method, a furnace annealing method may be used. 
     Next, as shown in  FIG. 3B , the crystalline silicon film  203  is etched by a first photolithography step to form island-like crystalline silicon films  204  to  207 . These are crystalline silicon films which subsequently become the active layers of transistors. 
     Note that, in this embodiment, although the crystalline silicon films are used as the active layers of the transistors, an amorphous silicon film can also be used as the active layer. 
     Here, in this embodiment, a protection film (not shown) made of a silicon oxide film and having a thickness of 130 nm is formed on the island-like crystalline silicon films  204  to  207  by a sputtering method, and an impurity element (hereinafter referred to as a p-type impurity element) to make a semiconductor a p-type semiconductor is added to the island-like crystalline silicon films  204  to  207 . As the p-type impurity element, an element (typically, boron or gallium) belonging to group 13 of the periodic table can be used. Note that, this protection film is provided to prevent the crystalline silicon film from directly being exposed to plasma when the impurity is added, and to enable fine concentration control. 
     The concentration of the p-type impurity element added at this time may be made 1×10 15  to 5×10 17  atoms/cm 3  (typically, 1×10 16  to 1×10 17  atoms/cm 3 ). The p-type impurity element added at this concentration is used to adjust the threshold voltage of the n-channel transistor. 
     Next, the surfaces of the island-like crystalline silicon films  204  to  207  are washed. First, the surface is washed by using pure water containing ozone. At that time, since a thin oxide film is formed on the surface, the thin oxide film is removed by using a hydrofluoric acid solution diluted to 1%. By this treatment, contaminants adhered to the surfaces of the island-like crystalline silicon films  204  to  207  can be removed. At this time, it is preferable that the concentration of ozone is 6 mg/L or more. The series of treatments are carried out without opening to the air. 
     Then, a gate insulating film  208  is formed to cover the island-like crystalline silicon films  204  to  207 . As the gate insulating film  208 , an insulating film having a thickness of 10 to 150 nm, preferably 50 to 100 nm and containing silicon may be used. This may have a single-layer structure or a laminate structure. In this embodiment, a silicon oxynitride film having a thickness of 80 nm is used. 
     In this embodiment, the steps from the surface washing of the island-like crystalline silicon films  204  to  207  to the formation of the gate insulating film  208  are carried out without opening to the air, so that contaminants and interface levels on the interface between the semiconductor film and the gate insulating film are lowered. In this case, a device of a multi-chamber system (or an inline system) including at least a washing chamber and a sputtering chamber may be used. 
     Next, a tantalum nitride film having a thickness of 30 nm is formed as a first conductive film  209 , and further, a tungsten film having a thickness of 370 nm is formed as a second conductive film  210 . In addition, a combination of a tungsten film as the first conductive film and an aluminum alloy film as the second conductive film, or a combination of a titanium film as the first conductive film and a tungsten film as the second conductive film may be used. 
     These metal films may be formed by a sputtering method. When an inert gas such as Xe or Ne is added as a sputtering gas, film peeling due to stress can be prevented. When the purity of a tungsten target is made 99.9999%, a low resistance tungsten film having a resistivity of 20 mΩcm or less can be formed. 
     Besides, the steps from the surface washing of the semiconductors  204  to  207  to the formation of the second conductive film  210  can also be carried out without opening to the air. In this case, a device of a multi-chamber system (or an inline system) including at least a washing chamber, a sputtering chamber for forming an insulating film, and a sputtering chamber for forming a conductive film may be used. 
     Next, the resist  211   a  to  211   e  is formed and the second conductive film  210  is etched. As an etching condition, it is preferable to perform a dry etching using ICP (Inductively Coupled Plasma). As an etching gas, a mixture gas of a carbon tetrafluoride (CF 4 ) gas, a chlorine (Cl 2 ) gas and an oxygen (O 2 ) gas is used. 
     As a typical etching condition, a gas pressure is made 1 Pa, and in this state, RF electric power (13.56 MHz) of 500 W is applied to a coil type electrode to produce plasma. Besides, RF electric power (13.56 MHz) of 150 W is applied as a self bias voltage to a stage on which the substrate is put, so that a negative self bias is applied to the substrate. At this time, it is appropriate that the amount of the flow of the respective gases is made such that the carbon tetrafluoride gas has a flow of 2.5×10 −5  m 3 /min, the chlorine gas has a flow of 2.5×10 −5  m 3 /min, and the oxygen gas has a flow of 1.0×10 −5  m 3 /min ( FIG. 3C ) 
     By this, the second conductive film (tungsten film)  210  is selectively etched, and electrodes  212  to  216  made of the second conductive film are formed. The reason why the second conductive film  210  is selectively etched is that the progress of etching of the first conductive film (tantalum nitride film) becomes extremely slow by addition of oxygen to the etching gas. 
     Note that, here, there is a reason why the first conductive film  209  is made to remain. Although the first conductive film can also be etched at this time, if the first conductive film is etched, the gate insulating film  208  is also etched in the same step and the film thickness is decreased. At this time, if the thickness of the gate insulating film  208  is 100 nm or more, there is no problem. However, if the thickness is less than that, a part of the gate insulating film  208  is removed in a subsequent step and the semiconductor film thereunder is exposed, and there is a possibility that the semiconductor film which becomes a source region or a drain region of a transistor is also removed. 
     However, the foregoing problem can be solved by leaving the first conductive film  209  as in this embodiment. 
     Next, an n-type impurity element (in this embodiment, phosphorus) is added in a self-aligning manner by using the resists  211   a  to  211   e  and the electrodes  212  to  216 . At this time, phosphorus passes through the first conductive film  209  is added. Impurity regions  217  to  225  formed in this way contain the n-type impurity element at a concentration of 1×10 20  to 1×10 21  atoms/cm 3  (typically, 2×10 20  to 5×10 21  atoms/cm 3 ). 
     Next, the first conductive film  209  is etched using a resists  211   a  to  211   e  as masks. This etching is performed by a dry etching method using the ICP, and a mixture gas of a carbon tetrafluoride (CF 4 ) gas and a chlorine (Cl 2 ) gas is used as an etching gas. A typical etching condition is such that a gas pressure is made 1 Pa, and RF electric power (13.56 MHz) of 500 W is applied to a coil type electrode to produce plasma in this state. Besides, RF electric power (13.56 MHz) of 20 W is applied as a self bias voltage to the stage on which the substrate is put, so that a negative self bias is applied to the substrate. At this time, it is appropriate that the flow of the respective gases is made such that the carbon tetrafluoride gas has a flow of 3.0×10 −5  m 3 /min, and the chlorine gas has a flow of 3.0×10 −5  m 3 /min. Thus, the electrodes  226  to  230  from the first conductive film are formed. ( FIG. 3D ) 
     Next, as shown in  FIG. 3E , the electrodes  212  to  216  from the second conductive film is etched selectively using the resists  211   a  to  211   e . This etching is performed by a dry etching method using the ICP, and a mixture gas of a carbon tetrafluoride (CF 4 ) gas, a chlorine (Cl 2 ) gas and an oxygen (O 2 ) gas is used as an etching gas. A typical etching condition is such that a gas pressure is made 1 Pa, and in this state, RF electric power (13.56 MHz) of 500 W is applied to a coil type electrode to produce plasma. Besides, RF electric power (13.56 MHz) of 20 W is applied as a self bias voltage to the stage on which the substrate is put, so that a negative self bias is applied to the substrate. At this time, it is appropriate that the amount of the flow of the respective gases is made such that the carbon tetrafluoride gas has a flow of 2.5×10 −5  m 3 /min, the chlorine gas has a flow of 2.5×10 −5  m 3 /min, and the oxygen gas has a flow of 1.0×10 −5  m 3 /min. The etching rate of the tantalum nitride film is suppressed by the existence of oxygen. Thus, the second gate electrodes  231  to  235  are formed. 
