Patent Publication Number: US-2023146966-A1

Title: Electroluminescence element and method of manufacturing electroluminescence element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Patent Application No. PCT/JP2021/023638, filed on Jun. 22, 2021, which claims the benefit of priority to Japanese Patent Application No. 2020-107890, filed on Jun. 23, 2020, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     An embodiment of the present invention relates to an element structure and a material of an electroluminescence element (hereinafter, also referred to as an “EL element”) using an electroluminescence material. An embodiment of the invention disclosed herein relates to an inverted structure electroluminescence element using a coating-type inorganic transparent oxide semiconductor material in a layer that transports electrons to a light emitting layer. 
     BACKGROUND 
     The EL element has a pair of electrodes called an anode and a cathode and has a structure in which a light emitting layer is arranged between the pair of electrodes. In the case where a potential is applied to the electrode of the EL element, an electron is injected from the cathode into the light emitting layer, and a hole is injected from the anode into the light emitting layer. The electron and hole recombine on a host-molecule of the light emitting layer. The energy emitted thereby excites a light emitting molecule in the light emitting layer, which then returns to the ground state to emit light. 
     In order for electrons to enter the light emitting layer, it is necessary to overcome an energy barrier created by the difference between the electron affinity of the light emitting layer and the work function of the cathode. In addition, in order for holes to enter the light emitting layer, it is necessary to overcome a barrier created by the difference between the ionization energy of the light emitting layer and the work function of the anode. In order to efficiently emit light, it is necessary to reduce the energy barrier. Therefore, an electron transport layer may be inserted between the cathode and the light emitting layer, and a hole transport layer may be inserted between the anode and the light emitting layer. For example, the electron transport layer may have an electron affinity between the work function of the cathode and the electron affinity of the light emitting layer. 
     On the other hand, although the basic structure of the EL element is a 2-terminal type element, a 3-terminal type EL element to which a third electrode is added is further disclosed. For example, an organic EL element including an anode, a layer formed of an organic electroluminescence material called a light emitting material layer, a cathode, and an auxiliary electrode arranged with respect to the cathode and the light emitting material layer via an insulating layer is disclosed (see Japanese Laid-Open Patent Publication No. 2002-343578). In addition, a light emitting transistor having a structure in which a hole injection layer, a carrier distribution layer, a hole transport layer, and a light emitting layer are stacked from the anode side to the cathode side and an auxiliary electrode is arranged with respect to the anode via an insulating film is disclosed (see International Patent Publication No. WO2007/043697). 
     In addition, an organic light emitting transistor element is disclosed that is configured to include an auxiliary electrode, an insulating film arranged on the auxiliary electrode, a first electrode arranged at a predetermined size on the insulating film, a charge injection suppressing layer on the first electrode, a charge injection layer arranged on the insulating film without the first electrode, a light emitting layer arranged on the charge injection suppressing layer and the charge injection layer or on the charge injection layer, and a second electrode arranged on the light emitting layer (see Japanese Laid-Open Patent Publication No. 2007-149922 and Japanese Laid-Open Patent Publication No. 2007-157871). 
     Since the electron mobility in the light emitting material layer of the organic EL element disclosed in Japanese Laid-Open Patent Publication No. 2002-343578 is low, the amount of electrons injected from the cathode is substantially determined by the potential difference between the anode and the cathode and the bias voltage applied from the auxiliary electrode hardly affects the carrier injection. Since the light emitting material layer has low electron mobility and high resistance, the injection of electrons into the light emitting material layer is concentrated exclusively in the vicinity of the cathode, and the bias voltage applied to the auxiliary electrode does not affect the amount of electron injection. 
     In the light emitting transistor described in International Patent Publication No. WO2007/043697, since the auxiliary electrode controls the state of light emission/non-light emission, the amounts of carriers (electrons, holes) of different polarities injected into the light emitting layer cannot be individually controlled independently even by using an external circuit. Furthermore, the light emitting transistor described in Japanese Laid-Open Patent Publication No. 2002-343578 cannot control the position of the area where the electron and the hole recombine in the light emitting layer, that is, the light emitting area. 
     In addition, in the organic light emitting transistor elements described in Japanese Laid-Open Patent Publication No. 2007-149922 and Japanese Laid-Open Patent Publication No. 2007-157871, since the carrier (electron) mobility of the electron transport layer formed of an organic material is low, there is a problem that a display panel having a large screen and high definition cannot be realized. 
     There is a problem that the thickness of the electron transport layer cannot be increased because there is no material that has sufficiently high mobility and is transparent. In the case of forming an electron transport layer by a sputtering method or a CVD method, an increase in the size of the substrate is costly, the process is complicated, and the productivity is problematic. In addition, in these methods, there is a problem that processing at high temperature is required. 
     SUMMARY 
     A method of manufacturing an electroluminescence element according to an embodiment of the present invention includes forming a first electrode on a substrate, forming a first electron transport layer in contact with the first electrode, forming a first insulating layer having an opening in a region overlapping with the first electrode, forming a second electron transport layer includes metal oxide semiconductor by applying a composition to the opening and removing a solvent after application, forming a light emitting layer overlapping with the second electron transport layer, the light emitting layer containing an electroluminescent material, forming a second electrode in a region overlapping with the light emitting layer. 
     An electroluminescence element according to an embodiment of the present invention includes a first electrode, a second electrode having a region facing the first electrode, a first insulating layer between the first electrode and the second electrode, an electron transport layer electrically connected to the first electrode, and a light emitting layer containing an electroluminescent material between the electron transport layer and the second electrode, wherein the first insulating layer has an opening, the opening has an overlapping region where the second electrode, the light emitting layer, the electron transport layer, and the first electrode overlap, the electron transport layer has a first electron transport layer in contact with the first electrode layer and a second electron transport layer arranged in the opening and in contact with the first electron transport layer, and a thickness of the second electron transport layer is larger at the edge of the opening than at the center of the opening, the second electron transport layer is in contact with a side surface of the opening and rises along the side surface, and the second electron transport layer is positioned within the opening without exceeding the opening of the first insulating layer. 
     A display device according to an embodiment of the present invention includes a pixel comprising the electroluminescence element, and a driving transistor connected to the electroluminescence element on a substrate, wherein the driving transistor comprises: an oxide semiconductor layer, a first insulating layer located under the oxide semiconductor layer, a first gate electrode having a region overlapping with the oxide semiconductor layer, the first gate electrode arranged on the substrate side of the oxide semiconductor layer with the first insulating layer interposed therebetween, a second gate electrode having a region overlapping with the oxide semiconductor layer and the first gate electrode, the second gate electrode arranged opposite to the substrate side of the oxide semiconductor layer, and the first electrode is electrically connected to the oxide semiconductor layer 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  2    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  3    is a diagram showing an operation of an EL element according to an embodiment of the present invention. 
         FIG.  4    is a diagram showing an operation of an EL element according to an embodiment of the present invention. 
         FIG.  5    is a diagram showing an operation of an EL element according to an embodiment of the present invention. 
         FIG.  6    is a diagram showing an example of an operation property of an EL element according to an embodiment of the present invention. 
         FIG.  7 A  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  7 B  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  7 C  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  8 A  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  8 B  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  8 C  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  9 A  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  9 B  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  9 C  is a diagram showing a method of manufacturing an EL element according to an embodiment of the present invention. 
         FIG.  10    is a diagram showing an example of an equivalent circuit of a pixel of a display device including an EL element according to an embodiment of the present invention. 
         FIG.  11    is a plan view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  12 A  is a cross-sectional view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  12 B  is a cross-sectional view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  13    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  14    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  15 A  is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  15 B  is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  16    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  17 A  is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  17 B  is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  18    is a diagram showing an example of an equivalent circuit of a pixel of a display device including an EL element according to an embodiment of the present invention. 
         FIG.  19    is a plan view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  20 A  is a cross-sectional view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  20 B  is a cross-sectional view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  21 A  is an energy band diagram of an oxide semiconductor layer according to an embodiment of the present invention. 
         FIG.  21 B  is an energy band diagram of an oxide semiconductor layer according to an embodiment of the present invention. 
         FIG.  22    is a diagram showing a relationship between an electrical characteristic of a transistor and an oxygen partial pressure at the time of deposition according to an embodiment of the present invention. 
         FIG.  23    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  24    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  25 A  is a cross-sectional view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  25 B  is a cross-sectional view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  26    is a plan view showing a configuration of a pixel of a display device using an EL element according to an embodiment of the present invention. 
         FIG.  27    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  28    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  29    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  30    is a cross-sectional view showing a structure of an EL element according to an embodiment of the present invention. 
         FIG.  31    is a band diagram of an inverted structure EL element according to an embodiment of the present invention. 
         FIG.  32    is a band diagram of an inverted structure EL element according to an embodiment of the present invention. 
         FIG.  33 A  is a diagram showing a voltage waveform applied to an electrode that control the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  33 B  is a diagram showing a voltage waveform applied to an electrode that control the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  33 C  is a diagram showing a voltage waveform applied to an electrode that control the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  33 D  is a diagram showing a voltage waveform applied to an electrode that control the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  34 A  is a diagram showing a relationship between the emission intensity and a voltage applied to an electrode for controlling the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  34 B  is a diagram showing a relationship between the emission intensity and a voltage applied to an electrode for controlling the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  34 C  is a diagram showing a relationship between the emission intensity and a voltage applied to an electrode for controlling the amount of carrier injection of an EL element according to an embodiment of the present invention. 
         FIG.  35    is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  36    is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  37    is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  38 A  is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  38 B  is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  39    is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  40    is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
         FIG.  41    is a cross-sectional view showing a configuration of an EL element according to a modification of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings and the like. However, the present invention includes many different aspects, and should not be construed as being limited to the embodiments exemplified below. In order to make the description clearer, although the drawings attached to the present specification may be schematically represented with respect to the width, thickness, shape, and the like of each part as compared with the actual embodiment, they are merely examples and does not necessarily limit the contents of the present invention. In addition, in the present invention, when a specific element described in a certain drawing and a specific element described in another drawing have the same or corresponding relationship, the same symbols (or symbols denoted as symbols followed by a, b, and the like) are given, and repeated description may be omitted as appropriate. Further, the letters “first” and “second” with respect to each element are convenient signs used to distinguish each element, and do not have any further meaning unless otherwise specified. 
     In the present specification, in the case where a member or area is referred to as being “above (or below)” another member or area, this includes not only the case where it is directly above (or below) another member or area but also the case where it is above (or below) another member or area. That is, it includes the case of including another component between a certain member or area above (or below) another member or area. 
     First Embodiment 
     1. Structure of EL Element 
     A structure of an EL element is classified into a bottom-emission type in which light is emitted through a substrate and a top-emission type in which light is emitted to a side opposite to the substrate. In addition, the structure of an EL element is classified into a stacked structure in which an anode, a hole transport layer, a light emitting layer, an electron transport layer, and a cathode are stacked in this order from the substrate side, and an inverted stacked structure in the reverse order, based on the stacking order in the manufacturing process. The EL element according to the present embodiment is classified into the inverted stacked structure and can be applied to both the bottom-emission type and the top-emission type. 
     1-1. Bottom-Emission Type EL Element 
       FIG.  1    shows a cross-sectional structure of an EL element  200   a  according to an embodiment of the present invention. The EL element  200   a  shown in  FIG.  1    is the bottom-emission type and has the inverted stacked structure. That is, the EL element  200   a  has a structure in which a first electrode  102 , a first insulating layer  104 , an electron transport layer  106  (a first electron transport layer  106   a , a second electron transport layer  106   b ), a light emitting layer  112 , and a third electrode  118  are stacked from a substrate  100  side. 
       FIG.  1    further shows a configuration in which a hole transport layer  114  and a hole injection layer  116  are arranged between the light emitting layer  112  and the third electrode  118 . In the EL element  200   a , one of the hole injection layer  116  and the hole transport layer  114  may be omitted or may be replaced with a hole injection transport layer having both functions of hole injection and hole transport. Although not shown in  FIG.  1   , a hole-blocking layer may be arranged between an electron injection layer  110  and the light emitting layer  112 , or an electron blocking layer may be arranged between the light emitting layer  112  and the hole transport layer  114 . 
      In the EL element  200   a  shown in  FIG.  1   , the first electrode  102 , the first insulating layer  104 , the electron transport layer  106  (the first electron transport layer  106   a , the second electron transport layer  106   b ), the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  are arranged overlapping in longitudinal direction. On the other hand, a second electrode  108  is arranged on the outer side of an area (overlapping area) where these layers overlap, and is arranged so as to be electrically connected to the first electron transport layer  106   a . That is, the first electron transport layer  106   a  is arranged wider than the first electrode  102 . At least a part of an outer end portion of the first electron transport layer  106   a  is arranged outside the first electrode  102 . As a result, the first electron transport layer  106   a  includes an area that overlaps with the first electrode  102  via the first insulating layer  104 , and further includes an area that does not overlap with the first electrode  102  on the outer side of that area. The second electrode  108  is arranged to be in contact with at least a part of the first electron transport layer  106   a  on the outer side of the overlapping area. For example, the second electrode  108  may be arranged so as to be sandwiched between the first electron transport layer  106   a  and the first insulating layer  104 . The second electrode  108  is preferably arranged so as to surround the outer periphery of the first electron transport layer  106   a . Such an arrangement of the second electrode  108  makes it possible to uniform a length from a center area of the first electron transport layer  106   a  to the second electrode  108  over the entire circumference. However, the EL element  200   a  according to an embodiment of the present invention is not limited to such an arrangement, and the second electrode  108  may be arranged in a part of the area at the peripheral portion of the first electron transport layer  106   a . Further, a wiring  111  may be arranged in a part of the area in contact with the second electrode  108 . The wiring  111  may be arranged to be sandwiched between the second electrode  108  and the first electron transport layer  106   a . 
     The contact area can be increased by arranging the upper surface of the second electrode  108  in contact with the first electron transport layer  106   a  of the EL element  200   a . As a result, a series resistance component of the EL element  200   a  is reduced and the drive voltage can be lowered. In addition, the current density flowing into the second electrode  108  of the EL element  200   a  may be reduced. Further, since the second electrode  108  is formed first, the second electrode  108  can be contacted with in an area with few surface defects of the first electron transport layer  106   a  of the EL element  200   a . 
     In  FIG.  1   , the third electrode  118  has a function of injecting a hole into the hole injection layer  116  and is an electrode also referred to as an “anode”. The second electrode  108  has a function of injecting an electron into the electron transport layer  106  and is an electrode also referred to as a “cathode”. The first electrode  102  has a function of controlling the amount of carriers (electrons) injected into the light emitting layer  112  and is also referred to as an “electrode for controlling the amount of carrier injection”. 
     The electron transport layer  106  of the EL element  200   a  is shown with two distinct layers of the first electron transport layer  106   a  and the second electron transport layer  106   b . Although the electron transport layer  106  will be described later, the first electron transport layer  106   a  and the second electron transport layer  106   b  have a common function of transporting electrons injected from the second electrode  108  to the light emitting layer  112 . On the other hand, the first electron transport layer  106   a  in contact with the second electrode  108  and the second electron transport layer  106   b  arranged on a side closer to the light emitting layer  112  have different electron concentrations and electron mobilities. For example, the carrier concentration (electron concentration) of the second electron transport layer  106   b  is preferably relatively lower than the carrier concentration (electron concentration) of the first electron transport layer  106   a  in order to prevent the deactivation of exciton of the light emitting layer. Further, the thickness of the second electron transport layer  106   b  is preferably thicker than the thickness of the first electron transport layer  106   a . 
     Also, the first electron transport layer  106   a  and the second electron transport layer  106   b  may be regarded as a single layer because of a common function of transporting electrons injected from the second electrode  108  to the light emitting layer  112 . 
     The EL element  200   a  includes a second insulating layer  120  arranged between the first electron transport layer  106   a  and the electron injection layer  110 . The second insulating layer  120  has an opening  124  that covers the periphery and exposes the upper surface of the first electron transport layer  106   a . A second electron transport layer  106   b  is arranged in the opening  124  of the second insulating layer  120 . An area where the second electron transport layer  106   b  is arranged is smaller than an area where the first electron transport layer  106   a  is arranged. The first electron transport layer  106   a  and the second electron transport layer  106   b  are contacted with each other at the opening  124  of the second insulating layer  120 . The first electron transport layer  106   a , the second electron transport layer  106   b , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  are stacked in an area  124   a  where the opening  124  is arranged. An area where these layers are stacked becomes the light emitting area of the EL element  200   a . In other words, the opening  124  of the second insulating layer  120  defines the light emitting area of the EL element  200   a . 
     The second electron transport layer  106   b  is arranged in the opening  124  of the second insulating layer  120 . The thickness of the second electron transport layer  106   b  is smaller than the thickness of the second insulating layer  120 . That is, the upper surface of the second electron transport layer  106   b  (the surface opposite to the substrate  100 ) is lower (the substrate  100  side) than the upper surface (the surface opposite to the substrate  100 ) of the second insulating layer  120 . The second electron transport layer  106   b  has a concave surface shape in which the thickness at the center portion is smaller than that at the end portion of the opening  124 . In this case, the thickness of the second electron transport layer  106   b  indicates a distance from the bottom of the opening  124  in the stacking direction (the direction perpendicular to the upper surface of the substrate  100 ). The concave surface shape of the second electron transport layer  106   b  is arranged inside the opening  124  of the second insulating layer  120 . Although the concave surface shape of the upper surface of the second electron transport layer  106   b  is not particularly limited, the concave surface shape of the second insulating layer  120  is preferably a contiguous concave surface shape and is rounded in the vicinity of the side wall. The upper surface of the second electron transport layer  106   b  has less unevenness and can be formed flatter than in the other film deposition methods. In order to prevent deactivation of the exciton of the light emitting layer, the thickness of the second electron transport layer  106   b  needs to be 150 nm or more. Preferably, the film is formed to have a thickness of 200 nm or more. 