     Next, an n-type impurity element (in this embodiment, phosphorus) is added. In this step, the second gate electrodes  231  to  235  function as masks, and phosphorus passes through part of the electrodes  226  to  230  made of the first conductive film and is added, and n-type impurity regions  236  to  245  containing phosphorus at a concentration of 2×10 16  to 5×10 19  atoms/cm 3  (typically, 5×10 17  to 5×10 18  atoms/cm 3 ) are formed. 
     Besides, as an addition condition here, an acceleration voltage is set quite high as 70 to 120 kV (in this embodiment, 90 kV) so that phosphorus passes through the first conductive film and the gate insulating film and reaches the island-like crystalline silicon films. 
     Next, as shown in  FIG. 4A , the electrodes  226  to  230  made of the first conductive film are etched to form first gate electrodes  246  to  250 . This etching is performed by a dry etching method using the ICP or a dry etching method with an RIE (Reactive Ion Etching) mode, and a mixture gas of a carbon tetrafluoride (CF 4 ) gas and a chlorine (Cl 2 ) gas is used as an etching gas. A typical etching condition is such that a gas pressure is made 1 Pa, and RF electric power (13.56 MHz) of 500 W is applied to a coil type electrode to produce plasma in this state. Besides, RF electric power (13.56 MHz) of 20 W is applied as a self bias voltage to the stage on which the substrate is put, so that a negative self bias is applied to the substrate. At this time, it is appropriate that the amount of the flow of the respective gases is made such that the carbon tetrafluoride gas has a flow of 2.5×10 −5  m 3 /min, the chlorine gas has a flow of 2.5×10 −5  m 3 /min, and the oxygen gas has a flow of 1.0×10 −5  m 3 /min. 
     At this time, the first gate electrodes  246  to  250  are etched so that they partially overlap the n-type impurity regions  236  to  245  through the gate insulating film  208 . For example, the n-type impurity region  236  is divided into a region  236   a  not overlapping the first gate electrode  246  and a region  236   b  overlapping there through the gate insulating film  208 . The n-type impurity region  237  is divided into a region  237   a  not overlapping the first gate electrode  246  and a region  237   b  overlapping there through the gate insulating film  208 . 
     Next, resists  251   a  and  251   b  are formed, and an impurity element (hereinafter referred to as a p-type impurity element) to make a semiconductor a p-type semiconductor is added. As the p-type impurity element, an element (typically, boron) belonging to group 13 of the periodic table may be added. Here, an acceleration voltage is set so that boron passes through the first gate electrodes  247  and  250  and the gate insulating film  208 , and reaches the semiconductor film. In this way, p-type impurity regions  252  to  255  are formed ( FIG. 4B ). 
     Next, as shown in  FIG. 4C , as a first inorganic insulating film  256 , a silicon nitride film or silicon oxynitride film having a thickness of 30 to 100 nm is formed. Thereafter, the added n-type impurity element and p-type impurity element are activated. As an activation means, a furnace annealing, a laser annealing, a lamp annealing, or a combination of those can be used. 
     Next, as shown in  FIG. 4D , a second inorganic insulating film  257  made of a silicon nitride film or a silicon oxynitride film is formed to a thickness of 50 to 200 nm. After the second inorganic insulating film  257  is formed, a heat treatment in the temperature range of 350 to 450° C. is carried out. Note that, it is effective to carry out a plasma treatment using a hydrogen (H 2 ) gas or an ammonia (NH 3 ) gas before the second inorganic insulating film  257  is formed. 
     Next, as an organic insulating film  258 , a resin film transparent to visible light is formed to a thickness of 1 to 2 μm. As the resin film, a polyimide film, a polyamide film, an acryl resin film, or a BCB (benzocyclobutene) film may be used. Besides, a photosensitive resin film can also be used. 
     Note that, in this embodiment, the laminate film of the first inorganic insulating film  256 , the second inorganic insulating film  257 , and the organic insulating film  258  is generically called an interlayer insulating film. 
     Next, as shown in  FIG. 5A , a pixel electrode (anode)  259  made of an oxide conductive film which has a large work function and is transparent to visible light is formed to a thickness of 80 to 120 nm on the organic insulating film  258 . In this embodiment, an oxide conductive film in which gallium oxide is added to zinc oxide is formed. Besides, as another oxide conductive film, it is also possible to use an oxide conductive film made of indium oxide, zinc oxide, tin oxide, or a compound of combination of those as other oxide conductive film. 
     Note that, after the oxide conductive film is formed, although patterning is carried out to form the pixel electrode  259 , a flattening treatment of the surface of the oxide conductive film can also be carried out before the patterning. The flattening treatment may be a plasma treatment or a CMP (Chemical Mechanical Polishing) treatment. 
     Next, contact holes are formed in the interlayer insulating film, and wiring lines  260  to  266  are formed. At this time, the wiring line  266  is formed to be connected with the pixel electrode  259 . In this embodiment, this wiring line is made as the laminate film of three-layer structure in which a titanium film having a thickness of 150 nm, an aluminum film containing titanium and having a thickness of 300 nm, and a titanium film having a thickness of 100 nm are continuously formed from the lower layer side by a sputtering method. 
     At this time, the wiring lines  260  and  262  function as source wiring lines of a CMOS circuit, and the wiring line  261  functions as a drain wiring line. The wiring line  263  is a source wiring line of a switching transistor, and the wiring line  264  is a drain wiring line of the switching transistor. The wiring line  265  is a source wiring line (equivalent to a current supply line) of a current controlling transistor, and the wiring line  266  is a drain wiring line of the current controlling transistor and is connected with the pixel electrode  259 . 
     Next, as shown in  FIG. 5B , a bank  267  is fowled. The bank  267  may be formed by patterning an insulating film having a thickness of 100 to 400 nm and containing silicon or an organic resin film. This bank  267  is formed to fill a portion between pixels (between pixel electrodes). Besides, it also has an object to prevent a subsequently formed organic EL film such as a light emitting layer from being brought into direct contact with the end portion of the pixel electrode  259 . 
     Incidentally, since the bank  267  is an insulating film, attention must be paid to electrostatic damage of a device at the time of film formation. When carbon particles or metal particles are added into the insulating film, which becomes a material of the bank  267 , to lower its resistivity, the generation of static electricity at the time of film formation can be suppressed. In that case, it is appropriate that the amount of addition of carbon particles or metal particles is adjusted so that the resistivity of the insulating film, which becomes a material of the bank  267 , becomes 1×10 6  to 1×10 12  Ωm (preferably, 1×10 8  to 1×10 10  Ωm). 
     When the carbon particles or the metal particles are added to the bank  267 , optical absorption is raised and transmissivity is lowered. That is, since light from the outside of the light emitting device is absorbed, it is possible to avoid such a disadvantage that an outside scene is reflected in the cathode surface of the EL element. 
     Next, an EL layer  268  is formed by evaporation. In this embodiment, a laminate of a hole injection layer and a light emitting layer is called an EL layer. An EL layer could be a laminate obtained by combining a light emitting layer with a hole injection layer, a hole transporting layer, a hole blocking layer, an electron transporting layer, and an electron injection layer. As long as the laminate includes a light emitting layer and a hole transporting layer, it fulfils the definition of the EL layer in this specification. 
     Described here is a method of forming a light emitting layer that emits green light from a triplet compound in the light emitting layer as the EL layer. 
     A copper phthalocyanine (CuPc) film with a thickness of 20 nm is formed first as a hole injection layer in this embodiment. Then, as a hole transporting layer, MTDATA that is an aromatic amine called star burst amine is deposited to a thickness of 20 nm and α-NPD that is also an aromatic amine-based compound is deposited to a thickness of 10 nm. Thus the hole transporting layer described in this embodiment has a two-layer structure of MTDATA and α-NPD. 
     Materials for forming the hole transporting layer are roughly divided into hole transporting low molecular weight compounds and hole transporting high molecular weight compounds. One or more compounds are selected from each of the two types of compounds to form a laminate hole transporting layer. Specifically, TPAC, PDA, TPD, and like other compounds can be used as the hole transporting low molecular weight compounds whereas various high polymers having polyvinyl carbazole (PVK) or TPD as their principal chains or side chains can be used as the hole transporting high molecular weight compounds. 
     The hole transporting layer thus can have layers formed of different materials. However, the total thickness of the hole transporting layer is preferably about 20 to 100 nm. When the layers that constitute the hole transporting layer are increased in number, the thickness of the individual layers has to be reduced. Therefore, two to four layers are preferable. 
     Then a light emitting layer is formed from CBP and Ir(ppy) 3  by co-evaporation to a thickness of 20 nm. After the light emitting layer is formed, a hole blocking layer is formed from BCP to a thickness of 10 nm and an electron transporting layer is formed from an aluminoquinolilate complex (Alq 3 ) to a thickness of 40 nm. 
     The case described here is of twining an EL layer that emits green light. Examples of other usable luminous materials emitting green light include an aluminoquinolilate complex (Alq 3 ), which is given in the above as the material of the electron transporting layer, and a beryllium benzoquinolilate complex (BeBq). Also included in the examples is an aluminoquinolilate complex (Alq 3 ) doped with coumarin 6 or quinacridon. 
     When an EL layer emitting red light is to be formed, examples of the usable luminous material include an Eu complex (Eu(DCM) 3  (Phen)) and an aluminoquinolilate complex (Alq 3 ) that is doped with DCM-1. 
     When an EL layer emitting blue light is to be formed, examples of the usable luminous material include DPVBi that is a distyril derivative, a zinc complex having an azomethine compound as a ligand, and DPVBi doped with perylene. 
     In carrying out the present invention, the luminous materials given in the above can be used to form EL layers respectively emitting red light, green light, and blue light, for example. A singlet compound and a triplet compound can be used in any combination as luminous materials in accordance with the need. Materials introduced in ‘Summary of the Invention’ may also be used as a triplet compound. 
     The EL layers respectively emitting red light, green light, and blue light formed here are merely an embodiment. Color of emitted light is not limited thereto and combinations of other colors can be chosen. 
     After the EL layer  268  is formed, a cathode  269  is formed to a thickness of 300 nm from a conductive film having a small work function. A conductive film containing an element belonging to Group 1 or 2 in the long-period periodic table and a transition element belonging to Groups 3 through 11 can be used as a conductive film having a small work function. This embodiment uses a conductive film formed of ytterbium (Yb). A conductive film formed of a compound of lithium and aluminum may also be used. Thus completed is an EL element  270  including the pixel electrode (anode)  259 , the EL layer  268 , and the cathode  269 . 
     After the cathode  269  is formed, it is effective to form a passivation film  271  so as to completely cover the EL element  270 . The passivation film  271  is a single layer of insulating film or a laminate of a combination of insulating films. Examples of the insulating film include a carbon film, a silicon nitride film, and a silicon oxynitride film. 
     A preferred passivation film is one that can cover a wide area, and a carbon film, especially a DLC (diamond-like carbon) film, is effective. A DLC film can be formed at a temperature range of from room temperature to 100° C., and it is easily be formed above the EL layer  268  that has a low heat resistance. In addition, a DLC film is high in oxygen blocking effect and can prevent oxidization of the EL layer  268 . Therefore, oxidization of the EL layer  268  during the subsequent sealing step can be avoided. 
     A seal (not shown in the drawing) is provided on the substrate  201  (or on the base film  202 ) so as to surround at least the pixel portion, thereby bonding a covering member  272 . The seal may be a UV-curable resin which allows less amount of gas to free and through which moisture and oxygen are hardly transmitted. A gap  273  is filled with inert gas (nitrogen gas or rare gas) or a resin (UV-curable resin or epoxy resin). 
     It is effective to place a substance having a hygroscopic effect or a substance having an antioxidizing effect in the gap  273 . The covering member  272  may be a glass substrate, a metal substrate (preferably a stainless steel substrate), a ceramic substrate, or a plastic substrate (including a plastic film). When a plastic substrate is used, it is preferable to foam carbon films (preferably diamond-like carbon films) on the front and back surfaces of the substrate to prevent transmission of oxygen or moisture. 
     A light emitting device structured as shown in  FIG. 5B  is thus completed. It is effective to use a film formation apparatus of multi-chamber type or inline type to process steps subsequent to formation of the bank  267  through formation of the passivation film  271  in succession without exposing the device to the air. The successive processing may be further extended to the step of bonding the covering member  272  while avoiding exposure to the air. 
     Thus formed on the substrate  201  are an n-channel transistor  601 , a p-channel transistor  602 , a switching transistor (a transistor functioning as a switching element for transferring a video data signal to a pixel)  603 , and a current controlling transistor (a transistor functioning as a current controlling element for controlling a current flowing into an EL element)  604 . 
     The driving circuit here includes as a basic circuit a CMOS circuit that combines the n-channel transistor  601  and the p-channel transistor  602  complementarily. The pixel portion is composed of a plurality of pixels each including the switching transistor  603  and the current controlling transistor  604 . 
     Up to this point, the manufacture process has needed the photolithography processing seven times, which is less than in a general active matrix light emitting device. In other words, the process of manufacturing transistors is greatly simplified to improve the yield and reduce the manufacture cost. 
     Moreover, as explained referring to  FIG. 4 , by preparing an impurity region that overlaps a first gate electrode with a gate insulating film interposed therebetween, the n-channel transistor can be thinned which is strong against degradation due to hot carrier injection. Therefore, a light emitting device of high reliability can be provided. 
     The light emitting device of this embodiment which has been finished up through the sealing (or enclosing) step for protecting the EL element is further described with reference to  FIGS. 6A and 6B . The symbols used in  FIGS. 3A to 5B  are mentioned when necessary. 
       FIG. 6A  is a top view showing the device that has been finished up through sealing the EL element, and  FIG. 6B  is a sectional view taken along the line A-A′ in  FIG. 6A . An area surrounded by a dotted line and denoted by  501  is a pixel portion, and  502  and  503  represent a source side driving circuit and a gate side driving circuit, respectively. Denoted by  504 ,  505 , and  506  are a covering member, a first seal, and a second seal. respectively. 
     Reference symbol  507  denotes a wiring line for transferring. signals to be inputted to the source side driving circuit  502  and the gate side driving circuit  503 . The wiring line  508  receives video signals and clock signals from an FPC (flexible printed circuit)  508  that is an external input terminal. Although the FPC alone is shown in FIG.  6 A, a printed wiring board (PWB) may be attached to the FPC. 
     The sectional structure is described next referring to  FIG. 6B . The pixel portion  501  and the source side driving circuit  502  are formed over the substrate  201 . The pixel portion  501  is composed of a plurality of pixels each including the current controlling transistor  604  and the pixel electrode  259  electrically connected to the drain of the transistor  604 . The source side driving circuit  502  is composed of a CMOS circuit that combines the n-channel transistor  601  and the p-channel transistor  602  (see  FIG. 5B ). A polarizing plate (typically a circular polarizing plate) may be bonded to the substrate  201 . 