     Configuring the second electron transport layer  106   b  in this manner makes it possible to increase the adhesion area between the second electron transport layer  106   b  and the second insulating layer  120 , and the adhesion is improved. In addition, since the thickness of the second electron transport layer  106   b  at the end portion is thicker than that of the central portion, the electric field can be suppressed from concentrating at the end portion of the third electrode  118  and the second electrode  108 , so that the withstand voltage can be improved, and the light emission can be prevented from concentrating at the peripheral part of the area  124   a  where the opening  124  is arranged, thereby prolonging the light emission lifetime of the element. 
     In addition, a side surface of the second insulating layer  120  in the opening  124  is preferred to be inclined so as to open upward. Such a cross-sectional shape of the opening  124  can reduce the steepness of a step. As a result, in the case where the light emitting layer  112 , the third electrode  118 , and the like are arranged overlapped with the opening  124 , each layer can be formed along the stepped portion. In other words, it is possible to prevent cracks in the light emitting layer  112 , the third-electrode  118 , and the like, and so-called step breakage can be prevented. 
     The second electrode  108  is sandwiched between the first insulating layer  104  and the second insulating layer  120 . The second electrode  108  is arranged at a position not exposed from the opening  124  by being sandwiched between the first insulating layer  104  and the second insulating layer  120 . The second electrode  108  is arranged so as to overlap with the third electrode  118  across the insulating layer. Since the end portion of the second electrode  108  is not exposed to the opening  124  of the second insulating layer  120 , it is configured so that an electric field concentration does not occur between the third electrode  118  and the second electrode  108  in the light emitting area. In the EL element  200   a , an offset area  126  is arranged so that the second electrode  108  is not exposed to the opening  124  of the second insulating layer  120 . The offset area  126  is an area from the end portion of the opening  124  to the end portion of the second electrode  108 , and corresponds to an area where the first electron transport layer  106   a  is sandwiched between the first insulating layer  104  and the second insulating layer  120 . For the purpose of preventing electric field concentration, in the case where the total thickness of the electron transport layer  106 , the electron injection layer  110 , the light emitting layer  112 , the hole transport layer  114 , the hole injection layer  116 , and the like is 100 nm to 1000 nm, the length of the offset area  126  (the direction in which carriers (electrons) flow) is preferably about 1 µm to 20 µm, for example, about 2 µm to 10 µm, as the length of 10 times the total thickness or more. 
     As described above, the second electrode  108  is sandwiched between the first insulating layer  104  and the second insulating layer  120 , and the end portion of the second electrode  108  is arranged outside the area  124   a  where the opening  124  of the second insulating layer  120  is arranged, so that the withstand voltage of the EL element  200   a  according to the present embodiment can be increased and the uniformity of the emission intensity in the light emitting area can be increased. Arranging the second insulating layer  120  makes it possible to increase the distance between the third electrode  118  and the second electrode  108  and reduce the parasitic capacitance. 
     The first electrode  102  is arranged so as to overlap with the area where the opening  124  of the second insulating layer  120  is arranged and is arranged so as to overlap with the first electron transport layer  106   a  via the first insulating layer  104 . The first electrode  102  is insulated from the first electron transport layer  106   a  by the first insulating layer  104 . Although there is no transfer of carriers between the first electrode  102  and the first electron transport layer  106   a , the first electron transport layer  106   a  is affected by an electric field generated by the application of a voltage to the first electrode  102 . 
     The first electron transport layer  106   a  is subjected to an electric field formed by the first electrode  102 . The amount of carriers (electrons) transported from the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ) to the light emitting layer  112  can be controlled by the electric field strength of the first electrode  102 . When the voltage applied to the first electrode  102  increases, the electric field acting on the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ) also increases. Since the electric field generated by applying a positive voltage to the first electrode  102  acts to draw carriers (electrons) from the second electrode  108  to the first electron transport layer  106   a , the amount of carriers (electrons) transported to the light emitting layer  112  can be increased. That is, the amount of carriers (electrons) transported from the first electron transport layers  106   a  to the light emitting layer  112  can be controlled by the magnitude of the voltage applied to the first electrode  102 . In other words, the balance (carrier balance) between the amount of electrons injected from the second electrode  108  and the amount of holes injected from the third electrode  118  can be adjusted by controlling the voltage applied to the first electrode  102 . 
     The first electrode  102  is preferably arranged so as to overlap with the offset area  126  of the first electron transport layer  106   a . Such an arrangement allows the first electrode  102  to apply an electric field to the offset area  126 . When a positive voltage is applied to the first electrode  102 , carriers (electrons) are induced in the first electron transport layer  106   a  forming the offset area  126 , thereby preventing the offset area  126  from becoming high in resistance. In the case where the length of the offset area  126  is about 2 µm to 10 µm, electrons can be prevented from flowing from the second electrode  108  to the first electron transport layer  106   a  when the first electrode  102  is connected to the earth potential. This is because the offset area  126  operates as a thin film transistor (Thin Film Transistor: TFT) having the first electrode  102  as a bottom gate. 
     Since the EL element  200   a  shown in  FIG.  1    is of the bottom-emission type, the first electrode  102  has light transmittance. For example, the first electrode  102  may be formed of a transparent conductive film. On the other hand, the third electrode  118  has a light-reflecting surface for reflecting light emitted from the light emitting layer  112 . The third electrode  118  is preferably formed of a material having a large work function for injecting a hole into the hole injection layer  116 . For example, the third electrode  118  is formed of a transparent conductive film such as indium-tin-oxide (ITO). For example, the light-reflecting surface of the third electrode  118  can be formed by stacking a metal film such as an aluminum alloy on a transparent conductive film. 
     As will be described later, the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ) is formed of an oxide semiconductor having light transmittance. The oxide semiconductor having light transmittance is an inorganic material and is thermally stable because it is an oxide. In the EL element  200   a , stable light emission without degradation of properties can be realized even in the inverted stacked structure by forming the electron transport layer  106  with an oxide semiconductor. 
     1-2. Top-Emission Type EL Element 
       FIG.  2    shows a top-emission type EL element  200   b . The top-emission type EL element  200   b  has the same structure as the bottom-emission type EL element  200   a  shown in  FIG.  1    except that the configurations of the third electrode  118  and the first electrode  102  are different. In the case where the EL element  200   b  is of the top-emission type, the first electrode  102  is formed of a metal film so as to form a light-reflecting surface, and the third electrode  118  is formed of a transparent conductive film so as to transmit light emitted from the light emitting layer  112 . Since the second electrode  108  is arranged on the outer side of the light emitting area, there is no need to change the structure and the constituent material. Although not shown in this diagram, a silicon nitride film (Si 3 N 4  film), a silicon oxide film (SiO 2  film), an aluminum oxide film (Al 2 O 3  film), or the like, which is formed by a plasma CVD (Chemical Vapor Deposition) method or a sputtering method as a thin film sealing layer, is usually formed on the upper layer of the third electrode  118 . 
     Since the first electrode  102  is formed of a metal film, it functions as a light-reflecting plate in the EL element  200   b . Since the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ) is formed of an oxide semiconductor film having light transmittance, the attenuation of light reflected by the first electrode  102  can be prevented, and light extraction efficiency (external quantum efficiency) can be increase. 
     In the top-emission type EL element  200   b , the configurations of the third electrode  118  and the first electrode  102  are different from that of the bottom-emission type EL element  200   a . That is, the EL element according to the present embodiment can realize both the bottom-emission type and the top emission type with a slight change while sharing the inverted stacked type. 
     In the bottom-emission type EL element  200   a  shown in  FIG.  1    and the top-emission type EL element  200   b  shown in  FIG.  2    have a structure in which the electron transport layer  106 , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  are stacked at least in the longitudinal direction, and the first electrode  102  is arranged so as to be close to the electron transport layer  106  with the first insulating layer  104  interposed therebetween, and the second electrode  108  is arranged on the outer periphery area of the area  124   a  where the opening  124  is arranged. In the EL element  200 , the potential of the first electrode  102  is controlled independently of the third electrode  118  and the second electrode  108 , so that the amount of carriers (electrons) transported from the electron transport layer  106  to the light emitting layer  112  can be controlled. The EL element  200  can realize both the bottom-emission type and the top emission type by appropriately selecting the materials of the first electrode  102  and the third electrode  118 . 
     Components of EL Element 
     2-1. First Electrode (Electrode for Controlling the Amount of Carrier Injection) 
     The first electrode  102  is formed using a metal material, a metal oxide material, a metal nitride material, or a metal oxynitride material having conductivity. The metal material is formed of a metal material such as aluminum (Al), silver (Ag), titanium (Ti), molybdenum (Mo), or tantalum (Ta), or an alloy material or a stacked metal using these metals. For example, indium tin oxide (ln 2 O 3 ·SnO 2 : ITO), indium zinc oxide (ln 2 O 3 ·ZnO: IZO), tin oxide (SnO 2 ), and zinc oxide (ZnO) can be used as the metal oxide materials. In addition, titanium oxide (TiOx: Nb) doped with niobium (Nb) or the like can be used as the metal oxide material. Titanium-nitride (TiN x ), zirconium-nitride (ZrN x ), or the like can be used as the metal nitride material. Titanium oxynitride (TiO x N y ), tantalum oxynitride (TaO x N y ), zirconium oxynitride (ZrO x N y ), hafnium oxynitride (HfO x N y ), or the like can be used as the metal-oxynitride material. A trace amount of metal elements that improve conductivity may be added to the metal oxide material, the metal nitride material, and the metal oxynitride material. For example, titanium oxide (TiOx: Ta) doped with tantalum (Ta) may be used. 
     The material for forming the first electrode  102  may be appropriately selected depending on whether the EL element  200  is the top-emission type or bottom-emission type. In the case of the bottom-emission type, the first electrode  102  is formed of a metal oxide material, a metal nitride material, or a metal oxynitride material having conductivity and having light transmittance. As a result, the EL element  200   a  can emit the light emitted from the light emitting layer  112  through the first electrode  102 . On the other hand, in the case of the top emission type, the first electrode  102  is formed of a metal material having high reflectance to visible light. In the EL element  200   b , the first electrode  102  is formed of a metal material, so that the light emitted from the light emitting layer  112  can be reflected and emitted from the third electrode  118 . 
     2-2. First Insulating Layer 
     The first insulating layer  104  is formed using an inorganic insulating material. Silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, or the like can be selected as the inorganic insulating material. The first insulating layer  104  is formed by a plasma CVD (Chemical Vapor Deposition) method, a sputtering method, or the like. The first insulating layer  104  is formed with a thickness of 50 nm to 900 nm, preferably 100 nm to 600 nm. Setting the thickness of the first insulating layer  104  within the above range makes it possible to apply the electric field generated by the first electrode  102  to the electron transport layer  106 , and even when the bias voltage increases, it is possible to prevent a tunneling current from flowing from the first electrode  102  to the electron transport layer  106  due to the tunneling effect. 
     The first insulating layer  104  may have both insulating properties and transparency by using such an insulating material. As a result, the first insulating layer  104  can be applied to both the bottom-emission type EL element  200   a  and the top-emission type EL element  200   b . In addition, the first electrode  102  and the electron transport layer  106  may be insulated from each other and the first electrode  102  and the second electrode  108  may be insulated from each other. 
     2-3. Second Insulating Layer 
     In the present embodiment, the second insulating layer  120  is formed of a polar organic insulating material. For example, a straight-chain fluorine organic material may be used as the second insulating layer  120 . For example, a fluoroalkylsilane (FAS)-based material is used as the straight-chain fluorine organic material. For example, H, 1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS), and the like are used as the fluoroalkylsilane (FAS)-based material. 
     In addition, for example, the second insulating layer  120  may include a fluorine liquid repellent in a photosensitive resin material. Positive-type photosensitive resin compositions and negative-type photosensitive resin compositions containing a straight-chain fluorine organic material as the main component are already commercially available, and it is possible to adjust the liquid repellent performance by mixing an appropriate amount of fluorine liquid repellent with these photosensitive resin compositions. 
     Since the second insulating layer  120  is formed using a straight-chain fluorine organic material, a high liquid-repellent surface with poor wettability is formed. In other words, negative charges appear on the upper surface of the second insulating layer  120  by the microphase-separation phenomenon by forming the second insulating layer  120  containing a bipolar molecule or a side chain. As will be described later, the opening  124  exposing the first electron transport layer  106   a  is formed on the second insulating layer  120 , and then the second electron transport layer  106   b  is formed in the opening  124  of the second insulating layer  120 . The side surface of the second insulating layer  120  in the opening  124  has high wettability and high lyophilic. The second electron transport layer  106   b  can be efficiently arranged in the opening  124  of the second insulating layer  120  by utilizing the liquid repellency of the upper surface and the lyophilic of the side surface of the opening of the second insulating layer  120 . 
     In addition, an organic insulating material such as polyimide, acryl, or epoxy may be used for the second insulating layer  120 . In this case, the liquid repellency of the upper surface of the second insulating layer  120  may be improved by, for example, a fluorine plasma treatment. 
     On the other hand, forming the second insulating layer  120  with an organic insulating material makes it easy to control the cross-sectional shape of the opening  124 . The opening  124  of the second insulating layer  120  is preferably tapered, and the cross-sectional shape of the opening  124  can be tapered by using a photosensitive organic insulating material. The thickness of the second insulating layer  120  is not particularly limited and may be, for example, 200 nm to 5000 nm. In the case of imparting the second insulating layer  120  of a function as a flattening film, the thickness is preferred to be about 2 µm to 5 µm. 
     2-4. Electron Transport Layer 
     The electron transport layer  106  is preferably formed of a material having high electron mobility in order to transport carriers (electrons) injected from the second electrode  108  into the surface of the light emitting area of the EL element  200 . In addition, in the case of the bottom-emission type, the electron transport layer  106  is preferably formed of a material having good visible light transmittance because it is arranged closer to the light emission side than the light emitting layer. In addition, in order to make the carrier concentration different between the first electron transport layer  106   a  and the second electron transport layer  106   b , the electron transport layer  106  is preferably formed of a material easy to control the carrier concentration. 
     In the present embodiment, a metal oxide material is used for the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ). An oxide semiconductor material having a bandgap of 2.8 eV or more, preferably 3.0 eV or more, and high electron mobility is preferably used as the metal oxide material. Even when a thin film is formed, such an oxide semiconductor material has semiconducting properties, transparent to visible light, and has n-type electrical conductivity. 
     A quaternary oxide material, a ternary oxide material, a binary oxide material, and a unitary oxide material are exemplified as the oxide semiconductor material applied to the electron transport layer  106 . The metal oxide material exemplified here is classified as the oxide semiconductor because the bandgap is 2.8 eV or more, exhibits n-type conductivity, and the donor concentration can be controlled by oxygen-deficiency or the like. 
     The quaternary oxide material includes ln 2 O 3 —Ga 2 O 3 —SnO 2 —ZnO—based oxide material, the ternary oxide material includes ln 2 O 3 —Ga 2 O 3 —ZnO— based oxide material, ln 2 O 3 —SnO 2 —ZnO—based oxide material, ln 2 O 3 —Ga 2 O 3 —SnO 2 —based oxide material, ln 2 O 3 —Ga 2 O 3 —SmO x —based oxide material, ln 2 O 3 —Al 2 O 3 —ZnO—based oxide material, Ga 2 O 3 —SnO 2 —ZnO—based oxide material, Ga 2 O 3 —Al 2 O 3 —ZnO—based oxide material, and SnO 2 —Al 2 O 3 —ZnO—based oxide material, the binary oxide material includes ln 2 O 3 —ZnO—based oxide material, ln 2 O 3 —Ga 2 O 3 —based oxide material, ln 2 O 3 —WO 3 —based oxide material, ln 2 O 3 —SnO 2 —based oxide material, SnO 2 —ZnO—based oxide material, Al 2 O 3 —ZnO—based oxide material, Ga 2 O 3 —SnO 2 —based oxide material, Ga 2 O 3 —ZnO—based oxide material, Ga 2 O 3 —MgO—based oxide material, MgO—ZnO—based oxide material, SnO 2 —MgO—based oxide material, and ln 2 O 3 —MgO—based oxide material, and the unitary oxide material includes In 2 O 3 -based metal oxide material, Ga 2 O 3 -based metal oxide material, SnO 2 -based metal oxide material, and ZnO-based metal oxide material. 
      In addition, the metal oxide material may contain silicon (Si), nickel (Ni), tungsten (W), hafnium (Hf), titanium (Ti), zirconium (Zr), and tantalum (Ta). For example, the In—Ga—Zn—O—based oxide material described above is a metal oxide material containing at least In and Ga and Zn and the composition ratio is not particularly limited. In other words, a material represented by the chemical formula lnMO 3 (ZnO) m  (m&gt;0) can be used as the first electron transport layer  106   a  and the second electron transport layer  106   b . In this case, M represents one or more metal elements selected from Ga, Al, Mg, Ti, Ta, W, Hf, and Si. Also, the quaternary oxide material, the ternary oxide material, the binary oxide material, and the unitary oxide material are not limited to those in which the contained oxide has a stoichiometric composition and may be composed of an oxide material having composition deviated from the stoichiometric composition. In addition, the oxide semiconductor layer as the electron transport layer  106  may have an amorphous phase, may have crystallinity, or may be a mixture of an amorphous phase and a crystalline phase. 
     The first electron transport layer  106   a  and the second electron transport layer  106   b  are preferably formed using oxide semiconductor materials having different compositions. For example, the first electron transport layer  106   a  is preferably formed of a tin (Sn)-based oxide semiconductor (InGaSnO x , InWSnO x , InAlSnO x , InSiSnO x ) with high electron mobility and high PBTS reliability evaluation. The second electron transport layer  106   b  is preferably formed of a zinc (Zn)-based oxide semiconductor (ZnSiO x , ZnMgO x , ZnAlO x , ZnIn, ZnGaO x  or the like), which is difficult to crystallize in large particle size and easy to form an amorphous film or a nano-microcrystalline film. In other words, the first electron transport layer  106   a  is preferably a metal oxide containing tin oxide and indium oxide as the main component, and at least one selected from gallium oxide, tungsten oxide, aluminum oxide, and silicon oxide, and the second electron transport layer  106   b  is preferably a metal oxide containing zinc oxide as the main component and at least one selected from silicon oxide, magnesium oxide, indium oxide, aluminum oxide, and gallium oxide. Selecting a material that is less likely to be crystallized in large particle size and easy to form an amorphous film or a nano-microcrystalline film as the second electron transport layer  106   b  makes it possible to flow a space-charge limited current, and an EL element having a long lifetime can be formed. 
     Selecting the oxide semiconductor materials having different compositions as described above makes it possible to optimize the bandgaps of the first electron transport layer  106   a  and the second electron transport layer  106   b . For example, the bandgap of the second electron transport layer  106   b  can be increased with respect to the bandgap of the first electron transport layer  106   a . Specifically, the bandgap of the first electron transport layer  106   a  may be 3.0 eV or more, and the bandgap of the second electron transport layer  106   b  may be equal to or larger than the bandgap of the first electron transport layer  106   a . The bandgap of the second electron transport layer  106   b  is preferably 3.4 eV or more. Setting the bandgap of the second electron transport layer  106   b  to 3.4 eV or more makes it possible to reduce the absorbance of blue light and improve reliability. 
     In addition, indium tin oxide (In 2 O 3 ·SnO 2 : ITO), indium zinc oxide (In 2 O 3 ·ZnO: IZO), tin oxide (SnO 2 ), titanium oxide (TiO x ), or the like is used as the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ). Gallium nitride (GaN), aluminum-gallium nitride (GaAlN x ), or the like is used as the metal nitride material. Titanium oxynitride (TiO x N y ), tantalum oxynitride (TaO x N y ), zirconium oxynitride (ZrO x N y ), hafnium oxynitride (HfO x N y ), or the like is used as the metal-oxynitride material. A trace amount of metal elements that improves conductivity may be added to the metal oxide material, the metal nitride material, and the metal oxynitride material. For example, titanium oxide doped with niobium (TiO x : Nb) may be used. In order to make the bandgap of these metal compounds at least 2.8 eV or more, the oxygen content or the nitrogen content may be adjusted. 
     The first electron transport layer  106   a  formed of the oxide semiconductor material can be formed by a sputtering method, a vacuum-deposition method, a coating method or the like. The second electron transport layer  106   b  formed of the oxide semiconductor material can be formed by a coating method. The first electron transport layer  106   a  preferably has a thickness of 10 nm to 70 nm, and the second electron transport layer  106   b  is formed with a thickness of 150 nm to 900 nm. In order to prevent a decrease in withstand voltage and breakdown due to foreign substances and particles, the thickness of the second electron transport layer  106   b  should be as thick as possible. 