     The pixel electrode  259  functions as the anode of the EL element. The bank  267  is formed on each end of the pixel electrode  259 . The EL layer  268  is formed on the pixel electrode  259  and the cathode  269  of the EL element is formed on the EL layer. The cathode  269  also functions as a wiring line common to all the pixels, and is electrically connected to the FPC  508  through the connection wiring line  507 . All the elements included in the pixel portion  501  and the source side driving circuit  502  are covered with the passivation film  271 . 
     The covering member  504  is bonded by the first seal  505 . A spacer may be provided to secure the distance between the covering member  504  and the EL element. The gap  273  is provided inside the first seal  505 . The first seal  505  is desirably a material that does not transmit moisture and oxygen. It is effective to place a. substance having a hygroscopic effect or a substance having an antioxidizing effect in the gap  273 . 
     On the front and back surfaces of the covering member  504 , carbon films (specifically, diamond-like carbon films)  509   a  and  509   b  each having a thickness of 2 to 30 nm are formed as protective films. The carbon films mechanically protect the surfaces of the covering member  504  as well as prevent permeance of oxygen and moisture. 
     After the covering member  504  is bonded, the second seal  506  is placed so as to cover the exposed surfaces of the first seal  505 . The same material may be used for the second seal  506  and the first seal  505 . 
     By enclosing the EL element with the structure as above, the EL element can be shut off from the surroundings completely and external substances that accelerate degradation by oxidization of the EL layer, such as moisture and oxygen, can be prevented from entering the EL element. Accordingly, a light emitting device of high reliability can be obtained. 
     A light emitting device in which a pixel portion and a driving circuit are on the same substrate and an FPC is attached to the substrate as shown in  FIGS. 6A and 6B  is specially called a light emitting device with built-in driving circuit in this specification. 
     The light emitting device manufactured in accordance with this embodiment can operate on both digital signals and analog signals. 
     Embodiment 2 
     This embodiment shows characteristics of EL elements having different EL layers that can be used in carrying out the present invention. Structures of the EL layers formed in this embodiment are shown in  FIGS. 7A to 7D . 
       FIG. 7A  shows the structure of an EL element a. First, a hole transporting layer is formed from α-NPD by evaporation to a thickness of 40 nm on an anode that is formed of a compound of indium oxide and tin oxide. On the hole transporting layer, a light emitting layer is formed from luminous materials consisting of Ir(ppy) 3  and CBP (triplet compounds) by co-evaporation to a thickness of 20 nm. On the light emitting layer, a BCP layer with a thickness of 10 nm and a Alq 3  layer with a thickness of 40 nm are formed by evaporation as an electron transporting layer. Then a cathode is formed from Yb to a thickness of 400 nm to complete the EL element a. Light emission from the EL element a utilizes triplet excitation energy by the triplet compounds. 
       FIG. 7B  shows the structure of an EL element b. First, a hole injection layer is formed from copper phthalocyanine by evaporation to a thickness of 20 nm on an anode that is formed of a compound of indium oxide and tin oxide. A hole transporting layer is formed thereon by depositing MTDATA to a thickness of 20 nm and then depositing α-NPD to a thickness of 10 nm by evaporation. On the hole transporting layer, a light emitting layer is formed from a luminous material consisting of Alq 3  (singlet compound) by evaporation to a thickness of 50 nm. Then a cathode is formed from Yb to a thickness of 400 nm to complete the EL element b by evaporation. Light emission from the EL element b utilizes singlet excitation energy by the singlet compound. 
       FIG. 7C  shows the structure of an EL element c. First, a hole transporting layer is formed from α-NPD by evaporation to a thickness of 50 nm on an anode that is formed of a compound of indium oxide and tin oxide. On the hole transporting layer, a light emitting layer is formed from a luminous material consisting of Alq 3  (singlet compound) by evaporation to a thickness of 50 nm. Then a cathode is formed from Yb to a thickness of 400 nm to complete the EL element c. Light emission from the EL element c utilizes singlet excitation energy by the singlet compound. The EL layer of the EL element c has no other layers than the light emitting layer and the hole transporting layer. 
       FIG. 7D  shows the structure of an EL element d. First, a hole transporting layer is formed from PEDOT that is a polythiophene derivative by spin coating to a thickness of 30 nm on an anode that is formed of a compound of indium oxide and tin oxide. Polyparaphenylenevinylene (hereinafter referred to as PPV) is then used as a luminous material to form a film with a thickness of 80 nm by spin coating on the hole transporting layer. Then a cathode is formed from Yb to a thickness of 400 nm to complete the EL element d by evaporation. Light emission from the EL element d utilizes singlet excitation energy by the singlet compound. The EL element d is different from the other EL elements a to c in that a high molecular weight material is used for the light emitting layer. 
     The EL elements illustrated in  FIGS. 7A to 7D  have been estimated for their electrical characteristics. Results are shown in  FIGS. 8A and 8B .  FIG. 8A  shows the luminance characteristic in relation to the current density. rough observation, there is a difference in characteristic in relation to the current density between the EL element that uses triplet compounds and the EL elements that use singlet compounds. To elaborate, when the current density is 60 mA/cm 2 , the EL element a that uses triplet compounds provides a luminance of about 6000 cd/m 2  whereas the EL elements b, c, and d that use singlet compounds each provide a luminance of about 2000 cd/m 2 , namely, one third of the luminance of the EL element a. 
       FIG. 8B  shows results of measuring the external quantum efficiency in relation to the current density. Similar to the case of the luminance characteristic, the EL element a that uses triplet compounds has exhibited a far better external quantum efficiency. The difference in external quantum efficiency between the EL element a and the EL elements b to d is seven times at the maximum. 
     As shown in the results in  FIGS. 8A and 8B , employing a triplet compound in an EL element improves light emission efficiency. 
     In order to further improve light emission provided by the EL element a of  FIG. 7A  which uses triplet compounds, another layer is added to the element. 
       FIG. 9A  shows the same EL element a as the one shown in  FIG. 7A . In  FIG. 9B , copper phthalocyanine is deposited by evaporation to a thickness of 20 nm on the anode of the EL element a. Electric characteristics of this EL element is shown in  FIGS. 10A and 10B . As shown in  FIG. 10A , providing the copper phthalocyanine layer on the anode does not change the luminance of the EL element itself much but the time during which the luminance is maintained is prolonged. 
       FIG. 10B  shows that the amount of current flowing in an early stage is changed by addition of one more layer but eventually reaches the same value. Therefore, it is clear from  FIGS. 10A and 10B  that the durability of the EL element when the same amount of current is flown is improved. Although copper phthalocyanine is usually known as a hole injection layer material that improves injection of holes from the anode, it is used here as a material that can improve the durability of the EL element. The results are obtained by measuring a change with time of the luminance of the EL element and a change with time of the amount of current flowing through the EL element when the EL element is continuously lit using a low voltage of 6.5 V. Instead of copper phthalocyanine shown in this embodiment, a polythiophene-based material, for example, PEDOT (poly(3,4-ethylene dioxythiophene)), may be used. 
     Then an EL element shown in  FIG. 9C  is fabricated. This EL element has, instead of the α-NPD hole transporting layer (40 nm) of  FIG. 9B , an MTDATA layer with a thickness of 20 nm and an α-NPD layer with a thickness of 10 nm which are formed by evaporation. In short, one more layer is formed between the copper phthalocyanine layer and the hole transporting layer, thereby reducing the energy difference in HOMO level between the two layers. The element in  FIG. 9C  is referred to as EL element a′ in this specification. 
     Electrical characteristics of the EL element of  FIG. 9C  is shown in  FIGS. 11A to 11C .  FIG. 11A  shows results of measuring the luminance of emitted light in relation to the current density. The measurement is made on the EL element a shown in  FIG. 9A  and the EL element a′ obtained by adding, to the EL element a, a hole injection layer formed of copper phthalocyanine and a hole transporting layer formed of MTDATA. From  FIG. 11A , it can be seen that the addition of the copper phthalocyanine layer and the MTDATA layer does not influence the luminance of light emitted from the EL element. 
       FIG. 11B  shows results of measuring the luminance of emitted light when a voltage is applied to the EL elements. An improvement is observed in luminance which is brought by the addition of the copper phthalocyanine layer and the MTDATA layer. The fact that a higher luminance is obtained from application of the same voltage means a lower voltage is needed to obtain the same level of luminance. 
       FIG. 11C  shows results of measuring the amount of current when a voltage is applied to the EL elements. When the same voltage is applied, the amount of current flowing is larger in the EL element a′ than in the EL element a. 
     The results above state that the voltage required to drive an EL element is reduced by adding to the EL element a the copper phthalocyanine layer and the MTDATA layer (EL element a′). 
     The EL element a′ has been measured also for its response speed. 
     In the measurement, DC (direct current) is applied by an arbitrary power supply. A period during which the voltage is applied is ‘ON’ (selected period) whereas a period during which 0 V is applied is ‘OFF’ (not-selected period), and ON and OFF take turns. Each period lasts 250 μs. 
     To be specific, estimation is made by using an oscilloscope to read outputs of a photomultiplier set in a microscope. In this measurement, a switching from OFF to ON is defined as rise and a switching from ON to OFF as drop. The rise response time is a time required for the emitted light to reach 90% luminance of full luminance in an optical response that follows switching of the power supply voltage from OFF to ON. On the other hand, the drop response time is a time required for the emitted light to decrease in luminance by 10% of the previous full luminance in an optical response that follows switching of the power supply voltage from ON to OFF. 
     The measurement is graphically shown in  FIG. 25 . In  FIG. 25 , an arrow a indicates the output (voltage) of the power supply and an arrow b indicates the optical response to the output. The photomultiplier used is of minus output type, and a negative electric potential is therefore outputted when a switching is made from OFF (0 V) to ON (6 V in the example shown here). 
     An arrow c in  FIG. 25  indicates the point at which the luminance reaches 90%. The rise response time at this point is 28 μs. In this embodiment, when the output of the power supply is 6 V, although there are slight fluctuations between the EL elements, the rise response time and the drop response time are both 1 to 100 μs, preferably 1 to 50 μs. Further measurement is made by changing the voltage during ON so that estimation is made for every voltage between 6 V and 10 V. Results thereof (the rise response time and the drop response time) are shown in Table 1. 
     [Table 1] 
     Table 1 shows that the response speed in this voltage range is very high and that the element therefore has no problem also when driven by normal digital driving. 
     Embodiment 3 
       FIG. 12  shows a sectional structure of a pixel portion in an active matrix light emitting device of this embodiment. In  FIG. 12 , reference symbol  10  denotes an insulator,  11 , the current controlling transistor (TFT)  604  of  FIG. 5B ,  12 , a pixel electrode (anode),  13 , a bank, and  14 , a known hole injection layer. Reference symbols  15 ,  16 , and  17  represent a light emitting layer that emits red light, a light emitting layer that emits green light, and a light emitting layer that emits blue light, respectively. Denoted by  18  is a known electron transporting layer, and  19 , a cathode. 
     In this embodiment, triplet compounds are used for the red light emitting layer  15  and the blue light emitting layer  17  whereas a singlet compound is used for the green light emitting layer  16 . In other words, an EL element that uses a singlet compound is an EL element that emits green light while EL elements that use triplet compounds are an EL element that emits red light and an EL element that emits blue light. 
     When a low molecular weight organic compound is used for a light emitting layer, a red light emitting layer and a blue light emitting layer have a lifetime shorter than that of a green light emitting layer under the present circumstances. This is because the red light emitting layer and the blue light emitting layer are inferior in light emission efficiency to the green light emitting layer and hence require higher operation voltage in order to emit light of the same luminance as the green light, to thereby accelerate their degradation that much. 
     However, the red light emitting layer  15  and the blue light emitting layer  17  in this embodiment use triplet compounds that are high in light emission efficiency and hence it is possible to obtain the same operation voltage as the green light emitting layer  16  in emitting light of the same level of luminance as the layer  16 . Accordingly, the red light emitting layer  15  and the blue light emitting layer  17  degrade not so much faster than the green light emitting layer  16 , and an image can be displayed in color while avoiding color displacement and like other problems. The lowered operation voltage is also preferable in terms of the margin for the withstand voltage of the transistor because the margin can be set low. 
     Although the case shown in this embodiment is of using triplet compounds for the red light emitting layer  15  and the blue light emitting layer  17 , the green light emitting layer  16  may also be formed of a triplet compound. 
     Next, the circuit structure of the pixel portion according to this embodiment is shown in  FIGS. 13A and 13B . Shown here are a pixel (pixel (RED))  20   a  having an EL element that emits red light, a pixel (pixel (GREEN))  20   b  having an EL element that emits green light, and a pixel (pixel (BLUE))  20   c  having an EL element that emits blue light. The three pixels have the same circuit structure. 
     In  FIG. 13A , reference symbol  21  denotes a gate wiring line,  22   a  to  22   c , source wiring lines (data wiring lines), and  23   a  to  23   c , current supplying lines. The current supplying lines  23  are wiring lines that determine the operation voltage of the EL elements, and apply the same voltage to the red light emitting pixel  20   a , the green light emitting pixel  20   b , and the blue light emitting pixel  20   c . Accordingly, the wiring lines may be designed to have the same width (thickness). 
     Denoted by  24   a  to  24   c  are switching transistors, which are n-channel transistors in this embodiment. Although shown here as an example is a structure in which two channel formation regions are placed between a source region and a drain region, the number of channel formation regions may be more than two or only one. 
     Denoted by  25   a  to  25   c  are current controlling transistors. A gate of each of the current controlling transistors is connected to one of the switching transistors  24   a  to  24   c , a source thereof is connected to one of the current supplying lines  23   a  to  23   c , and a drain thereof is connected to one of EL elements  26   a  to  26   c .  27   a  to  27   c  denote condensers for holding the voltage applied to gates of the current supplying lines  25   a  to  25   c . However, the condensers  27   a  to  27   c  may be omitted. 
     The case shown in  FIG. 13A  is of using n-channel transistors for the switching transistors  24   a  to  24   c  and p-channel transistors for the current controlling transistors  25   a  to  25   c . As shown in  FIG. 13B , it is also possible to use p-channel transistors for switching transistors  28   a  to  28   c  and n-channel transistors for current controlling transistors  29   a  to  29   c  in each of a pixel (RED)  30   a , a pixel (GREEN)  30   b , and a pixel (BLUE)  30   c.    
       FIGS. 13A and 13B  show a case in which two transistors are provided in one pixel. However, the number of transistors may be more than two (typically, three to six). Any combination of n-channel transistors and p-channel transistors may be employed also when more than two transistors are provided in each pixel. 
     In this embodiment, the EL element  26   a  is a red light emitting EL element and the EL element  26   c  is a blue light emitting EL element, and both of them use triplet compounds for their light emitting layers. The EL element  26   b  is a green light emitting EL element and a singlet compound is used for its light emitting layer. 
     By choosing between a triplet compound and a singlet compound in this way, the El elements  26   a  to  26   c  can have the same operation voltage (10 V or less, preferably 3 to 10V). Thus the power supply required in the light emitting device can uniformly be set to, for example, 3 V or 5 V, to make the circuit design simpler. 