     The carrier concentration of the first electron transport layer  106   a  is preferably 10 times or more, preferably  100  times or more higher than the carrier concentration of the second electron transport layer  106   b . For example, the carrier concentration (electron concentration) of the first electron transport layer  106   a  is preferably in the range of 10 14 /cm 3  to 10 19 /cm 3 , the carrier concentration (electron concentration) of the second electron transport layer  106   b  is preferably in the range of 10 11 /cm 3  to 10 17 /cm 3 , and the difference between both carrier concentrations is preferably one order of magnitude or more, more preferably two orders of magnitude or more, as described above. 
     In addition, the electron mobility of the second electron transport layer  106   b  with respect to the electron mobility of the first electron transport layer  106   a  is preferably 1/10 or less. For example, the electron mobility of the first electron transport layer  106   a  is preferably 10 cm 2 /V sec to 200 cm 2 /V ·sec, and the electron mobility of the second electron transport layer  106   b  is preferably 0.001 cm 2 /V·sec to 10 cm 2 /V·sec. 
     Since the first electron transport layer  106   a  has high carrier concentration and high electron mobility as described above, the resistance can be reduced in a short time when a positive voltage is applied to the first electrode  102 . The first electron transport layer  106   a  can uniform the in-plane distribution of electrons injected from the second electrode  108  by having such physical properties. In other words, carriers (electrons) injected from the peripheral part of the first electron transport layer  106   a  can be transported toward the center by the second electrode  108 , and it is possible to uniform the electron concentration in the light emitting area. As a result, the in-plane uniformity of the emission intensity of the EL element  200  can be achieved. In addition, using the first electron transport layer  106   a  having high electron mobility makes it possible to transport the carriers (electrons) injected from the second electrode  108  to the area where the electric field of the first electrode  102  acts in a short time. 
     The second electron transport layer  106   b  is arranged proximate to the light emitting layer  112 . Therefore, in the case where the carrier concentration (electron concentration) of the second electron transport layer  106   b  is 10 20 /cm 3  or more, the excited state in the light emitting layer  112  is deactivated and the luminous efficiency is lowered. On the other hand, in the case where the carrier concentration (electron concentration) of the second electron transport layer  106   b  is 10 11 /cm 3  or less, the carrier supplied to the light emitting layer  112  is reduced, and a satisfactory brightness cannot be obtained. As described above, the emission efficiency of EL element  200  can be increased and the in-plane uniformity of the emission intensity can be achieved by making the carrier concentrations and the electron mobilities of the first electron transport layer  106   a  and the second electron transport layer  106   b  different from each other. The current flowing through the second electron transport layer  106   b  is required to be a space-charge-limited current to obtain uniform emission intensity. 
     2-5. Second Electrode (Cathode) 
     Conventionally, materials such as an aluminum-lithium alloy (AlLi) and a magnesium-silver alloy (MgAg) have been used as the cathode material of the EL element. However, these materials are easily deteriorated under the influence of oxygen and moisture in the atmosphere and are difficult to handle. These materials are metals or alkaline metals, and in order to have light transmittance, it is necessary to reduce the thickness of the film to form a semi-transmissive film. However, if the cathode is made to be a thin film, the seat resistance becomes high. Since the resistance of the electrode acts as a series resistance component in the EL element, thinning of the cathode increases the drive voltage and increases the power consumption. Further, it also becomes a cause of non-uniformity of the emission intensity (brightness) in the plane of the light emitting area of the EL element. 
     In the EL element  200  according to the present embodiment, the second electrode  108  is formed of a metal oxide material, a metal nitride material, or a metal oxynitride material having conductivity. In other words, the second electrode  108  is formed of a low resistance oxide conductor film. For example, the metal oxide conductive material includes indium tin oxide (In 2 O 3 ·SnO 2 : ITO), indium tin zinc oxide (In 2 O 3 ·SnO 2 ·ZnO: ITZO), indium tin silicon oxide (In 2 O 3 ·SnO 2 ·SiO 2 : ITSO), tin oxide (SnO 2 ), aluminum zinc tin oxide (Al 2 O 3 · ZnO· SnO 2 : AZTO), gallium zinc tin oxide (Ga 2 O 3 ·ZnO·SnO 2 : GZTO), zinc tin oxide (ZnO·SnO 2 : ZTO), and gallium tin oxide (Ga 2 O 3 ·SnO 2 : GTO). Such a metal oxide material can form a good ohmic contact with the first electron transport layer  106   a . 
     In addition, in the second electrode  108 , titanium oxide (TiO X : Nb) doped with niobium (Nb) or the like can be applied as the metal oxide material, and titanium nitride (TiN x ), zirconium nitride (ZrN x ), or the like can be applied as the metal nitride material, and titanium oxynitride (TiO x N y ), tantalum oxynitride (TaO x N y ), zirconium oxynitride (ZrO x N y ), hafnium oxynitride (HfO x N y ), or the like can be applied as the metal oxynitride material. In addition, a trace amount of metal elements that improves conductivity may be added to the metal oxide material, the metal nitride material, and the metal oxynitride material. For example, titanium oxide doped with tantalum (TiO x : Ta) may be used. A refractory metal silicide oxide such as TiSiO x  may be used. Using such a metal oxide material, a metal nitride material, or a metal oxynitride material exhibiting n-type electrical conductivity makes it possible to ensure bonding stability even when the material contacted the wiring  111 . That is, using such a metal oxide material, a metal nitride material, or a metal oxynitride material makes it possible to prevent an oxidation-reduction reaction (local battery reaction) with aluminum (Al) having a base potential. 
     The carrier concentration of the second electrode  108  is preferably 10 20 /cm 3  to 10 21 /cm 3 . Since the second electrode  108  has such a carrier concentration, it is possible to reduce resistance and suppress the series resistance loss. As a result, the power consumption of the EL element  200  can be reduced, and the current efficiency can be improved. 
     2-6. Electron Injection Layer 
     In the EL element, an electron injection layer is used to reduce the energy barrier for injecting electrons from the cathode to the electron transport material. In the EL element  200  according to the present embodiment, the electron injection layer  110  is preferably arranged in order to facilitate injection of electrons from the electron transport layer  106  formed of the oxide semiconductor to the light emitting layer  112 . The electron injection layer  110  is arranged between the electron transport layer  106  and the light emitting layer  112 . As shown in  FIG.   1    and  FIG.  2   , forming the electron injection layer  110  on the entire surface of the display area is important to improve yield and reliability. 
     The electron injection layer  110  is preferably formed using a material having a small work function in order to inject electrons into the light emitting layer  112  formed including the electroluminescence material. The electron injection layer  110  contains calcium (Ca) oxide and aluminum (Al) oxide. For example, a C12A7 (12CaO·7Al 2 O 3 ) electride is preferably used as the electron injection layer  110 . Since the C12A7 electride has semiconducting properties and can be controlled from a high resistance to a low resistance, and has a work function of 2.4 eV to 3.2 eV, which is about the same as an alkali metal, it can be suitably used as the electron injection layer  110 . 
     The electron injection layer  110  formed by the C12A7 electride is formed by a sputtering method using a polycrystal of C12A7 electride as a target. Since the C12A7 electride has semiconducting properties, the thickness of the electron injection layer  110  can be within 1 nm to 10 nm. Also, it is preferred that the molar ratio of Ca:Al of the C12A7 electride is in the range of 13:13 to 11:16. The electron injection layer  110  using the C12A7 electrode can be formed by a sputtering method. The electron injection layer  110  formed of the C12A7 electron is preferably amorphous but may be crystalline. 
     Since the C12A7 electride is stable in the atmosphere, it is convenient to handle as compared with an alkali metal compound such as lithium fluoride (LiF), lithium oxide (Li 2 O), sodium chloride (NaCl), and potassium chloride (KCl), which are conventionally used as an electron injection layer. This eliminates the need to operate in dry air or an inert gas in the manufacturing process of the EL element  200  and reduces the limitation of the manufacturing process. 
     In addition, because of the large ionization potential, the C12A7 electride can be used as a hole-blocking layer by arranging the C12A7 electride on a side opposite to the hole transport layer  114  with the light emitting layer  112  interposed therebetween. That is, arranging the electron injection layer  110  formed of the C12A7 electride between the electron transport layer  106  and the light emitting layer  112  suppresses the holes injected into the light emitting layer  112  penetrating to the second element  108  side, and the luminous efficiency can be increased. In addition, magnesium-zinc oxide (Mg x Zn y O, e.g., Mg 0.3 Zn 0.7 O), Zn 0.75 Si 0.25 O x , LaMgO x , MgSiO x , and the like can also be used as an electron injection layer because they have small work function of 3.1 eV and are highly stable in the atmosphere. They can also be used as good electron injection layers as long as the thickness is within 1 nm to 10 nm as in the C12A7. Since the bandgap of Zn 0.7 Mg 0.3 O x  or Zn 0.75 Si 0.25 O x  is as large as 3.9 eV to 4.1 eV, hole injection from the light emitting layer  112  can be prevented. A ternary metal oxide semiconductor material in which Zn 0.7 Mg 0.3 O x  and Zn 0.75 Si 0.25 O x  are mixed in the range of 1:4 to 1:10 can also be used as the electron injection layer. Since the electron injection layer is formed by a sputter deposition method, cross-talk with adjacent pixels can be prevented by adjusting the oxygen partial pressure of the mixed gases of Ar and O 2  so that the specific resistance is 10 7  Ω·cm ore more when the thickness is 10 nm. 
      2-7. Light Emitting Layer 
     The light emitting layer  112  is formed using an electroluminescence material. For example, a fluorescent compound material that emits fluorescence, a phosphorescent compound material that emits phosphorescence, or a thermally activated delayed fluorescence material (TADF) can be used as the electroluminescence material. 
     For example, N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (YGAPA), or the like can be used as a blue light emitting material. N-(9,10-diphenyl-2-anthryl) N,9-diphenyl-9H-carbazole-3-amine (2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazole-3-amine (2PCABPhA), N-(9,10-diphenyl-2-anthryl)N,N′,N′-triphenyl-1,4-phenylenediamine (2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1 ,4-phenylenediamine (2DPABPhA), N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazole-9-yl)phenyl]-N-phenylanthracene-2-amine (2YGABPhA), N,N,9-triphenylanthracene-9-amine (DPhAPhA), or the like can be used as a green light emitting material. N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (p-mPhTD), 7,13-diphenyl-N,N,N′, N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (p-mPhAFD), or the like can be used as a red light emitting material. In addition, a phosphorescent material such as bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C3′]iridium(III)acetylacetonate(Ir(btp) 2 (acac)) can be used. 
     In addition, various known materials such as Quantum dot (QD), a perovskite-based inorganic light emitting material, and a perovskite-based inorganic-organic hybrid light emitting material can be used as the light emitting layer  112 . The light emitting layer  112  can be produced by a vapor deposition method, a transcription method, a spin coating method, a spray coating method, a printing method (ink jet printing method, a gravure printing method), or the like. The thickness of the light emitting layer  112  may be appropriately selected, for example, it is arranged in a range of 10 nm to 100 nm. 
     In  FIG.  1    and  FIG.  2   , although an example in which the light emitting layer  112  is separated for each EL element is shown, in the case where the plurality of EL elements is arranged on the same plane, the light emitting layer  112  may be arranged so as to be contiguous over a plurality of light emitting elements. Although an electron-blocking layer may be formed on the entire surface between the light emitting layer  112  and the hole transport layer  114 , it is omitted in  FIG.  1    and  FIG.  2   . 
     2-8. Hole Transport Layer 
     The hole transport layer  114  is formed using a hole transporting material. The hole transport layer  114  may be, for example, an arylamine compound, an amine compound containing a carbazole group, and an amine compound containing a fluorene derivative, or the like. The hole transport layer  114  may be an organic material such as 4,4′-bis[N-(naphthyl)-N-phenyl-amino]biphenyl(α-NPD), N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), 2-TNATA, 4′,4′4″-tris(N-(3-methylphenyl)N-phenylamino)triphenylamine (MTDATA), 4,4′-N,N′-dicarbazole biphenyl (CBP), 4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl (DFLDPBi), 4,4′-bis [N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (BSPB), spiro-NPD, spiro-TPD, spiro-TAD, TNB, and the like. 
     The hole transport layer  114  is formed by a general film forming method such as a vacuum-deposition method, a coating method or the like. The hole transport layer  114  is manufactured with a thickness of 10 nm to 500 nm. 
     2-9. Hole Injection Layer 
     The hole injection layer  116  includes a material with high hole-injection properties with respect to an organic layer. A metal oxide such as molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, or manganese oxide can be used as the material with high hole-injection properties. In addition, an organic compound such as phthalocyanine (H 2 Pc), copper (II) phthalocyanine (abbreviation: CuPc), vanadyl phthalocyanine (VOPc), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (DPAB), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl (DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (DPA3B), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (PCzPCA2), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (PCzPCN1), and 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN) and the like can be used. 
     The hole injection layer  116  is formed by a general film forming method such as a vacuum-deposition method, a coating method or the like. The hole injection layer  116  is manufactured with a thickness of 1 nm to 100 nm. 
     2-10. Third Electrode (Anode) 
     The third electrode  118  is made of a metal, an alloy, or a conductive compound having a large work function (specifically, 4.0 eV or more). For example, indium tin oxide (ITO), indium zinc oxide (IZO), tungsten oxide, and indium oxide (IWZO) containing zinc oxide are used as the third electrode  118 . The third electrode  118  using these the conductive metal oxide materials is used is manufactured by a vacuum-deposition method or a sputtering method. 
     As described with reference to  FIG.  1   , in the case of the bottom-emission type EL element  200   a , since the third electrode  118  is located back surface of the emission surface, it is preferred to have a light-reflecting surface. In this case, the third electrode  118  is preferably formed by stacking a metal film on a transparent conductive film. On the other hand, as described with reference to  FIG.  2   , in the top-emission type EL element  200   b , the third electrode  118  can be formed using a transparent conductive film as described above. 
     2-11. Wiring 
     A high conductive metal material such as aluminum (Al) or copper (Cu) is used as the wiring  111 . For example, the wiring  111  is made using an aluminum alloy, a copper alloy, or a silver alloy. An aluminum-neodymium alloy (Al—Nd), an aluminum-titanium alloy (Al—Ti), an aluminum-silicon alloy (Al—Si), an aluminum-neodymium-nickel alloy (Al—Nd—Ni), an aluminum-carbon-nickel alloy (Al—C—Ni), a copper-nickel alloy (Cu-Ni), or the like can be used as the aluminum alloy. The wiring resistance can be reduced while having heat resistance by using such a metal material. In addition, a three-layer stacked structure electrode, such as Mo/Al/Mo, Mo/Cu/Mo, is also useful. That is, a three-layer stacked structure in which the above-described metal material is sandwiched by an antioxidant layer containing molybdenum (Mo), zirconium (Zr), titanium (Ti), or an alloy material thereof can also be applied. 
     Operation of EL Element 
     An operation of the EL element according to the present embodiment will be described with reference to  FIG.  3   ,  FIG.  4   , and  FIG.  5   . The EL element  200  shown in this section has a schematic configuration. 
     3-1. Light Emitting and Non-Light Emitting Operations 
       FIG.  3    schematically shows a configuration of the EL element  200  according to the present embodiment.  FIG.  3    shows a structure in which the second electrode  108 , the electron transport layer  106 , the electron injection layer  110 , the light emitting layer  112 , the hole transport layer  114 , the hole injection layer  116 , the third electrode  118 , the first insulating layer  104 , and the first electrode  102  are arranged as members constituting the EL element  200 . 
     The EL element  200  is a type of light emitting diode and emits light by flowing a forward current between the third electrode (anode)  118  and the second electrode (cathode)  108 .  FIG.  3    shows an embodiment in which a third power  128   c  is connected between the third electrode  118  and the ground, a second power source  128   b  is connected between the second electrode  108  and the ground, a second switch  130   b  is connected in series between the first electrode  102  and the first power source  128   a , and a first switch  130   a  is connected between the first electrode  102  and the ground.  FIG.  3    shows a state in which the first switch  130   a  and the second switch  130   b  of the switch  130  are off and no bias is applied to the first electrode  102 . 
     That is,  FIG.  3    shows a state in which the first switch  130   a  for controlling the conduction state between the first electrode  102  and the earth (ground) is off, and the second switch  130   b  for controlling the connection between the first electrode  102  and the first power source  128   a  is off. In this state, since the EL element  200  is biased in the forward direction, if the bias voltage is a light emission start voltage or higher, holes are injected from the third electrode  118 , and electrons are injected from the second electrode  108 . In the EL element  200 , a positive voltage is applied between the third electrode  118  and the second electrode  108  by the third power  128   c . The emission intensity can be controlled by the magnitude of the forward current flowing through the EL element  200 . 
     However, in a configuration in which the electron transport layer  106  and the third electrode  118  are arranged on the first insulating layer  104  so as to face each other with the light emitting layer  112  interposed therebetween, and the second electrode  108  is connected at the peripheral portion of the first electron transport layer  106   a , it is not possible to uniformly emit light in the light emitting area (area where the electron transport layer  106 , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  overlap). In this case, the electric field generated between the third electrode  118  and the second electrode  108  is not uniform in the light emitting area, and the electric field is concentrated on the end portions of the second insulating layer  120  and the second electrode  108 . In the state where both of the first switch  130   a  and the second switch  130   b  are off, carriers (electrons) injected from the second electrode  108  are not uniformly distributed in the plane of the first electron transport layer  106   a  but are injected into the end portion of the second electron transport layer  106   b . In the second electron transport layer  106   b  according to the present embodiment, since the thickness at the end portion is larger than the thickness at the central portion, the electric field concentration at the end portion of the second electron transport layer  106   b  is moderated. Therefore, the offset area  126  can be shortened, and accordingly, an area of the light emitting area can be increased. 
     However, in  FIG.  3   , since the first switch  130   a  and the second switch  130   b  are off, no voltage is applied to the first electrode  102  from the second power source  128   b . The carriers (electrons) injected from the second electrode  108  to the first electron transport layer  106   a  are not affected by the first electrode  102  and therefore do not spread to the center area of the first electron transport layer  106   a . That is, the carriers (electrons) injected into the peripheral portion of the first electron transport layer  106   a  are not drifted due to the fact that no voltage is applied to the first electrode  102 , and therefore do not spread over the entire center area of the first electron transport layer  106   a . Therefore, in the biased state shown in  FIG.  3   , the central portion of the light emitting area of the EL element  200  is dark, and only the peripheral portion thereof emits light brightly. 
       FIG.  4    shows a state in which the first switch  130   a  is turned on and the potential of the first electrode  102  becomes the ground potential in the switch  130 . In this state, no carriers (electrons) are present in the first electron transport layer  106   a , and the first electron transport layer  106   a  is in an insulating state. As a result, no current flows through the EL element  200 , and no light is emitted (non-light emitting state). This is because the first electrode  102  operates as the bottom gate, so that the first electron transport layer  106   a  of the offset area  126  becomes a depletion layer and no current flows. 
     As shown in  FIG.  5   , in the case where a forward bias is applied between the third electrode  118  and the second electrode  108  of the EL element  200  and the second switching  130   b  is turned on, an electric field formed by the first electrode  102  acts on the first electron transport layer  106   a . Since a positive voltage is applied to the first electrode  102 , carriers (electrons) injected from the second electrode  108  to the first electron transport layer  106   a  are drifted to the center area of the first electron transport layer  106   a . Thereby, carriers (electrons) are transported from the peripheral portion of the first electron transport layer  106   a  to the central area of the first electron transport layer  106   a . The electric field generated by the positive-voltage-applied first electrode  102  acts to spread carriers (electrons) injected from the second electrode  108  over the entire surface of the first electron transport layer  106   a . 
      Since the EL element  200  is biased in the forward direction, carriers (electrons) transported to the central area of the first electron transport layer  106   a  move from the first electron transport layer  106   a  toward the light emitting layer  112 . The carriers (holes) injected from the third electrode  118  and the carriers (electrons) injected from the second electrode  108  recombine in the light emitting layer  112  to generate excitons, and photons are emitted and observed as light emission when the excitons in the excited state transition to the ground state. 
     In the biased state shown in  FIG.  5   , the amount of carriers (electrons) injected into the first electron transport layer  106   a  can be controlled by the voltage of the second electrode  108 . Increasing the voltage of the second electrode  108  makes it possible to increase the amount of carriers (electrons) injected into the first electron transport layer  106   a . The amount of carriers (electrons) injected into the light emitting layer  112  from the first electron transport layer  106   a  may be controlled by the voltage of the first electrode  102 . Increasing the voltage of the first electrode  102  makes it possible to draw a large amount of carriers (electrons) injected from the second electrode  108  into the central area of the first electron transport layer  106   a , and the amount of carriers injected into the light emitting layer  112  can be increased. 