     The structure of this embodiment may be combined with any of the structures of Embodiments 1 and 2. 
     Embodiment 4 
     This embodiment describes a case in which n-channel transistors are used for all of transistors that constitute a pixel portion and a driving circuit. The n-channel transistors are fabricated in accordance with Embodiment 1, and explanations thereof are omitted. 
     The sectional structure of a light emitting device according to this embodiment is shown in  FIG. 14 . The basic structure thereof is the same as the sectional structure of  FIG. 5B  which is described in Embodiment 1. Therefore only differences are picked up and explained here. 
     In this embodiment, an n-channel transistor  1201  is provided instead of the p-channel transistor  602  of  FIG. 5B  and a current controlling transistor  1202  that is an n-channel transistor is provided in place of the current controlling transistor  604 . 
     A wiring line  266  connected to a drain of the current controlling transistor  1202  functions as a cathode of an EL element. Formed on the wiring line are an EL layer  1203 , an anode  1204  formed of an oxide conductive film, and a passivation film  1205 . The wiring line  266  is desirably formed from a metal film containing an element belonging to Group 1 or 2 in the periodic table. If not, at least a surface of the wiring line  266  that is in contact with the EL layer  1203  is formed of a metal film containing an element belonging to Group 1 or 2 in the periodic table. 
     The n-channel transistors used in this embodiment may be all enhancement type transistors or depression type transistors. Alternatively, enhancement type transistors and depression type transistors may be used in combination. 
     Now, the circuit structure of pixels is shown in  FIG. 15 . For the parts denoted by the same reference symbols as those in  FIGS. 13A and 13B , refer to explanations of  FIGS. 13A and 13B . 
     As shown in  FIG. 15 , the switching transistors  24   a  to  24   c  and the current controlling transistors  36   a  to  36   c  provided in a pixel (RED)  35   a , a pixel (GREEN)  35   b , and a pixel (BLUE)  35   c , respectively, are all n-channel transistors. 
     According to the structure of this embodiment, the photolithography step for forming the p-channel transistors and the photolithography step for forming the pixel electrodes (anodes) in the process of manufacturing a light emitting device of Embodiment 1 corresponding to the photolithography step for forming. cathodes in this embodiment are eliminated. Therefore the manufacture process can be simplified even more. 
     The structure of this embodiment may be combined with any of the structures of Embodiments 1 through 3. 
     Embodiment 5 
     This embodiment describes a case in which p-channel transistors are used for all of transistors that constitute a pixel portion and a driving circuit. The sectional structure of a light emitting device according to this embodiment is shown in  FIG. 16 . For the parts denoted by the same reference symbols as those in  FIG. 5B , refer to explanations of Embodiment 1. 
     In this embodiment, the driving circuit is composed of a PMOS circuit that has a p-channel transistor  1401  and a p-channel transistor  1402  whereas the pixel portion has a switching transistor  1403  that is a p-channel transistor and a current controlling transistor  1404  that is a p-channel transistor. An active layer of the p-channel transistor  1401  includes a. source region  41 , a drain region  42 , LDD regions  43   a  and  43   b , and a channel formation region  44 . The p-channel transistor  1402 , the switching transistor  1403 , and the current controlling transistor  1404  have the same active layer structure as the p-channel transistor  1401 . 
     Now, a process of manufacturing a p-channel transistor in accordance with this embodiment will be described with reference to  FIGS. 17A to 17C . First, the manufacture process of Embodiment 1 are finished up through the step of  FIG. 3B . 
     Next, electrodes  212  to  216  are formed from a second conductive film using resists  211   a  to  211   e . The resists  211   a  to  211   e  and the electrodes  212  to  216  formed of the second conductive film are then used as masks to dope a semiconductor film with an element belonging to Group 13 in the periodic table (boron, in this embodiment). As a result, regions  301  to  309  containing boron in a concentration of 1×10 20  to 1×10 21  atoms/cm 3  (hereinafter referred to as p type impurity regions (a)) are formed ( FIG. 17A ). 
     The electrodes  212  to  216  formed of the second conductive film are then etched using the resists  211   a  to  211   e  under the same etching conditions as those in  FIG. 3E  to form second gate electrodes  310  to  314  ( FIG. 17B ). 
     Next, the resists  211   a  to  211   e  and the second gate electrodes  310  to  314  are used as masks to etch a first conductive film  209  under the same etching conditions as those in  FIG. 3D  to form first gate electrodes  315  to  319 . 
     The resists  211   a  to  211   e  and the second gate electrodes  310  to  314  are then used as masks to dope the semiconductor film with an element belonging to Group 13 in the periodic table (boron, in this embodiment). As a result, regions  320  to  329  containing boron in a concentration of 1×10 16  to 1×10 19  atoms/cm 3 , typically 1×10 17  to 1×10 18  atoms/cm 3  (hereinafter referred to as p type impurity regions (b)) are formed ( FIG. 17C ). 
     The subsequent steps are the same as the step of  FIG. 4C  and the following steps thereof in Embodiment 1. A light emitting device structured as shown in  FIG. 16  is manufactured through the above process. 
     The p-channel transistors used in this embodiment may be all enhancement type transistors or depression type transistors. Alternatively, enhancement type transistors and depression type transistors may be used in combination. 
     The circuit structure of pixels is shown in  FIG. 18 . For the parts denoted by the same reference symbols as those in  FIGS. 13A and 13B , refer to explanations of  FIGS. 13A and 13B . 
     As shown in  FIG. 18 , switching transistors  51   a  to  51   c  and current controlling transistors  52   a  to  52   c  provided in a pixel (RED)  50   a , a pixel (GREEN)  50   b , and a pixel (BLUE)  50   c , respectively, are all p-channel transistors. 
     According to the structure of this embodiment, one photolithography step in the process of manufacturing a light emitting device of Embodiment 1 is omitted. Therefore the manufacture process is more simplified than Embodiment 1. 
     The structure of this embodiment may be combined with any of the structures of Embodiments 1 through 4. 
     Embodiment 6 
     An active matrix light emitting device of the present invention can also employ an MOS (metal oxide semiconductor) transistor for a semiconductor element. In this case, a MOS transistor formed on a semiconductor substrate (typically a silicon wafer) by a known method is used. 
     The structure of this embodiment, except for the semiconductor element, may be combined with any of the structures of Embodiments 1 through 5. 
     Embodiment 7 
     Embodiment 1 shows in  FIGS. 6A and 6B  the light emitting device with built-in driving circuit as an example of the light emitting device in which a pixel portion and a driving circuit are integrally formed on the same insulator. However, it is also possible to use an external IC (integrated circuit) for the driving circuit. In this case, the structure thereof is as shown in  FIG. 19A . 
     In a module shown in  FIG. 19A , an FPC  63  is attached to an active matrix substrate  60  (including a pixel portion  61  and wiring lines  62   a  and  62   b ), and a printed wiring board  64  is attached to the substrate through the FPC  63 . A functional block diagram of the printed wiring board  64  is shown in  FIG. 19B . 
     As shown in  FIG. 19B , the printed wiring board  64  is provided with an IC functioning as at least I/O ports (also called input or output units)  65  and  68 , a source side driving circuit  66 , and a gate side driving circuit  67 . 
     A module in which an FPC is attached to an active matrix substrate with a pixel portion formed thereon and a printed wiring board functioning as a driving circuit is attached to the substrate through the FPC, as in the module above, is specially called a light emitting module with external driving circuit in this specification. 
     In a module shown in  FIG. 20A , an FPC  74  is attached to a light emitting device with built-in driving circuit  70  (including a pixel portion  71 , a source side driving circuit  72 , a gate side driving circuit  73 , and wiring lines  72   a  and  73   a ), and a printed wiring board  75  is attached to the light emitting device with built-in driving circuit  70  through the FPC  74 . A functional block diagram of the printed wiring board  75  is shown in  FIG. 20B . 
     As shown in  FIG. 20B , the printed wiring board  75  is provided with an IC functioning as at least I/O ports  76  and  79  and a controlling unit  77 . Although a memory unit  78  is provided here, it is not always necessary. The controlling unit  77  has a function of controlling the driving circuits and correcting video data. 
     A module in which a printed wiring board having a function as a controller is attached to a light emitting device with built-in driving circuit with the driving circuit and a pixel portion formed on a substrate, as in the module above, is specially called a light emitting module with external controller in this specification. 
     Embodiment 8 
     The light-emitting device (including the module at the state of which is shown in Embodiment 9) formed by implementing this invention may be built in various electrical appliances and thereof pixel portion is used as a image display portion. As electrical appliances of this invention, there are a video camera, a digital camera, a goggle type display (head mounted display), a navigation system, an audio apparatus, a note type personal computer, a game apparatus, a portable information terminal (such as a mobile computer, a portable telephone, a portable game apparatus or an electronic book), and an image playback device with a recording medium. Specific examples of the electronic equipment are shown in  FIGS. 21 and 22 . 
       FIG. 21A  shows a display and includes a casing  2001 , a supporting base  2002  and a display portion  2003 . The light-emitting device of this invention may be used for the display portion  2003 . When using the light-emitting device having the EL element in the display portion  2003 , since the EL element is a self-light emitting type backlight is not necessary and the display portion may be made thin. 
       FIG. 21B  shows a video camera, which contains a main body  2101 , a display portion  2102 , a sound input portion  2103 , operation switches  2104 , a battery  2105 , and an image receiving portion  2106 . The light-emitting device of this invention can be applied to the display portion  2102 . 
       FIG. 21C  shows a digital camera, which contains a main body  2201 , a display portion  2202 , an eye contact portion  2203 , and operation switches  2204 . The light emitting-device and the liquid crystal display device of this invention can be applied to the display portion  2202 . 
       FIG. 21D  shows an image playback device equipped with a recording medium (specifically, a DVD playback device), which contains a main body  2301 , a recording medium (such as a CD, LD or DVD)  2302 , operation switches  2303 , a display portion (a)  2304 , a display portion (b)  2305 . The display portion (a) is mainly used for displaying image information. The display portion (b)  2305  is mainly used for displaying character information. The light-emitting device of this invention can be applied to the display portion (a) and the display portion (b). Note that, the image playback device equipped with the recording medium includes devices such as CD playback device, and game machines. 
       FIG. 21E  shows a portable (mobile) computer, which contains a main body  2401 , a display portion  2402 , an image receiving portion  2403 , operation switches  2404  and a memory slot  2405 . The light-emitting device of this invention can be applied to the display portion  2402 . This portable computer may record information to a recording medium that has accumulated flash memory or involatile memory, and playback such information. 
       FIG. 21F  shows a personal computer, which contains a main body  2501 , a casing  2502 , a display portion  2503 , and a keyboard  2504 . The light-emitting device of this invention can be applied to the display portion  2503 . 
     The above electronic appliances more often display information sent through electron communication circuits such as Internet or the CATV (cable television), and especially image information display is increasing. When using the light-emitting device having the EL element in the display portion, since the response speed of the EL element is extremely fast, it becomes possible to display pictures without delay. 
     Further, since the light emitting portion of the light-emitting device consumes power, it is preferable to display information so that the light emitting portion is as small as possible. Therefore, when using the light-emitting device in the display portion where character information is mainly shown in the portable information terminal, especially in a portable phone or an audio apparatus, it is preferable to drive so that the character information is formed of a light emitting portion with the non-light emitting portion as a background. 
     Here,  FIG. 22A  shows a portable telephone, which contains a main body  2601 , a sound output portion  2602 , a sound input portion  2603 , a display portion  2604 , an operation switch  2605  and an antenna  2606 . The light-emitting device of this present invention can be applied to the display portion  2604 . Note that, when using the light-emitting device to the display portion  2604 , the consumption power of the portable telephone may be suppressed by displaying white letters in the background of the black color. 
       FIG. 22B  shows also a portable telephone, but it is a folding twice type different from that of  FIG. 22A , and contains a main body  2611 , a sound output portion  2612 , a sound input portion  2613 , a display portion (a)  2614 , a display portion (b)  2615  and an antenna  2616 . The operation switch is not adhered to this type portable telephone, but its function is provided to the portable telephone by displaying a character information shown in  FIGS. 22C ,  22 D and  22 E by either of the display portion (a) or (b). Further, another display portion displays mainly the image information. The light-emitting device of the present invention can be used as the display portion (a)  2614  or a display portion (b)  2615 . 
     In the case of the portable telephone shown in  FIG. 22B , the light-emitting device used in the display portion  2604  is incorporated with a sensor by a CMOS circuit (a CMOS sensor), and may be used as an authentication system terminal for authenticating the user by reading the fingerprints or the hand of the user. Further, light emission may be performed by taking into consideration the brightness (illumination) of outside and making information display at a contrast that is already set. 
     Further, the low power consumption may be attained by decreasing the brightness when using the operating switch  2605  and increasing the brightness when the use of the operation switch is finished. Further, the brightness of the display portion  2604  is increased when a call is received, and low power consumption is attained by decreasing the brightness during a telephone conversation. Further, when using the telephone continuously, by making it have a function so that display is turned off by time control unless it is reset, low power consumption is realized. It should be noted that this control may be operated by hand. 
     Further,  FIG. 22F  shows an audio reproduction devices, concretely a car audio which contains a main body  2621 , a display portion  2622 , and operation switches  2623  and  2624 . The light-emitting device of this invention can be applied to the display portion  2622 . Further, in this embodiment, a car mounted audio (car audio) is shown, but it may be used in a portable type or domestic type audio (audio component). Note that, when using a light-emitting device in the display portion  2622 , by displaying white characters in a black background, power consumption may be suppressed. It is especially effective for the portable type audio reproduction device. 
     In the case of the portable type electronic apparatuses shown in this embodiment, the sensor portion is provided to perceive the external light and the function to lower the brightness of display when it is used in the dark area as a method to lower the power consumption. 
     As in the above, the applicable range of this invention is extremely wide, and may be used for various electrical equipment. Further, the electrical equipment of this embodiment may use the electronic device containing any of the structures of Embodiments 1 to 8. 
     Embodiment 9 
     Embodiment 1 describes a case where the transistors are top gate transistors. However, the transistor structure of the present invention is not limited thereto and bottom gate transistors (typically reverse stagger transistors) may also be used in carrying out the present invention as shown in  FIGS. 23A and 23B . The reverse stagger transistors may be formed by any method. 
       FIG. 23A  is a top view of an EL module formed in manufacture of a light emitting device that uses bottom gate transistors. A source side driving circuit  3001 , a gate side driving circuit  3002 , and a pixel portion  3003  are formed therein.  FIG. 23B  shows in section a region a  3004  of the pixel portion  3003 . The sectional view is obtained by cutting the light emitting device along the line x-x′ in  FIG. 23A . 
       FIG. 23B  illustrates only a current controlling transistor out of transistors that constitute a pixel transistor. Reference symbol  3011  denotes a substrate and  3012  denotes an insulating film to serve as a base (hereinafter referred to as a base film). A transparent substrate is used for the substrate  3011 , typically, a glass substrate, a quartz substrate, a glass ceramic substrate, or a crystallized glass substrate. However, the one that can withstand the highest process temperature during the manufacture process has to be chosen. 