     In order for the light emitting layer  112  to emit light substantially uniformly over the entire surface, electrons flowing in the second electron transport layer  106   b  preferably form a space-charge limited current. Therefore, the second electron transport layer  106   b  is preferably in an amorphous state, a nano-sized microcrystalline state, or a mixed state thereof. The first electron transport layer  106   a  preferably contains nano-sized microcrystals and is a dense film. 
     As described above, the EL element  200  according to the present embodiment has the first electrode  102  in addition to the third electrode  118  and the second electrode  108 , so that the density of carriers injected into the light emitting layer  112  can be controlled. 
     Further, the second electron transport layer  106   b  according to the present embodiment has a larger thickness at the end portion than at the central portion. Therefore, the electric field can be suppressed from concentrating at the end portion of the second electron transport layer  106   b , and the withstand voltage can be improved. Further, in order to suppress leakage from the side wall of the second electron transport layer  106   b  and improve reliability, a third insulating layer  122  may be stacked on the lower layer of the second insulating layer  120  as shown in  FIGS.  17 A and  17 B . 
     Although  FIG.  5    shows an example in which the first electrode  102  is arranged on the second electrode  108  side, the first electrode  102  may be arranged on the third electrode  118  side. In addition to the first electrode  102 , an electrode that controls the injection amount of carriers (holes) may be further arranged on the third electrode  118  side. 
     3-2. Career Balance Control 
     In order for the EL element to emit light, holes need to be injected from the anode and electrons need to be injected from the cathode. In order to increase the current efficiency (luminous efficiency) of the EL element, it is necessary to balance the amount of electrons transported from the anode to the light emitting layer and the amount of positive holes transported from the cathode to the light emitting layer (hereinafter, also referred to as “carrier balance”). The EL element can improve the current efficiency by balancing the carriers. 
     However, in the conventional EL element, since the electronic mobility is lower than the hole mobility of the light emitting layer, the carrier balance is lost, and the luminous efficiency is lowered. In addition, the EL element has a problem in that, when the carrier balance is lost and the number of holes is excessively large in the light emitting layer, holes are accumulated at the interface between the light emitting layer and the electron transport layer, which causes the degradation of the current efficiency (luminous efficiency). Therefore, attempts have been made to balance the holes injected into the light emitting layer with electrons by adjusting the materials and thicknesses of the hole transport layer and the electron transport layer. However, even if the element structure itself of the EL element is adjusted, it is not possible to follow the temporal change or the thermal change of the light emission property. 
     In contrast, in the EL element  200  according to the embodiment of the present invention, the carrier balance can be controlled by the first electrodes  102 . That is, the carrier balance can be controlled by controlling the transport amount of carriers (electrons) to the light emitting layer  112  by the first electrode  102  arranged on the electron transport layer  106  side. That is, the number of holes and electrons in the light emitting layer  112  can be controlled to be the same by increasing the amount of electrons transported by the first electrode  102  so that the amount of electrons transported from the second electrode  108  to the light emitting layer  112  is not insufficient with respect to the amount of positive holes transported from the third electrode  118  to the light emitting layer  112 . In other words, as shown in  FIG.  32   , the EL element  200  according to the present embodiment is capable of keeping the carrier balance in the light emitting layer  112  constant by increasing the electron current by the first electrode  102  so that the magnitude of the electron current injected from the electron injection layer  110  to the light emitting layer  112  is the same with respect to the hole current injected from the hole transport layer  114  to the light emitting layer  112 . 
       FIG.  6    is a diagram schematically showing a relation between currents (Ie) flowing between the third electrode  118  and the second electrode  108  when a voltage (Vac) applied between the third electrode  118  and the second electrode  108  of the EL element  200  is constant and a voltage (Vg) applied to the first electrode  102  is changed. As shown in  FIG.  6   , in the case where the voltage (Vg) applied to the first electrode  102  is 0V, the electronic current (Ie) is small, and light emission over the entire surface of EL element  200  is not observed. When the voltage of the first electrode  102  is increased from this condition, carriers (electrons) injected from the second electrode  108  into the electron transport layer  106  become electron currents (Ie) and flow from the first electron transport layer  106   a  toward the light emitting layer  112 . In this case, the electronic current (Ie) increases exponentially as the forward current of the diode (“I area” shown in  FIG.  6   ). 
      In the case where the voltage (Vg) applied to the first electrode  102  is further increased, the increasing amount of the electronic current (Ie) with respect to the change amount of the voltage (Vg) tends to saturate, and the slope of the curve of Ie vs Vg property becomes gentle (“II area” shown in  FIG.  6   ). In the case where the magnitude of the voltage (Vg) applied to the first electrode  102  at the area B is varied between the first voltage (Vg1) and the second voltage (Vg2), the electronic current (Ie) varies between the first current (Ie1) and the second current (Ie2). The area where the voltage (Vg) of the first electrode  102  varies from the first voltage (Vg1) to the second voltage (Vg2) is an area where the electronic current (Ie) does not change abruptly, and it an area where the emission intensity of the EL element  200  is saturating. 
     A change in the electron current (Ie) means an increase or decrease in the amount of holes and electrons injected into the light emitting layer  112 . In the case where the voltage (Vg) of the first electrode  102  is changed between the first voltage (Vg1) and the second voltage (Vg2), the amount of electrons injected into the light emitting layer  112  is changed. That is, changing the voltage (Vg) of the first electrode  102  makes it possible to control the carrier balance between the electrons and the holes in the light emitting layer  112 . As shown in  FIGS.  34 A to  34 C , changing the amount of electrons injected into the light emitting layer  112  makes it possible to shift the center position (the position of the light emitting area in the thickness direction of the light emitting layer  112 ) of the area to which the electrons and the holes are recombined. For example, in  FIG.  34 A , in the case where the first electrode  102  is at the first voltage (V102 = Vg1), the electronic current becomes relatively smaller than the hole current, and the position of the light emitting area in the light emitting layer  112  becomes the cathode side (EL (b), the “A” side shown in  FIG.  6   ). On the other hand, in  FIG.  34 C , in the case where the first electrode  102  is at the second voltage (V102 = Vg2), the electronic current becomes relatively larger than the hole current, and the position of the light emitting area in the light emitting layer  112  is shifted to the anode side (EL (t), the “B” side shown in  FIG.  6   ). In  FIG.  34 B , in the case where the first electrode  102  is at the half of the first voltage plus the second voltage (V102 = (Vg1 + Vg2) / 2), the electronic current and the hole current become equal, and the position of the light emitting area in the light emitting layer  112  is shifted to the central portion (EL (m)). 
     In this way, the EL element  200  can control the position of the light emitting area in the light emitting layer  112  in the thickness direction by the voltage of the first electrode  102 . For example, in the case where the voltage of the first electrode  102  is changed between the first voltage (Vg1) and the second voltage (Vg2), the position of the light emitting area in the light emitting layer  112  can be swung between the cathode side A and the anode side B. The entire light emitting layer  112  can be utilized as a light emitting area by controlling the voltage of the first electrode  102 . As a result, the entire area of the light emitting layer  112  can be used as a light emitting area, so that the lifetime of the brightness degradation (for example, the time for the initial brightness drops to 70%) can be extended. The voltage of the first electrode  102  varies between Vg1 and Vg2 shown in  FIG.  6   , and the intensity of the brightness can be controlled by the potential difference (voltage) between the second electrode  108  and the third electrode  118 . 
     As described above, in the EL element according to the present embodiment, the electron transport layer is formed of the oxide semiconductor layer, the first electrode for controlling the amount of carrier injection is arranged on the electron transport layer with the insulating layer interposed therebetween, and the first electrode is arranged opposite to the third electrode  118  which is the anode, whereby the electron injection amount into the light emitting layer can be controlled. The EL element according to the present embodiment can control the carrier balance between the electrons and the holes in the light emitting layer by the action of the first electrode that controls the amount of carrier injection. As a result, the current efficiency of the EL element can be increased, and the lifetime can be extended. 
     In the conventional EL element structure, the entire thickness of the light emitting layer is not uniformly deteriorated, and the light emitting layer is unevenly deteriorated, so that it is difficult to suppress the brightness deterioration, and therefore the lifetime of the EL element cannot be extended. However, in the EL element  200  according to the embodiment of the present invention, the entire light emitting layer  112  can be made to be a light emitting area by controlling the voltage of the first electrode  102 , whereby the entire thickness of the light emitting layer  112  can uniformly deteriorate, and therefore the lifetime for brightness degradation can be extended. As a result, even if the thickness of the light emitting layer  112  is increased from the conventional thickness (e.g., 30 nm) to 45 nm to 90 nm that is 1.5 times to 3.0 times, the entire area in the thickness direction of the light emitting layer  112  can be emitted, so that the lifetime of the EL element  200  can be further increased. As shown in  FIGS.  33 A to  33 D , although the waveform of the voltage V102 applied to the first electrode  102  may be (A) a sinusoidal waveform, (B) a rectangular stepped waveform, (C) a trapezoidal stepped waveform, or (D) a triangular waveform, the best waveform can be selected according to the circuit system. There is no limit on how many times to repeat in one field period. In order to maximize the light emission lifetime, the emission time ratio of the central light emitting area may be increased. A rectangular stepped wave or a trapezoidal stepped wave in  FIGS.  33 B or  33 C  is preferable as the voltage waveform. 
     Method of Manufacturing EL Element 
     An example of a method of manufacturing an EL element according to an embodiment of the present invention will be described with reference to  FIGS.  7 A to  7 C ,  FIGS.  8 A to  8 C , and  FIGS.  9 A to  9 C . In the following, a method of manufacturing the bottom-emission type EL element  200   a  shown in  FIG.  1    will be described. 
       FIG.  7 A  shows a step of forming the first electrode  102 , the first insulating layer  104 , the second electrode  108 , and the wiring  111  on the substrate  100 . For example, a transparent insulating substrate is used as the substrate  100 . A quartz substrate and alkali-free glass substrate exemplified by aluminosilicate glass and aluminoborosilicate glass are used as the transparent insulating substrate. In addition, a resin substrate such as polyimide, para-aramid, or polyethylene naphthalate (PEN) can be used as the transparent insulating substrate. 
     The first electrode  102  is formed of a transparent conductive film such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO). The transparent conductive film is formed with a thickness of 30 nm to 200 nm using a sputtering method. The first electrode  102  is formed by forming a resist mask by a photolithography process and performing etching with respect to the transparent conductive film formed on the first surface of the substrate  100 . The first electrode  102  is preferably formed to have a tapered end surface in a cross-sectional view. 
     The first insulating layer  104  is formed of an inorganic transparent insulating film such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride film. The inorganic transparent insulating film is formed by a plasma CVD (Chemical Vapor Deposition) method or a sputtering method. The first insulating layer  104  is formed to have a thickness of about 100 nm to 500 nm. The first insulating layer  104  is formed so as to bury the first electrode  102 . In this case, since the end surface of the first electrode  102  is formed in a tapered shape, the first electrode  102  including the stepped portion can be surely covered. 
     The second electrode  108  is formed by sputtering a film of a metal oxide material, a metal nitride material, a metal oxynitride material, or a refractory metal silicide oxide material having conductivity. For example, the second conductive film  107  forming the second electrode  108  is made of a conductive metal oxide film with a thickness of 30 nm to 200 nm. In addition, a third conductive film  109  forming the wiring  111  is formed by sputtering a film of a metal material or an alloy material. The third conductive film  109  forming the wiring  111  is made of a metal film with a thickness of 200 nm to 2000 nm in order to achieve low resistance. 
       FIGS.  8 A to  8 C  shows a lithography process for forming the second electrode  108  and the wiring  111 . In this case, a multi-tone exposure method (halftone exposure method) is applied, and patterns of the second electrodes  108  and the wiring  111  are formed by a single photomask. 
     As shown in  FIG.  8 A , a positive photoresist film  205  is formed on the third conductive film  109 . A multi-tone mask  201  is used to expose the photoresist film  205 . In the multi-tone mask  201 , although a gray tone mask in which a slit having a resolution equal to or lower than that of an exposure machine is arranged and an intermediate exposure is realized by blocking a part of light by the slit, and a halftone mask in which an intermediate exposure is realized by using a semi-transmissive film are known as a multi-tone mask pattern, both multi-tone masks  201  can be used in the present embodiment. Exposure through the light-transmitting area, a semi-transmitting area  202 , and a non-transmitting area  203  of the multi-tone mask  201  forms three types of portions on the photoresist film  205 , i.e., an exposed portion, an intermediate exposed portion, and an unexposed portion. 
     After that, as shown in  FIG.  8 A , the photoresist film  205  is developed to form a resist mask  207   a  with areas of different thicknesses.  FIG.  8 A  shows an embodiment in which the resist mask  207   a  has a thicker film thickness in a portion corresponding to the region where the wiring  111  is formed, and a relatively thinner film thickness in a portion corresponding to the region where the second electrode  108  is formed. 
      The third conductive film  109  and the second conductive film  107  are etched using the resist mask  207   a . Although the etching conditions are not limited, for example, the third conductive film  109  formed of a metal material is subjected to wet etching using a mixed acid etchant, and the second conductive film  107  formed of a metal oxide material or the like is subjected to dry etching using a chlorine-based gas or wet etching using an oxalic acid-based gas. At this step, the second electrode  108  is formed. After this etching, an area with thin thickness of the resist mask  207   a  is removed by an ashing process to expose the surface of the third conductive film  109 .  FIG.  8 B  shows a resist mask  207   b  after the ashing process is performed. The resist mask  207   b  remains on the third conductive film  109 . 
     Next, the exposed third conductive film  109  is etched. This etching is performed, for example, by wet etching using a mixed acid etchant. If the second conductive film  107  formed of a metal oxide or the like contains tin (Sn) in an amount of 10 atm% or more, the second conductive film  107  is less likely to be etched by a mixed acid etchant, so that the selectivity can be relatively high. Therefore, the shape of the second electrode  108  in the lower layer is maintained.  FIG.  8 C  shows a step that the third conductive film  109  is etched to form the wiring  111 . After the third conductive film  109  is etched, the resist mask  207   b  is removed by a resist stripping solution or ashing. 
     The surface of the second electrode  108  that has already been formed is exposed to the oxygen plasma by the resist stripping solution or ashing treatment. However, tin (Sn), zinc (Zn), indium (In), gallium (Ga), tungsten (W), titanium (Ti), tantalum (Ta), hafnium (Hf), and zirconium (Zr), which are contained as components of the second electrode  108 , do not generate defects that trap carriers (electrons) even when they become oxides and become an n-type oxide semiconductor without expressing the role of a carrier (electron) killer. Therefore, even when exposed to an oxygen-plasma, a good ohmic contact can be formed with the first electron transport layer  106   a  formed in a later process. 
       FIG.  7 B  shows a step of forming the first electron transport layer  106   a . The first electron transport layer  106   a  is formed on substantially the entire surface of the substrate  100  so as to cover the second electrode  108  and the wiring  111 . The first electron transport layer  106   a  can be produced by a sputtering method using a sputtering target obtained by sintering a metal oxide, an atomic layer deposition (ALD) method, or a mist CVD (Mist Chemical Vapor Deposition) method. The first electron transport layer  106   a  is formed with a thickness of 10 nm to 200 nm, for example, a thickness of 30 nm to 50 nm. 
     Although the first electron transport layer  106   a  is formed of a metal oxide material, as described above, the first electron transport layer  106   a  is preferably formed of a tin (Sn)-based oxide semiconductor (InGaSnO x , InWSnO x , InSiSnO x , InGaSnSmO x ) with high electron mobility and high PBTS (Positive Bias temperature Stress) reliability evaluation. Also, if tin (Sn) is contained in the tin (Sn)-based oxide semiconductor in an amount of 10 atm% or more, even if zinc (Zn) is contained, the compositional change of zinc (Zn) in the process becomes small, and therefore, the inclusion of zinc (Zn) is not completely denied. 
       FIG.  7 C  shows a step of forming the second insulating layer  120  on the first electron transport layer  106   a . For example, the second insulating layer  120  is formed using a straight-chain fluorine organic material. A high liquid-repellent surface with poor wettability is formed by forming the second insulating layer  120  using a straight-chain fluorine organic material. In addition, the second insulating layer  120  may be formed of an organic insulating material such as polyimide, acryl, or epoxy siloxane. In this case, the upper surface of the second insulating layer  120  may be improved in water repellency by, for example, a fluorine plasma treatment. The second insulating layer  120  is formed with a thickness of 100 nm to 5000 nm. For example, in the case where the flattening process is performed, it is preferred to be formed with a thickness of 2000 nm to 5000 nm. As a result, a high liquid-repellent surface with poor wettability is formed on the second insulating layer  120 . 
       FIG.  9 A  shows a step of forming the opening  124  in the second insulating layer  120 . The opening  124  may be formed by etching the second insulating layer  120 . In the case where the second insulating layer  120  is formed of a photosensitive organic resin material, the opening  124  can be formed by exposure using a photomask and development. In any cases, in order to form the EL element  200   a , the opening  124  is preferably processed so that the inner wall surface is tapered shape. As a result, the upper surface of the second insulating layer  120  with high liquid repellency is removed from the opening  124 , and a side surface with a high wettability and high lyophilic is formed. 
       FIG.  9 B  and  FIG.  9 C  show steps of forming the second electron transport layer  106   b . The second electron transport layer  106   b  according to the present embodiment is prepared by a coating method using a composition containing a metal salt, a first amide, and a solvent as a metal oxide material 106b′. The metal oxide material 106b′ of the second electron transport layer  106   b  can be applied by using any of methods such as spin coating, dip coating, ink jet coating, flexographic printing, roll coating, die coating, transfer printing, spraying, and slit coating. 
     As described above, the metal-oxide-material 106b′ preferably contains a zinc (Zn)-based oxide semiconductor (ZnSiO x , ZnMgO x , InZnSiO x , InZnGeO x , InZnMgO x , InZnMgGaO x , InZnGaO x , InGaSnZnO x  or the like) that is difficult to crystallize in large particle size and easy to form an amorphous film or a nano-microcrystalline film. That is, the metal oxide material 106b′ preferably contains zinc oxide and at least one selected from silicon oxide, germanium oxide, magnesium oxide, indium oxide, tin oxide, and gallium oxide. The carrier concentration of the oxide can be adjusted by doping divalent Zn or Mg, trivalent In or Ga or tetravalent Sn. 
     Adjusting the ratio (B/(A+B)) between the number (A) of indium ions and the sum (B) of the number of magnesium ions and the number of zinc ions in the metal oxide material  106   b ′ as a coating liquid for metal oxide thin film formation makes it possible to change the carrier concentration n and the carrier mobility µ, and control the specific resistance value ρ (Ω·cm) of the second electron transport layer  106   b . Adjusting the value of B/(A+B) in the range of 0.35 to 0.65 makes it possible to control the specific resistance value of the second electron transport layer  106   b  in the range of 10 2  Ω·cm to 10 6  Ω·cm. More preferably, the specific resistance value of the second electron transport layer  106   b  is controlled in a range of 10 3  Ω·cm to 10 5  Ω·cm. In the case where the thickness of the second electron transport layer  106   b  is as thin as 200 nm, the specific resistance value is preferably adjusted to about 10 5  Ω·cm, and in the case where the thickness of the second electron transport layer  106   b  is as thick as 2000 nm, the specific resistance value is preferably adjusted to about 10 3  Ω·cm. In order to improve the yield, the thickness of the second electron transport layer  106   b  is preferably increased. In the case where the thickness of the second electron transport layer  106   b  is the range of 500 nm to 1000 nm, it is possible to suppress the upper and lower shorting caused by the particles. In the case where the thickness of the second electron transport layer  106   b  is in the range of 500 nm to 1000 nm, the specific resistance value of the second electron transport layer  106   b  is about 10 4  Ω·cm, and the voltage required for light emission can be sufficiently reduced. As shown in  FIG.  31   , the function of the second electron transport layer  106   b  can be improved by multilayering the second electron transport layer  106   b  into two or more layers, enlarging the bandgap in a stepwise manner, and reducing the work function. The bandgap of the second electron transport layer  106   b  is preferably at least 3.0 eV or more, and preferably 3.4 eV or more. The value of the work function of the second electron transport layer  106   b  is also able to carry out good electron transport by selecting the material so that the value of the work function gradually decreases from 3.8 eV to 3.3 eV as the value approaches the light emitting layer. As a result, the applied voltage for emitting light can be reduced, and heat generation can be suppressed, thereby achieving a longer life. 
     In addition, the metal salt contained in the metal oxide  106   b ′ is preferably an inorganic acid salt of the above-mentioned metal. For example, at least one selected from the group consisting of nitrate, sulfate, phosphate, carbonate, bicarbonate, borate, hydrochloride, and hydrofluoric acid can be used as the inorganic acid salt. In addition, hydrochloride and nitrate are preferable as the inorganic acid salt in order to perform the heat treatment after the coating at a lower temperature. 
     Examples of the first amide contained in the metal oxide material 106b′ include compounds represented by Chemical Formula (1) below.  
     