     The base film  3012  is effective especially when a substrate containing a movable ion or a conductive substrate is used. If a quartz substrate is used, the base film may be omitted. An insulating film containing silicon is used for the base film  3012 . The insulating film containing silicon herein refers to an insulating film containing oxygen or nitrogen in a given ratio to the content of silicon, specifically, a silicon oxide film, a silicon nitride film, or a silicon oxynitride film (SiOxNy: x and y are arbitrary integers). 
     Reference symbol  3013  denotes a current controlling transistor that is a p-channel transistor. When an EL emits light toward the top face of the substrate (the face on which transistors and an EL layer are formed) as shown in this embodiment, it is desirable to use n-channel transistors for a switching transistor and a current controlling transistor as well. However, the present invention is not limited to thereto. The switching transistor may be an n-channel transistor or a p-channel transistor and the same applies to the current controlling transistor. 
     The current controlling transistor  3013  is composed of an active layer, a gate insulating film  3017 , a gate electrode  3018 , a first interlayer insulating film  3019 , a source wiring line  3020 , and a drain wiring line  3021 . The active layer includes a source region  3014 , a drain region  3015 , and a channel formation region  3016 . The current controlling transistor  3013  in this embodiment is an n-channel transistor. 
     The switching transistor has a drain region connected to the gate electrode  3018  of the current controlling transistor  3013 . The gate electrode  3018  of the current controlling transistor  3013  is electrically connected to the drain region (not shown) of the switching transistor through a drain wiring line (not shown), to be exact. The gate electrode  3018  has a single gate structure but may take a multi-gate structure. The source wiring line  3020  of the current controlling transistor  3013  is connected to a current supplying line (not shown). 
     The current controlling transistor  3013  is an element for controlling the amount of current supplied to the EL element, and a relatively large amount of current flows through this transistor. Therefore, it is preferable to design the current controlling transistor to have a channel width (W) wider than the channel width of the switching transistor. It is also preferable to design the current controlling transistor to have a rather long channel length (L) in order to avoid excessive current flow in the current controlling transistor  3013 . Desirably, the length is set such that the current is 0.5 to 2 μA (preferably 1 to 1.5 μA) per pixel. 
     If the active layer (channel formation region, in particular) of the current controlling transistor  3013  is formed thick (desirably 50 to 100 nm, more desirably 60 to 80 nm), degradation of the transistor can be slowed. 
     After the current controlling transistor  3013  is formed, the first interlayer insulating film  3019  and a second interlayer insulating film (not shown) are formed to fond a pixel electrode  3023  that is electrically connected to the current controlling transistor  3013 . In this embodiment, the pixel electrode  3023  formed of a conductive film functions as a cathode of the EL element. 
     Specifically, the pixel electrode is formed of an alloy film of aluminum and lithium. Any conductive film formed of an element belonging to Group 1 or 2 in the periodic table or a conductive film doped with the Group 1 (or 2) element can be used. 
     After the pixel electrode  3023  is formed, a third interlayer insulating film  3024  is formed. The third interlayer insulating film  3024  serves as a so-called bank. 
     An EL layer  3025  is formed next. Shown in  FIG. 23B  in section is a column of pixels that be formed the same EL layer. 
     The EL layer in this embodiment uses Alq 3  for an electron injection layer, BCP for an electron transporting layer, and CBP doped with Ir(ppy) 3  for a light emitting layer. A hole transporting layer thereof is formed of α-NPD. 
     Next, an anode  3026  is formed from a transparent conductive film on the EL layer. The transparent conductive film used in this embodiment is a conductive film formed from a compound of indium oxide and tin oxide, or a compound of indium oxide and zinc oxide. 
     A passivation film is further formed on the anode from an insulating material to thereby complete an EL module having a reverse stagger transistor structure. The Light emitting device manufactured in accordance with this embodiment emits light in the direction indicated by the arrow in  FIG. 23B  (toward the top face). 
     A reverse stagger transistror can be fabricated with a smaller number of manufacture steps than needed to fabricate a top gate transistor. Therefore it is very advantageous for cost down, which is one of the objects of the present invention. 
     The structure of this embodiment may be combined freely with any of the structures of Embodiments 1 through 8. 
     Embodiment 10 
     Described next in this embodiment is a case of introducing an SRAM to a pixel portion.  FIG. 24  shows an enlarged view of a pixel  3104 . In  FIG. 24 , reference symbol  3105  denotes a switching transistor. The switching transistor  3105  has a gate electrode connected to a gate signal line  3106  that is one of gate signal lines (G 1  to Gn) to which gate signals are inputted. The switching transistor  3105  has a source region and a drain region one of which is connected to a source signal line  3107  that is one of source signal lines (S 1  to Sn) to which source signals are inputted, and the other of which is connected to an input side of an SRAM  3108 . An output side of the SRAM  3108  is connected to a gate electrode of a current controlling transistor  3109 . 
     The current controlling transistor  3109  has a source region and a drain region one of which is connected to a current supplying line  3110  that is one of current supplying lines (V 1  to Vn), and the other of which is connected to an EL element  3111 . 
     The EL element  3111  is composed of an anode, a cathode, and an EL layer interposed between the anode and the cathode. When the anode is connected to the source region or the drain region of the current controlling transistor  3109 , in other words, when the anode is a pixel electrode, the cathode serves as an opposite electrode. On the other hand, when the cathode is connected to the source region or the drain region of the current controlling transistor  3109 , in other words, when the cathode is a pixel electrode, the anode serves as the opposite electrode. 
     The SRAM  3108  has two p-channel transistors and two n-channel transistors. Source regions of the p-channel transistors are connected to Vddh on the high voltage side whereas source regions of the n-channel transistors are connected to Vss on the low voltage side. One p-channel transistor and one n-channel transistor forms a pair, and one SRAM has two pairs of p-channel transistors and n-channel transistors. 
     A drain region of one p-channel transistor is connected to a drain region of the n-channel transistor of the pair. A gate electrode of one p-channel transistor is connected to a gate electrode of the n-channel transistor of the pair. Drain regions of the p-channel transistor and the n-channel transistor of one pair are kept at the same level of electric potential as gate electrodes of the p-channel transistor and the n-channel transistor of the other pair. 
     Drain regions of the p-channel transistor and the n-channel transistor of one pair receive input signals (Vin) and serve as the input side. Drain regions of the p-channel transistor and the n-channel transistor of the other pair send out output signals (Vout) and serve as the output side. 
     The SRAM is designed to hold Vin and output Vout that is a signal obtained by inverting Vin. When Vin is Hi, Vout is a Lo signal corresponding to Vss. When Vin is Lo, Vout is a Hi signal corresponding to Vddh. 
     In the case where one SRAM is provided in the pixel  3104  as shown in this embodiment, a still image can be displayed while stopping the operation of most of the external circuit because the memory data in the pixel is kept. This makes it possible to reduce power consumption. One pixel may have a plurality of SRAMs. A plurality of data can be held when plural SRAMs are provided in one pixel, making gray scale display by time gray scale possible. 
     The structure of this embodiment may be combined freely with any of the structures of Embodiments 1 through 9. 
     By carrying out the present invention, the luminance of Light emitted from EL elements formed on the same substrate can readily be equalized and a low power consumption light emitting device that can emit light of high luminance with a low voltage can be obtained. Also, a low power consumption electric machine can be provided when this light emitting device is used in a display portion thereof. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 voltage (V) 
                 rise response time(μs) 
                 drop response time(μs) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 6 
                 28 
                 3 
               
               
                 7 
                 6 
                 3.24 
               
               
                 8 
                 3.5 
                 4.2 
               
               
                 9 
                 2.36 
                 4.04 
               
               
                 10 
                 1.64 
                 4.52