       
         
         
             
             
         
       
     
     (In Formula (I), R1 represents a hydrogen atom, a branched or straight-chain alkyl group having 1 to 6 carbon atoms, an oxygen atom in which a hydrogen atom or a branched or straight-chain alkyl group having 1 to 6 carbon atoms is bonded, or a nitrogen atom in which a hydrogen atom, an oxygen atom, or a branched or straight-chain alkyl group having 1 to 6 carbon atoms are bonded.) 
      Also, the oxygen atom in which a hydrogen atom or a branched or straight-chain alkyl group having 1 to 6 carbon atoms is bonded is —OH or —OR 2  (R 2  is a branched or straight-chain alkyl group having 1 to 6 carbon atoms). In addition, the nitrogen atom in which a hydrogen atom, an oxygen atom, or a branched or straight-chain alkyl group having 1 to 6 carbon atoms is bonded is, for example, —NH 2 , —NHR 3  or —NR 4 R 5  (R3, R 4 , and R 5  are each independently being a branched or straight-chain alkyl group having 1 to 6 carbon atoms). 
     Specific examples of the first amide include acetamide, acetylurea, acrylamide, adipoamide, acetaldehyde semicarbazone, azodicarbonamide, 4-amino-2,3,5,6-tetrafluorobenzamide, β-alaninamide hydrochloride, L-alaninamide hydrochloride, benzamide, benzylurea, biurea, biuret, butylamide, 3-bromopropionamide, butylurea, 3,5-bis(trifluoromethyl)benzamide, tert-butyl carbamate, hexanamide, ammonium carbamate, ethyl carbamate, 2-chloroacetamide, 2-chloroethylurea, crotonamide, 2-cyanoacetamide, butyl carbamate, isopropyl carbamate, methyl carbamate, cyanoacetylurea, cyclopropanecarboxamide, cyclohexylurea, 2,2-dichloroacetamide, dicyandiamidine phosphate, guanylurea sulfate, 1,1-dimethylurea, 2,2-dimethoxypropionamide, ethyl urea, fluoroacetamide, formamide, fumaramide, glycinamide hydrochloride, hydroxyurea, hydantoic acid, 2-hydroxylethylurea, heptafluorobutyramide, 2-hydroxyisobutyramide, isobutyramide, lactamide, malenamide, malonamide, 1-methylurea, nitrourea, oxamic acid, oxamic acid ethyl, oxamide, oxamic acid hydrazide, oxamic acid butyl, phenylurea, phthalamide, propionic acid amide, pivalamide, pentafluorobenzamide, pentafluoropropionamide, semicarbazide hydrochloride, succin acid amide, trichloroacetamide, trifluoroacetamide, urea nitrate, urea, and valeramide. Among them, formamide, urea, and ammonium carbamate are preferable. These may be used in one kind or in a combination of two or more kinds. 
     The solvent contained in the metal oxide material  106   b ′ is intended mainly water. That is, it means that 50 mass% or more of the solvent is water. It is only necessary to use water as the main component, and the solvent may be water only, or a mixed solvent of water and an organic solvent may be used. Specific examples of the organic solvents other than water include ethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, propylene glycol monopropyl ether, methyl ethyl ketone, ethyl lactate, cyclohexanone, γ-butyrolactone, N-methyl pyrrolidone, formamide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-2-pyrrolidone, N-methyl caprolactam, dimethyl sulfoxide, tetramethyl urea, pyridine, dimethyl sulfone, hexamethyl sulfoxide, methanol, ethanol, 1-propanol, isopropanol, n-butanol, 2-butanol, tert-butanol, 1-pentanol, 2-pentanol, 3-pentanol, n-hexanol, cyclohexanol, 2-methyl-2-butanol, 3-methyl-2-butanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol, 3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol, 4-methyl-1-pentanol, 4-methyl-2-pentanol, 2,2-dimethyl-3-pentanol, 2,3-dimethyl-3-pentanol, 2,4-dimethyl-3-pentanol, 4,4-dimethyl-2-pentanol, 3-ethyl-3-pentanol, 1-heptanol, 2-heptanol, 3-heptanol, 2-methyl-2-hexanol, 2-methyl-3-hexanol, 5-methyl-1-hexanol, 5-methyl-2-hexanol, 2-ethyl-1-hexanol, 4-methyl-3-heptanol, 6-methyl-2-heptanol, 1-octanol, 2-octanol, 3-octanol, 2-propyl-1-pentanol, 2,4,4-trimethyl-1-pentanol, 2,6-dimethyl-4-heptanol, 3-ethyl-2,2-dimethyl-pentanol, 1-nonanol, 2-nonanol, 3,5,5-trimethyl-1-hexanol, 1-decanol, 2-decanol, 4-decanol, 3,7-dimethyl-1-octanol, and 3,7-dimethyl-3-octanol. Two or more of these organic solvents may be used in combination. 
     In the present embodiment, although ethylene glycol monomethyl ether (boiling point 124° C.) and propylene glycol monomethyl ether (boiling point 120° C.) are listed as the solvent contained in the metal oxide material  106   b ′ forming the second electron transport layer  106   b , pinholes may occur or roughness of unevenness on the surface of the film may increase depending on coating conditions, drying conditions, and burning conditions. This is due to the low boiling point. On the other hand, using a high boiling solvent such as ethylene glycol (boiling point 198° C.) makes it possible to less likely to occur pinholes and reduces the roughness of the unevenness on the surface of the film. However, in this case, it is easy to remain in the film after burning, and a film with good electron mobility cannot be obtained. 
     For this reason, the compound represented by Chemical Formula (2) is preferable as the solvent contained in the metal oxide material 106b′ forming the second electron transport layer  106   b .  
     
       
         
         
             
             
         
       
     
     (In the formula, R2 represents a straight-chain or branched alkylene group having 2 to 3 carbon atoms, and R3 represents a straight-chain or branched alkyl group having 1 to 3 carbon atoms.) 
     Preferred examples of the solvent represented by Chemical Formula (2) include dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, and dipropylene glycol monopropyl ether. Among them, dipropylene glycol monomethyl ether (boiling point: 188° C.) is particularly preferable as the solvent according to the present embodiment. 
     The content of the first amide in the metal oxide material  106   b ′ is 0.1 to 80 mass%, preferably 5 to 50 mass% with respect to the metal salt. The solid content concentration in the metal oxide material  106   b ′ is 0.1 mass% or more, preferably 0.3 mass% or more, and more preferably 0.5 mass% or more. In addition, the solid content concentration in the metal oxide material 106b′ is 30.0 mass% or less, preferably 20.0 mass% or less, and more preferably 15.0 mass% or less. Here, the solid content concentration is the total concentration of the metal salt and the first amide. 
     The metal oxide material  106   b ′ according to the present embodiment is preferably acidic. In addition, pH of the metal oxide material  106   b ′ is preferably 1 to 3. At least one selected from the group consisting of nitric acid, sulfuric acid, phosphoric acid, carbonic acid, boric acid, hydrochloric acid and hydrofluoric acid can be used to make pH acidic. For this reason, the first electron transport layer  106   a  of the present embodiment is a tin (Sn)-based oxide semiconductor having excellent acid resistance. If the content of tin (Sn) is 10 atm% or more, the first electron transport layer  106   a  is not corroded by the acid liquid. Further, the content of tin (Sn) in the first electron transport layer  106   a  is preferably 20 atm% or more in view of electron mobility and PBTS reliability. 
     In the case where the temperature at which the second electron transport layer  106   b  is formed is reduced to 200° C. or lower, it is preferred to use a metal oxide semiconductor film-forming composition in which an organic group 2 metal compound represented by the following Chemical Formula (3) and an organic group 3 metal compound represented by the following Chemical Formula (4) are dissolved in an organic solvent.  
     
       
         
         
             
             
         
       
     
     (In the formula, R4 represents an alkyl group. M1 represents a group 2 metal element.)  
     
       
         
         
             
             
         
       
     
     (In the formula, R5, R6, and R7 independently represent a hydrogen or alkyl-group. M2 represents a group 3 metal element.) 
     Examples of the organic group 2 metal compound represented by Chemical Formula (3) include diethylzinc and dibutyl magnesium. The bandgap can be controlled in a range of 3.2 eV to 3.6 eV by adding 0 wt% to 40 wt% of dibutyl magnesium or diethylzinc. Adding dibutyl magnesium or diethylzinc makes it possible to control the work function in a range of 3.2 eV to 3.8 eV. 
     Diethylzinc and dibutyl magnesium are ignitable in the atmosphere and must be stored and used with great care. For this reason, without diluting the above materials, it is difficult to apply them by a flexographic printing method, an inkjet printing method, a slit coating method, or the like in an atmosphere where water (H 2 O) is usually present. Diethylzinc and dibutyl magnesium can be dissolved in an organic solvent to reduce the risk of ignition and the like. However, in order to form a thin film using a coating solution in which diethylzinc or dibutyl magnesium is dissolved while reacting with an alcohol-based organic solvent, a high-temperature heat treatment at 400° C. or higher is required. In order to realize low-temperature heat treatment at 200° C. or lower, diisopropyl ether has been proposed as an ether-based solvent. However, the boiling point of diisopropyl ether is as low as 69° C. and clogging of the inkjet head is likely to occur when the inkjet coating method is used. In addition, since the flash point is as low as -28° C., it is difficult to ensure safety when used in a mass production factory. 
     In the present embodiment, it is preferred to use an ether-based solvent with a boiling point in the range of 162° C. to 189° C. and a flash point of 56° C. or higher. Specific examples thereof include diethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, diethylene glycol ethyl methyl ether, and diethylene glycol diethyl ether. Among them, dipropylene glycol dimethyl ether and diethylene glycol diethyl ether with low surface tension is preferred in consideration of applicability. 
     In the case where diethylzinc or dibutyl magnesium is added to enlarge the bandgap and reduce the work function, the specific resistance value of the formed film is 10 6  Ω·cm or more. The specific resistance of the second electron transport layer  106   b  according to the present embodiment is preferably controlled within a range of 10 3  Ω·cm to 10 5  Ω·cm. Therefore, the specific resistance value can be adjusted to the above-described range by doping a small amount of the organic group 3 metal compound represented by Chemical Formula (4). 
     Examples of the organic group 3 metal compound represented by Chemical Formula (4) include triethylaluminum, trimethylgallium, and trimethylindium. Using triethylaluminum, trimethylgallium, trimethylindium, or the like makes it possible to control the specific resistance value of the second electron transport layer  106   b . The amount of doping can be adjusted in the molar ratio of 10 -8  to 10 -4  with respect to diethylzinc. In the case where the thickness of the second electron transport layer  106   b  is as thin as 200 nm, the doping amount is preferably adjusted to about 10 -8  to 10 -–6 , and in the case where the thickness of the second electron transport layer  106   b  is as thick as 2000 nm, the doping amount is preferably adjusted to about 10 -5  to 10 -4 . The doping material may be one described above or a mixture of two or more kinds. In the case where the second electron transport layer  106   b  is as thick as 700 nm to 2000 nm, it is essential that the second electron transport layer  106   b  has a multi-layer structure of two or more layers as shown in  FIG.  31   , and that the bandgap of the layer adjacent to the light emitting layer is increased and the work function is reduced. 
       FIG.  9 B  shows a step of applying the metal oxide material  106   b ′ to the substrate. The metal oxide  106   b ′ is applied to the opening  124  of the second insulating layer  120 . The upper surface of the metal oxide material  106   b ′ is formed higher than the upper surface of the second insulating layer  120  in the opening  124  of the second insulating layer  120 . The metal oxide material  106   b ′ containing the solvent can be formed into a convex shape in which the thickness at the center portion of the opening  124  is larger than that at the end portion due to the surface tension of the metal oxide material  106   b ′. Since the second insulating layer  120  has the upper surface with high liquid repellency and the side surface of the opening  124  with high lyophilic, it is possible to efficiently arrange a sufficiently large amount of the metal oxide material  106   b ′ at a desired position in the opening  124 . 
       FIG.  9 C  shows a step of heat-treating the metal oxide material 106b′. The applied metal oxide  106   b ′ can be heat-treated at a low temperature, for example, at 150° C. or higher and lower than 300° C. to produce a flat and dense amorphous metal oxide semiconductor layer. The temperature at which the metal oxide material  106   b ′ is heat-treated is more preferably 150° C. or higher and 275° C. or lower. The heat treatment time is not particularly limited, it may be, for example, 10 minutes to 2 hours. Also, in order to remove the residual solvent before the heat treatment step, it is preferred to perform a drying step as a pretreatment at 50° C. or higher and lower than 150° C. Performing the heat treatment in this manner makes it possible to form the amorphous metal oxide semiconductor layer at lower temperatures than the conventional method, and the reliability of the EL element  200  can be improved. 
     The second electron transport layer  106   b  is formed with a thickness of 50 nm to 2000 nm, for example, a thickness of 200 nm to 1000 nm. The second electron transport layer  106   b  is formed such that the upper surface of the second electron transport layer  106   b  is lower than the upper surface of the second insulating layer  120  by heat-treating the metal oxide material  106   b ′ containing the solvent so that the solvent volatilizes and the volume shrinks. The second electron transport layer  106   b  may be formed in a single concave surface shape having a larger thickness at the end portion of the opening  124  than that at the central portion due to friction caused by shrinkage of the metal oxide material  106   b ′ containing the solvent while contacting the side surface of the opening  124  and interfacial tension between the metal oxide material  106   b ′ and the second insulating layer  120 . In this way, the upper surface of the second electron transport layer  106   b  can be formed with a less uneven structure. The second electron transport layer  106   b  has a larger thickness at the end portion than that at the central portion of the opening  124 , so that the electric field can be suppressed from concentrating at the end portions of the third electrode  118  and the second electrode  108 , and therefore, the withstand voltage can be improved. In addition, the upper surface of the second electron transport layer  106   b  has one consecutive concave surface shape, so that a corner portion formed by the side surface and the bottom surface of the opening  124  is buffered. Therefore, adhesion at the opening of the electron injection layer  110 , the light emitting layer  112 , the hole transport layer  114 , the hole injection layer  116 , and the third electrode  118  formed on the second electron transport layer  106   b  can be improved. 
     Thereafter, the electron injection layer  110 , the light emitting layer  112 , the hole transport layer  114 , the hole injection layer  116 , and the third electrode  118  are formed, whereby the EL element  200   a  shown in  FIG.  1    is manufactured. The electron injection layer  110  can be formed by a sputtering method using a sputtering target such as the C12A7 electride, Mg 0.3 Zn 0.7 O x , Zn 0.75 Si 0.25 O x , LaMgO X , or MgSiO x . The electron injection layer  110  is formed on substantially the entire surface of the substrate  100  so as to cover the opening  124 . The light emitting layer  112  is formed by a vacuum-deposition method or a printing method. As shown in  FIG.  1    and  FIG.  2   , the light emitting layer  112  may be formed separately for the EL element or may be formed contiguously over a plurality of EL elements formed in the same plane. The hole transport layer  114  and the hole injection layer  116  are formed by a vacuum-deposition method or a coating method. The thickness of the hole transport layer  114  is preferably 200 nm or more in order to reduce surface plasmon loss. Since the EL element  200   a  is of the bottom-emission type, the third electrode  118  is formed by a sputtering method such that a metal film such as aluminum (Al) is stacked on a transparent conductive film such as indium-tin-oxide (ITO). 
     As described above, according to the present embodiment, the EL element  200  can be formed by stacking thin films. Although the EL element  200  according to the present embodiment has the inverted stacked structure in which films are stacked from the cathode side, it has a structure in which the electron transport layer  106  and the electron injection layer  110  are formed of a metal oxide so as not to be damaged in the manufacturing process. In particular, forming the second electron transport layer  106   b  by a coating method makes it possible to increase the thickness of the film, and therefore, it is possible to make the EL element  200  less likely to be short-circuited. In addition, forming a good ohmic contact with the electron transport layer  106  without using an alkali metal in the second electrode  108  makes it possible to reduce the burden on the manufacturing process, and therefore, a chemically stable structure as an element can be obtained. In particular, it can also ensure sufficient reliability even in a thin film sealing method used for a flexible EL panel. 
     Second Embodiment 
     In the present embodiment, an example of a display device (EL display device) in which pixels are formed by the EL element according to an embodiment of the present invention is described. 
       FIG.  10    shows an example of an equivalent circuit of a pixel  302  arranged in the display device according to the present embodiment. The pixel  302  includes a select transistor  136 , a drive transistor  138 , and a capacitive element  140  in addition to the EL element  200 . In the select transistor  136 , a gate is electrically connected to scan signal line  132 , a source is electrically connected to a data signal line  134 , and a drain is electrically connected to a gate of the drive transistor  138 . In the drive transistor  138 , a source is electrically connected to a common potential line  144  and a drain is electrically connected to the second electrode  108  of the EL element  200 . The capacitive element  140  is electrically connected between the gate of the drive transistor  138  and the common potential line  144 . In the EL element  200 , the first electrode  102  is electrically connected to a signal line for controlling the amount of carrier injection  146 , and the third electrode  118  is electrically connected to a power line  142 .  FIG.  10    shows the case where the select transistor  136  and the drive transistor  138  are of a double-gate type. 
     In an equivalent circuit of the pixel  302  shown in  FIG.  10   , a scan signal is supplied to the scan signal line  132 , and a data signal (video signal) is supplied to the data signal line  134 . A power potential (Vdd) is applied to the power line  142 , and a ground potential or a potential (Vss) lower than a ground potential (earth potential) is applied to the common potential line  144 . A voltage (Vg) for controlling the amount of carriers injected into the light emitting layer  112  is applied to the signal line for controlling the amount of carrier injection  146 , as described with reference to  FIG.  6   . The voltage (Vg) for controlling the amount of carrier injection may be a constant positive voltage or may be a voltage that varies between predetermined voltages Vg1 and Vg2 as shown in  FIG.  6    and  FIGS.  33 A to  33 D . A sine waveform, a staircase waveform, a trapezoidal waveform, a triangular waveform, or the like is used as the fluctuating potential waveform. Light emission of the EL element  200  can be stopped by bringing the value of the carrier-amount-control signal voltage (Vg) closer to the ground potential. That is, the light emitting period can also be controlled. That is, the signal line for controlling the amount of carrier injection  146  can function as an enable line for controlling light emission (ON) and non-light emission (OFF) of the EL element  200 . 
     In the pixel  302  shown in  FIG.  10   , a voltage based on the data signal is applied from the data signal line  134  to the gate of the drive transistor  138  when the select transistor  136  is turned on. The capacitive element  140  holds the source-gate voltage of the drive transistor  138 . When the drive transistor  138  is turned on, a current flows into the EL element  200  from the power line  142  to emit light. In the case where a voltage (Vg) for controlling the amount of carrier injection is applied to the first electrode  102 , not only the emission intensity of the EL element  200  can be controlled, but also the position of an area (in other words, the light emitting area) at which the electrons and the holes in the light emitting layer  112  recombine with each other can be controlled. That is, the carrier balance in the light emitting layer  112  can be controlled. 
     According to the present embodiment, the light emission state of the EL element can be controlled by forming the pixel  302  in the EL element arranged with the electrode for controlling the amount of carrier injection (the first electrode  102 ), arranging the signal line for controlling the amount of carrier injection  146 , and connecting to the electrode for controlling the amount of carrier injection (the first electrode  102 ). That is, the recombination area of the holes and the electrons injected into the light emitting layer  112  can be concentrated on the central portion area of the light emitting layer  112  by controlling the light emission of the EL element not only by the drive transistor  138  but by controlling the amount of electrons injected into the light emitting layer  112  by the electrode for controlling the amount of carrier injection (the first electrode  102 ). As a result, degradation of the EL element can be suppressed, and the reliability of the EL display device can be improved. 
       FIG.  11    shows a plan view of the pixel  302  of the display device according to the present embodiment.  FIG.  12 A  shows a cross-sectional structure along a line of A1-A2 shown in  FIG.  11   , and  FIG.  12 B  shows a cross-sectional structure along a line of B1-B2 shown in  FIG.  11   . In the following description, reference is made to these drawings. 
     In the pixel  302 , the select transistor  136 , the drive transistor  138 , the capacitive element  140 , and the EL element  200  are arranged. In a plan view of the pixel  302  shown in  FIG.  11   , the arrangement of the first electrode  102 , the first electron transport layer  106   a , and the opening  124  is shown as the components of the EL element  200 . 
     The drive transistor  138  includes a first oxide semiconductor layer  152   a , a first gate electrode  154   a , and a second gate electrode  156   a . The first gate electrode  154   a  and the second gate electrode  156   a  are arranged so as to have an area overlapping with each other with the first oxide semiconductor layer  152   a  interposed therebetween. The first insulating layer  104  is arranged between the first oxide semiconductor layer  152   a  and the substrate  100 . The first gate electrode  154   a  is arranged between the first insulating layer  104  and the substrate  100 . In addition, the third insulating layer  122  is arranged between the first oxide semiconductor layer  152   a  and the second gate electrode  156   a . In other words, the first oxide semiconductor layer  152   a  is arranged between the first insulating layer  104  and the third insulating layer  122 , and the first insulating layer  104 , the first oxide semiconductor layer  152   a , and the third insulating layer  122  are interposed between the first gate electrode  154   a  and the second gate electrode  156   a . 
      In the present embodiment, the drive transistor  138  has a double-gate structure in which the first oxide semiconductor layer  152   a  is sandwiched between the first gate electrode  154   a  and the second gate electrode  156   a . In the drive transistor  138 , a channel area is formed in an area where the first oxide semiconductor layer  152   a  overlaps with one or both of the first gate electrode  154   a  and the second gate electrode  156   a . In the first oxide semiconductor layer  152   a , the carrier concentration of the area as the channel area is preferably 1 × 10 14  to 5 × 10 18 /cm 3 . 
     In the drive transistor  138 , the second electrode  108   a  and the second electrode  108   b  are arranged between the first oxide semiconductor layer  152   a  and the first insulating layer  104 . The second electrode  108   a  and the second electrode  108   b  are arranged apart from each other. The second electrode  108   a  and the second electrode  108   b  are arranged so as to be in contact with the first oxide semiconductor layer  152   a , thereby functioning as a source area and a drain area. In addition, a conductive layer  150   a  is arranged between the second electrode  108   a  and the first oxide semiconductor layer  152   a , and a conductive layer  150   b  is arranged between the second electrode  108   b  and the first oxide semiconductor layer  152   a . The conductive layer  150   a  is arranged inside the second electrode  108   a  which does not reach the end portion, and the conductive layer  150   b  is arranged inside the second electrode  108   b  which does not reach the end portion. 
     The first gate electrode  154   a  and the second gate electrode  156   a  are arranged so as to overlap with each other at an area where the second electrode  108   a  and the second electrode  108   b  are separated from each other. The second electrode  108   a  and the second electrode  108   b  may be arranged such that a part of area overlaps one or both of the first gate electrode  154   a  and the second gate electrode  156   a . At least one of the second electrodes  108   a  and  108   b  may be arranged so as to overlap with one or both of the first gate electrode  154   a  and the second gate electrode  156   a  so as to adjacent to the channel area of the first oxide semiconductor layer  152   a , thereby increasing the drain current of the drive transistor  138 . Also, the first gate electrode  154   a  is arranged in the same layer-structure as the common potential line  144 . 
     As shown in  FIG.  12 B , the select transistor  136  has a structure similar to that of the drive transistor  138 . That is, the select transistor  136  includes a second oxide semiconductor layer  152   b , a first gate electrode  154   b , and a second gate electrode  156   b . In addition, a second electrode  108   c  and a conductive layer  150   c , and the second electrode  108   d  and a conductive layer  150   d  are included in contact with the second oxide semiconductor layer  152   b . The second electrode  108   c  and the second electrode  108   d  are arranged apart from each other. The second electrode  108   c  and the second electrode  108   d  are arranged so as to be in contact with the second oxide semiconductor layer  152   b , thereby functioning as a source area and a drain area. The conductive layer  150   c  forms the data signal line  134 . 
     The capacitive element  140  is formed in an area where the second  108   d  overlaps with a capacitance electrode  162  via the first insulating layer  104 . The capacitance electrode  162  is also formed as the common potential line  144 . 
      Also, in the present embodiment, the oxide semiconductor layer  152  may be made of the same material as the oxide semiconductor material of the first electron transport layer  106   a  described in the first embodiment. In addition, an inorganic insulating material is used as the first insulating layer  104  and the third insulating layer  122 . Examples of the inorganic insulating material include silicon oxide, silicon nitride, silicon oxynitride, and aluminum oxide. 
     The EL element  200  has a similar configuration as that shown in the first embodiment. The EL element  200  is electrically connected to the drive transistor  138 . In the EL element  200 , an area corresponding to the second electrode  108  is contiguously formed from the drive transistor  138 . With such a configuration, the routing of wirings is simplified, and the opening ratio of the pixel  302  (the ratio of the area where the EL element actually emits light with respect to the area occupied by one pixel) can be increased. 
     Also, in order to increase the opening ratio of the pixel, the electron mobility of the first electron transport layer  106   a  needs to be increased. For example, a pixel pitch in an 85-inch display panel with a resolution of 8K × 4K (7680 × 4320 pixels) is about 244 µm. In this case, when a rectangular pixel is assumed, the length in the longitudinal direction is about 732 µm. When it is assumed that the time from the application of the voltage to the EL element until the light emission is about 4 to 5 µsec, and in the case where an area corresponding to the second electrode  108  is arranged at one end in the longitudinal direction of the rectangular pixel, the carrier (electron) mobility of the first electron transport layer  106   a  is 10 cm 2 /V·sec or more, preferably 20 cm 2 /V· sec or more, otherwise it is difficult for carriers (electrons) to reach the other end in the longitudinal direction of the pixel. 
     Of course, if the area corresponding to the second electrode  108  is arranged so as to surround the four sides of the rectangular pixel, the carrier drifts from the four sides toward the center, so that the carrier (electron) mobility may be about 2.5 cm 2 /V·sec. However, in this case, the effective light emitting area per pixel is reduced, and the opening rate is lowered. 
     Under such circumstances, the oxide semiconductor material as exemplified in the present embodiment can realize the required carrier (electron) mobility. On the other hand, the electron transport layers formed of the organic materials as described in Japanese Laid-Open Patent Publication No. 2007-149922 and Japanese Laid-Open Patent Publication No. 2007-157871 have a carrier (electron) mobility of 2.5 cm 2 /V·sec or less, and therefore cannot realize a large-screen and high-definition display panel. In the case of a tin (Sn)-based oxide semiconductor TFT, since the electron mobility can be 20 cm 2 /V·sec or more, the opening rate can be increased even in the bottom-emission type, and the life of the EL element can be prolonged. 
     As described above, the display device according to the present embodiment can be manufactured through the same manufacturing process as elements such as the drive transistor and the select transistor manufactured using the oxide semiconductor layer by using the oxide semiconductor layer as the electron transport layer for forming the EL element. In addition, in the display device according to the present embodiment, an electrode for controlling the amount of carrier injection is arranged in the EL element, and an electron transport layer with a high carrier (electron) mobility is arranged with respect to the electrode for controlling the amount of carrier injection via an insulating layer, so that the emission intensity in the pixel plane can be made uniform and high definition can be achieved. 
       FIG.  12 A  and  FIG.  12 B  show a cross-sectional structure of the pixel  302  shown in  FIG.  11   .  FIG.  12 A  shows a cross-sectional structure along a line of A1-A2 shown in  FIG.  11   , and  FIG.  12 B  shows a cross-sectional structure along a line of B1-B2. The EL element  200  is arranged such that the second electrode  108  is in contact with the lower layer of the first electron transport layer  106   a . In the EL element  200 , an area corresponding to the second electrode  108  is contiguously formed from the drive transistor  138 . The drive transistor  138  and the select transistor  136  have a bottom contact structure in which the second electrodes  108   a ,  108   b ,  108   c , and  108   d  are arranged in contact with the lower layer of the first oxide semiconductor layer  152   a  and the second oxide semiconductor layer  152   b  in which the channel is formed. The drive transistor  138  and the select transistor  136  have a double-gate structure in which the first gate electrodes  154   a ,  154   b  and the second gate electrodes  156   a ,  156   b  are arranged so as to sandwich the first oxide semiconductor layer  152   a  and the second oxide semiconductor layer  152   b . 
     As shown in  FIG.  12 A , the first electron transport layer  106   a  of the EL element  200  may be arranged so as to be contiguous with the first oxide semiconductor layer  152   a  of the drive transistor  138 . The first electron transport layer  106   a  may be formed of an oxide semiconductor material, so that it can be formed using the same layer as the first electron transport layer  152   a  arranged for the drive transistor  138 . Also, the present embodiment is not limited to the structure shown in  FIG.  12 A , and the first electron transport layer  106   a  and the first oxide semiconductor layer  152   a  may be separated from each other or may be formed of different layers. 
     In cross-sectional view of  FIG.  23    and  FIG.  24   , the first electron transport layer  106   a  and the first oxide semiconductor layer  152   a  are completely separated and formed in different layers. The EL element of  FIG.  23    and  FIG.  24    is classified into an EL element  200   c  of  FIG.  13   . The first electrode (electrode for controlling the amount of carrier injection)  102  of the EL element of  FIG.  23    and  FIG.  24    is formed of the same material in the same layer as the source electrode and drain electrode of the transistor element for driving the EL element. The first electron transport layer  106   a  is formed on the third insulating layer  122 . Different materials can be used for the first electron transport layer  106   a  and the first oxide semiconductor layer  152   a  of the transistor element for driving the EL element in the EL element structure of  FIG.  23    and  FIG.  24   , and materials suitable for each of them can be freely selected. Although there is a disadvantage in that the number of photolithography processes is increased by one, there is an advantage in that the performance of the EL element can be improved. In  FIG.  23    and  FIG.  24   , the second electrode (cathode)  108  and the second gate electrode  156   a  of a driving transistor element are formed of the same material and simultaneously in the same layer. Although not shown in this drawing, the data signal line  134  connected to a select transistor element is also formed of the same material and simultaneously in the same layer as the second electrode (cathode)  108  and the second gate electrode  156   a  of the driving transistor element. This is the same structure as a cross-sectional view shown in  FIG.  25 B . 
     The drive transistor  138  and the select transistor  136  in  FIG.  12 A  are covered by the third insulating layer  122  and the second insulating layer  120 . The opening  124  that penetrates the second insulating layer  120  and the third insulating layer  122  and exposes the first electron transport layer  106   a  is arranged in an area where the EL element  200  is arranged. When the second electron transport layer  106   b  is formed by the coating method, the drive transistor  138  and the select transistor  136  are covered by the second insulating layer  120  and the third insulating layer  122 , and therefore, the coating film is prevented from being directly attached. The pixel  302  having such a configuration can prevent degradation in the manufacturing process of the drive transistor  138  and the select transistor  136 . When the second electron transport layer  106   b  formed by the coating method is heat- burning, the select transistor  136  and the drive transistor  138  can also be annealed at the same time, and therefore, the cost can be reduced. 
     As described above, according to the present embodiment, it is possible to obtain a display device including the pixel  302  using the EL element  200  shown in the first embodiment. Further, according to the present embodiment, since the EL element  200  has a configuration in which a short-circuit defect is less likely to occur, it is possible to provide a high-quality display device with few pixel defects. 
      Third Embodiment 
       FIG.  13    and  FIG.  14    show EL elements having cathode structures different from the EL element shown in the first embodiment. In the following description, portions different from those of the first embodiment will be described, and description of common portions will be omitted. 
       FIG.  13    shows a cross-sectional structure of the EL element  200   c  according to another embodiment of the present invention. The EL element  200   c  is different from the EL element described in the first embodiment in that the second electrode  108  is arranged in contact with the upper layer of the first electron transport layer  106   a . The first electron transport layer  106   a  includes an area that overlaps with the second electron transport layer  106   b , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  via the opening  124 , and further includes, an area that does not overlap with the second electron transport layer  106   b  on the outer side of the area. The second electrode  108  is arranged in an area where the second electron transport layer  106   a  does not overlap with the second electron transport layer  106   b , that is, in an area between the end portion of the first electron transport layer  106  and the end portion of the second electron transport layer  106   b . The second electrode  108  is arranged on a surface of the first electron transport layer  106   a  opposite to the side of the first insulating layer  104 . Further, the wiring  111  may be arranged in contact with the second electrode  108 . The wiring  111  is arranged between the second electrode  108  and the second insulating layer  120 . However, the present invention is not limited thereto, and as shown in  FIG.  14   , the wiring  111  may be arranged between the second electrode  108  and the first electron transport layer  106   a . 
     The contact area can be increased by arranging the second electrode  108  in contact with the surface of the first electron transport layer  106   a  of the EL element  200   c . As a result, the series resistance component of the EL element  200   c  is reduced and the drive voltage can be lowered. In addition, the EL element  200   c  can reduce the current density flowing into the second electrode  108 . 
     In the EL element  200   c , an area corresponding to the second electrode  108  is contiguously formed from the drive transistor  138 . That is, although not shown in the drawings, the drive transistor  138  and the select transistor  136  may have a top-contact structure in which the second electrodes  108   a ,  108   b ,  108   c , and  108   d  are arranged in contact with the upper layer of the first oxide semiconductor layer  152   a  and the second oxide semiconductor layer  152   b  in which the channels are formed. 
     The EL element  200   c  and an EL element  200   d  are the same as those of the first embodiment except that the second electrode  108  has the top contact structure. In addition, the EL element  200   c  and the EL element  200   d  can be replaced with the EL element  200  shown in  FIG.  12 A . 
     Fourth Embodiment 
       FIGS.  15 A and  15 B  shows an EL element having no first electrode and having a different cathode configuration from the EL element shown in the first embodiment. In the following description, portions different from those of the first embodiment will be described, and description of common portions will be omitted. 
       FIG.  15 A  shows a cross-sectional structure of an EL element  200   e  according to another embodiment of the present invention. The EL element  200   e  is different from the EL element shown in the first embodiment in that the first electrode  102  is not arranged. Although the first electron transport layer  106   a  is arranged on the second electrode  108 , the wiring  111  may not be arranged. The first electron transport layer  106   a  is arranged on the lower layer side of the second insulating layer  120  and exposed by the opening  124  of the second insulating layer  120 . The second electron transport layer  106   b  is in contact with the first electron transport layer  106   a  in the opening  124 . The outer end portion of the first electron transport layer  106   a  is arranged on the outer side of the area  124   a  where the opening  124  of the second insulating layer  120  is arranged. As a result, the first electron transport layer  106   a  includes an area that overlaps with the second electron transport layer  106   b , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  via the opening  124 , and further includes an area that does not overlap with the second electron transport layer  106   b  on the outer side of the area. The end portion of the second electrode  108  is arranged in an area where the first electron transport layer  106   a  does not overlap with the second electron transport layer  106   b , that is, in an area between the end portion of the first electron transport layer  106   a  and the end portion of the second electron transport layer  106   b . The wiring  111  may be arranged between the second electrode  108  and the first insulating layer  104 . The wiring  111  is arranged in an outer peripheral portion that is apart from the periphery of the area  124   a  where the opening  124  is arranged by the offset area  126 . 
     The contact area can be increased by arranging the first electron transport layer  106   a  in contact with the surface of the second electrode  108  of the EL element  200   e . As a result, the series resistance component of the EL element  200   e  is reduced and the drive voltage can be lowered. In addition, the EL element  200   e  can reduce the current density flowing into the second electrode  108 . 
     In the EL element  200   e  shown in  FIG.  15 A , a transparent conductive film formed of indium-tin oxide (In 2 O 3 ·SnO 2 : ITO) or the like is used as the second electrode  108 . That is, the EL element  200   e  may have a configuration in which the first electron transport layer  106   a  is in direct contact with the second electrode  108  formed of the transparent conductive film. In the EL element  200   e , even if the second electrode  108  is replaced with a transparent conductive film, since the electron transport layer  106  is made thick by the two-layer stacked structure of the first electron transport layer  106   a  and the second electron transport layer  106   b , a short circuit can be prevented, and an electrically stable structure can be obtained. 
       FIG.  25 A  and  FIG.  25 B  show a pixel structure cross-sectional view in the case where the EL element  200   e  having the same structure as that of  FIG.  15 A  is used, and  FIG.  26    shows a pixel structure plan view. Although a fourth insulating layer (passivation film)  170  is not shown in  FIG.  1   ,  FIG.  2   ,  FIG.  12    to  FIG.  17   ,  FIG.  20   ,  FIG.  29   , and  FIG.  30   , the fourth insulating layer (passivation film)  170  is preferably formed as shown in  FIG.  23    to  FIG.  25   ,  FIG.  27   , and  FIG.  28    in order to improve the long-term reliability of each element. A silicon-nitride film, an alumina film, or the like is used as the fourth insulating layer (passivation film)  170 . 
     Although the bottom-emission type EL element of  FIG.  15 A  is used in  FIG.  25 A , the top-emission type EL element of  FIG.  15 B  can also be used. The top-emission type EL element can be formed in  FIG.  25 A  by forming the common potential line  144  so as to cover the entire area of the opening  124 . As shown in  FIG.  25 B , the data signal line  134  and the second gate electrode  156  are formed of the same material simultaneously in the same layer. 
     In the EL element  200   e  shown in  FIG.  15 B , a top-emission type EL element can also be formed by coating the wiring  111  with the second electrode  108  and the first electron transport layer  106   a .  FIG.  27    to  FIG.  30   ,  FIG.  40   , and  FIG.  41    show cross-sectional views of the EL element in which the second electrode (cathode)  108  of the EL element  200   e  in  FIG.  15 A  is completely separated from the source electrode and drain electrode of the driving transistor element and formed in a different layer. The second electrode (cathode)  108  is formed of the transparent conductive film such as ITO or IZO to form a bottom-emission type EL element, and the second electrode (cathode)  108  is formed of a metal film having a high visible-light reflectance to form a top-emission type EL element. 
     The EL element  200   e  according to the present embodiment is the same as the EL element  200   a  shown in  FIG.  1    except that the first electrode is omitted and the configuration of the second electrode  108  is different, and the same advantageous effects can be obtained. In addition, the EL element  200   e  can be replaced with the EL element  200  shown in  FIG.  12 A . 
     Fifth Embodiment 
     An EL element having a cathode structure different from that of the EL element shown in the first embodiment is shown. In the following description, portions different from those of the first embodiment will be described, and description of common portions will be omitted. 
       FIG.  16    shows a cross-sectional view of an EL element  200   f  according to another embodiment of the present invention. The EL element  200   f  is different from the EL element described in the first embodiment in that the second electrode  108  arranged below the first electron transport layer  106   a  is arranged only in a part of area of the peripheral portion of the first electron transport layer  106   a . The first electron transport layer  106   a  includes an area that overlaps with the second electron transport layer  106   b , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  via the opening  124 , and further includes an area that does not overlap with the second electron transport layer  106   b  at the outer side of the area  124   a  where the opening  124  is arranged. The second electrode  108  is arranged in an area where the second electron transport layer  106   a  does not overlap with the second electron transport layer  106   b , that is, in an area between the end portion of the first electron transport layer  106   a  and the end portion of the second electron transport layer  106   b . The second electrode  108  is preferably arranged so as to surround the outer periphery of the first electron transport layer  106   a . However, as shown in  FIG.  16   , in the EL element  200   f  according to the present embodiment, the second electrode  108  is arranged only in a part of area of the peripheral portion of the first electron transport layer  106   a . The second electrode  108  is arranged between the first electron transport layer  106   a  and the first insulating layer  104 . Further, the wiring  111  may be arranged in contact with the second electrode  108 . The wiring  111  may be arranged between the second electrode  108  and the first electron transport layer  106   a . As shown in  FIG.  16   , a wiring  146  may be arranged between the first electrode  102  and the first insulating layer  104 . The wiring  146  may be arranged between the substrate  100  and the first electrode  102 . 
     The EL element  200   f  is the same as that of the first embodiment except that the configuration of the second electrode  108  is different, and the same advantageous effects can be obtained. In addition, the EL element  200   f  can be replaced with the EL element  200  shown in  FIG.  12 A . 
     Sixth Embodiment 
     The present embodiment shows an EL element in which the configuration of the cathode is different from that of the EL element shown in the first embodiment. In the following description, portions different from those of the first embodiment will be described, and description of common portions will be omitted. 
       FIGS.  17 A and  17 B  shows a cross-sectional view of an EL element  200   g  according to the present embodiment. The EL element  200   g  is different from the EL element shown in the first embodiment in that a fourth electrode  105  is further arranged on the first electron transport layer  106   a  via the third insulating layer  122 . The first electron transport layer  106   a  includes an area that overlaps with a second electron transport layer  106   b , the electron injection layer  110 , the light emitting layer  112 , and the third electrode  118  via the opening  124 , and further includes an area that does not overlap with the second electron transport layer  106   b  at the outer side of the area. The second electrode  108  is arranged in an area where the second electron transport layer  106   a  does not overlap with the second electron transport layer  106   b , that is, in an area between the end portion of the first electron transport layer  106   a  and the end portion of the second electron transport layer  106   b . The second electrode  108  is arranged between the first electron transport layer  106   a  and the first insulating layer  104 . Further, the wiring  111  may be arranged in contact with the second electrode  108 . The wiring  111  may be arranged between the second electrode  108  and the first electron transport layer  106   a . The wiring  146  may be arranged in contact with the first electrode  102 . As shown in  FIGS.  17 A and  17 B , the wiring  146  may be arranged between the first electrode  102  and the first insulating layer  104 . 
     The fourth electrode  105  is arranged in an area where the first electron transport layer  106   a  does not overlap with the second electron transport layer  106   b , that is, in an area between the end portion of the first electron transport layer  106   a  and the end portion of the second electron transport layer  106   b . The fourth electrode  105  is arranged so as to overlap with the first electrode  102  with the third insulating layer  122 , the first electron transport layer  106   a , and the first insulating layer  104  interposed therebetween. The fourth electrode  105  is arranged between the third insulating layer  122  and the second insulating layer  120  on the first electron transport layer  106   a . The fourth electrode  105  is electrically connected to the first electrode  102  via a contact hole. With such a configuration, it is possible to prevent the current from concentrating only at the end portion of the second electrode  108 , and it is possible to greatly improve the reliability of the element. The fourth electrode  105  is arranged so as to overlap with the third electrode  118  with the second insulating layer  120  interposed therebetween. Since the end portion of the fourth electrode  105  is not exposed to the opening  124  of the second insulating layer  120 , it is configured so that an electric field concentration does not occur between the third electrode  118  and the fourth electrode  105  in the light emitting area. 
     In the EL element  200   g  shown in  FIGS.  17 A and  17 B , the fourth electrode  105  is arranged so as to overlap with the first electrode  102  with the first electron transport layer  106   a  interposed therebetween. The fourth electrode  105  is preferably controlled to have the same potential as the first electrode  102 . Since the fourth electrode  105  and the first electrode  102  are controlled to have the same potential, an electric field is applied from both the front and back surfaces of the first electron transport layer  106   a , and the amount of carriers (electrons) injected into the light emitting layer  112  can be controlled by the same principles as those of the double-gate transistor. The amount of carriers (electrons) transported from the electron transport layer  106  (the first electron transport layer  106   a  and the second electron transport layer  106   b ) to the light emitting layer  112  can be controlled by the electric field strength of the first electrode  102  and the fourth electrode  105 . When the voltage applied to the first electrode  102  and the fourth electrode  105  increases, the electric field acting on the first electron transport layer  106   a  also increases. Since the electric field generated by applying a positive voltage to the first electrode  102  and the fourth electrode  105  acts to draw carriers (electrons) from the second electrode  108  to the first electron transport layer  106   a , the amount of carriers (electrons) transported to the light emitting layer  112  can be further increased. That is, the balance (carrier balance) between the amount of electrons injected from the second electrode  108  and the amount of positive holes injected from the third electrode  118  can be adjusted by controlling the voltage applied to the first electrode  102  and the fourth electrode  105 . On the other hand, since the potentials of the first electrode  102  and the fourth electrode  105  become the common potential (Vss), leakage of carriers (electrons) in the first electron transport layer  106   a  can be suppressed, and the first electron transport layer  106   a  becomes an insulating state (depletion state). As a result, no current flows through the EL element  200   g , and no light is emitted (non-light emitting state). 
     The EL element  200   g  is the same as that of the first embodiment except that it has the fourth electrode  105 , and the same advantageous effects can be obtained. The EL element  200   g  can be replaced with the EL element  200  shown in  FIG.  12 A . 
     Seventh Embodiment 
     An example of the display device (EL display device) in which a pixel is formed by the EL element  200   g  according to the present embodiment will be described. In the following description, portions different from those in the second embodiment will be described, and description of common portions will be omitted. 
       FIG.  18    shows an example of an equivalent circuit of a pixel  302   g  arranged in the display device according to the present embodiment. In addition to the EL element  200   g , the pixel  302   g  includes the select transistor  136 , the drive transistor  138 , and the capacitive element  140 . In the select transistor  136 , a gate is electrically connected to the scan signal line  132 , a source is electrically connected to the data signal line  134 , and a drain is electrically connected to a gate of the drive transistor  138 . In the drive transistor  138 , a source is electrically connected to the common potential line  144  and a drain is electrically connected to the second electrode  108  of the EL element  200   g . The capacitive element  140  is electrically connected between the gate of the drive transistor  138  and the common potential line  144 . In the EL element  200   g , the first electrode  102  and the fourth electrode  105  are electrically connected to the signal line for controlling the amount of carrier injection  146 , and the third electrode  118  is electrically connected to the power line  142 .  FIG.  18    shows the case where the EL element  200   g , the select transistor  136 , and the drive transistor  138  are of a double-gate type. 
     In the pixel  302   g  shown in  FIG.  18   , when the select transistor  136  is turned on, a voltage based on the data signal is applied from the data signal line  134  to the gate of the drive transistor  138 . The capacitive element  140  holds the source-gate voltage of the drive transistor  138 . When the drive transistor  138  is turned on, a current flows into the EL element  200   g  from the power line  142  to emit light. In this case, when a voltage (Vg) for controlling the amount of carrier injection is applied to the first electrode  102  and the fourth electrode  105 , it is possible not only to control the emission intensity of the EL element  200   g  but also to control the position of the area at which the electrons and the holes in the light emitting layer  112  recombine with each other (in other words, the light emitting area). That is, the carrier balance in the light emitting layer  112  can be controlled. 
     According to the present embodiment, the light emission state of the EL element can be controlled by forming the pixel  302   g  with the EL element  200   g  in which the electrodes for controlling the amount of carrier injection (the first electrode  102  and the fourth electrode  105 ) are arranged and arranging the signal line for controlling the amount of carrier injection to connect to the electrodes for controlling the amount of carrier injection (the first electrode  102  and the fourth electrode  105 ). That is, degradation of the EL element can be suppressed by controlling the amount of electrons injection to the light emitting layer  112  by the electrodes for controlling the amount of carrier injection, rather than controlling the light emission of the EL element only by the drive transistor, and the reliability of the EL display device can be further enhanced. 
       FIG.  19    shows a plan view of the pixel  302   g  of the display device according to the present embodiment. The select transistor  136 , the drive transistor  138 , the capacitive element  140 , and the EL element  200   g  are arranged in the pixel  302   g . In a plan view of the pixel  302   g  shown in  FIG.  19   , the arrangement of the first electrode  102 , the fourth electrode  105 , the first electron transport layer  106   a , and the opening  124  is shown as the components of the EL element  200   g . 
     The drive transistor  138  includes a first oxide semiconductor layer  152   a , the first gate electrode  154   a , and the second gate electrode  156   a . The first gate electrode  154   a  and the second gate electrode  156   a  are arranged so as to have an area overlapping with each other with the first oxide semiconductor layer  152   a  interposed therebetween. That is, the drive transistor  138  has a double-gate structure in which the first oxide semiconductor layer  152   a  is sandwiched between the first gate electrode  154   a  and the second gate electrode  156   a . 
     The select transistor  136  has a structure similar to that of the drive transistor  138 . That is, the select transistor  136  includes the second oxide semiconductor layer  152   b , the first gate electrode  154   b , and the second gate electrode  156   b . 
     The capacitive element  140  is formed in an area where the second electrode  108   d  overlaps with the capacitance electrode  162  via the first insulating layer  104 . The capacitance electrode  162  is also formed as the common potential line  144 . 
     The EL element  200   g  has a similar configuration as that of the EL element  200   g  shown in  FIGS.  17 A and  17 B . The EL element  200   g  is electrically connected to the drive transistor  138 . In the EL element  200   g , an area corresponding to the second electrode  108  is contiguously formed from the drive transistor  138 . With such a configuration, the routing of wirings is simplified, and the opening ratio of the pixel  302  (the ratio of the area where the EL element actually emits light with respect to the area occupied by one pixel) can be increased. 
     Also, in the present embodiment, the oxide semiconductor layer  152  may be made of the same material as the oxide semiconductor material of the first electron transport layer  106   a  described in the first embodiment. In addition, inorganic insulating material is used as the first insulating layer  104 , the third insulating layer  122 , and a fourth insulating layer  119 . Silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, or the like may be used as the inorganic insulating material. The fourth electrode  105  may be made of the same material as that of the second gate electrode  156   a . 
     As described above, the display device according to the present embodiment can be manufactured through the same manufacturing process as elements such as the drive transistor and the select transistor manufactured using the oxide semiconductor layer by using the oxide semiconductor layer as the electron transport layer for forming the EL element. In addition, in the display device according to the present embodiment, an electrode for controlling the amount of carrier injection is arranged in the EL element, and an electron transport layer with a high carrier (electron) mobility is arranged with respect to the electrode for controlling the amount of carrier injection via an insulating layer, so that the emission intensity in the pixel plane can be made uniform and high definition can be achieved. 
     Eighth Embodiment 
     An example of the display device (EL display device) in which a pixel is formed by an EL element  200   h  according to the present embodiment will be described. In the following description, portions different from those in the second embodiment will be described, and descriptions of common portions will be omitted. 
       FIGS.  20 A and  20 B  shows a cross-sectional view of a display device in which the pixel is formed by the EL element  200   h  according to the present embodiment. The select transistor  136 , the drive transistor  138 , the capacitive element  140 , and the EL element  200   h  are arranged in the pixel  302 . 
     The drive transistor  138  includes the first oxide semiconductor layer  152   a , the first gate electrode  154   a , and the second gate electrode  156   a . The first gate electrode  154   a  and the second gate electrode  156   a  are arranged so as to have an area overlapping with each other with the first oxide semiconductor layer  152   a  interposed therebetween. That is, the drive transistor  138  has a double-gate structure in which the first oxide semiconductor layer  152   a  is sandwiched between the first gate electrode  154   a  and the second gate electrode  156   a . 
     The first oxide semiconductor layer  152   a  is a transparent oxide semiconductor containing one or more elements selected from indium (In), zinc (Zn), gallium (Ga), tin (Sn), aluminum (Al), tungsten (W), and silicon (Si). For example, a quaternary oxide material, a ternary oxide material, and a binary oxide material exhibiting semiconducting properties are used as the oxide semiconductor material for forming the first oxide semiconductor layer  152   a . For example, it is preferred to use In 2 O 3 —Ga 2 O 3 —SnO 2 —ZnO—based oxide material as the quaternary oxide material, In 2 O 3 —Ga 2 O 3 —SnO 2 —based oxide material, In 2 O 3 —Ga 2 O 3 —ZnO—based oxide material, In 2 O 3 —SnO 2 —ZnO—based oxide material, In 2 O 3 —Al 2 O 3 —ZnO—based oxide material, Ga 2 O 3 —SnO 2 —ZnO—based oxide material, Ga 2 O 3 —Al 2 O 3 —ZnO—based oxide material, and SnO 2 —Al 2 O 3 —ZnO—based oxide material as the ternary oxide material, and In 2 O 3 —SnO 2 —based oxide material, In 2 O 3 —ZnO—based oxide material, SnO 2 —ZnO—base oxide material, Al 2 O 3 —ZnO—based oxide material, Ga 2 O 3 —ZnO—based oxide material, SnO 2 —SiO 2 —based oxide material, and In 2 O 3 —W 2 O 3 —based oxide material as the binary oxide material, and it is particularly preferred to use In 2 O 3 —Ga 2 O 3 —SnO 2 —based oxide material. In addition, the oxide semiconductor may include tantalum (Ta), scandium (Sc), nickel (Ni), lanthanum (La), magnesium (Mg), hafnium (Hf), idithrium (Y), titanium (Ti), and samarium (Sm). For example, the In—Ga—Sn—O—based oxide material described above is an oxide material containing at least In, Ga, and Sn, and the composition ratio thereof is not particularly limited. More preferably, the composition ratio with respect to In, Ga, and Sn of the In—Ga—Sn—O—based oxide material is such that atm% of In is 50 to 80%, atm% of Ga is 10 to 25%, and atm% of Sn is 10 to 30%. In other words, a thin film represented by the chemical formula InMO 3 (ZnO) m  (m &gt; 0) can be used as the first oxide semiconductor layer  152   a . In this case, M represents one or a plurality of metal elements selected from Sn, Ga, Zn, Sc, La, Y, Ni, Al, Mg, Ti, Ta, W, Hf, and Si. Also, the quaternary oxide material, the ternary oxide material, and the binary oxide material described above are not limited to those in which the contained oxide has a stoichiometric composition and may be composed of an oxide material with a composition deviated from the stoichiometric composition. 
     In the present embodiment, the first oxide semiconductor layer  152   a  has a structure in which a first area  152   a   1  and a second area  152   a   2  are stacked from the substrate  100  side. In the first oxide semiconductor layer  152   a , the thickness of the first area  152   a   1  is larger than the thickness of the second area  152   a   2 . The thickness of the first area  152   a   1  of the first oxide semiconductor layer  152   a  is preferably 30 nm to 100 nm. The thickness of the second area  152   a   2  of the first oxide semiconductor layer  152   a  is preferably 2 nm to 10 nm. However, the thickness of the first oxide semiconductor layer  152   a  including the first area  152   a   1  and the second area  152   a   2  may be 20 nm to 100 nm, for example, 30 nm to 60 nm. 
     The first oxide semiconductor layer  152   a  has different carrier concentrations (concentrations of majority carriers) of the first area  152   a   1  and the second area  152   a   2 . The carrier concentration of the second area  152   a   2  is smaller than the carrier concentration of the first area  152   a   1 . The carrier concentration of the first area  152   a   1  is preferably about 1×10 15 /cm 3  to 5×10 18 /cm 3 , and the carrier concentration of the second area  152   a   2  is preferably about 1×10 11 /cm 3  to 1×10 15 /cm 3 . Correspondingly, the first area  152   a   1  of the first oxide semiconductor layer  152   a  preferably has a specific resistance value of about 10 -1  Ω·cm to 10 3  Ω·cm. The second area  152   a   2  of the first oxide semiconductor layer  152   a  preferably has a specific resistance value of about 10 4  Ω·cm to 10 9  Ω·cm. In addition, the carrier mobility of the second area  152   a   2  of the first oxide semiconductor layer  152   a  is also preferably smaller than the carrier mobility of the first area  152   a   1  of the first oxide semiconductor layer  152   a . 
     In addition, the first oxide semiconductor layer  152   a  may have different crystallinities between the first area  152   a   1  and the second area  152   a   2 . The crystallinity rate of the second area  152   a   2  of the first oxide semiconductor layer  152   a  is preferably higher than the crystallinity rate of the first area  152   a   1 . The first area  152   a   1  of the first oxide semiconductor layer  152   a  may be in the form of amorphous, microcrystalline phase, or a mixed phase of amorphous and nano microcrystalline phase. The second area  152   a   2  of the first oxide semiconductor layer  152   a  may be in the form of amorphous, nanocrystalline phase, or a mixed phase of amorphous and nanocrystalline phase. In this case, the second area  152   a   2  has a higher mixing ratio of the microcrystalline phase than the first area  152   a   1  and may be in the form of a mixed phase with a polycrystalline phase. 
     The first oxide semiconductor layer  152   a  can be manufactured by a sputtering method. The first area  152   a   1  and the second area  152   a   2  can be manufactured by changing the sputtering conditions. For example, the first area  152   a   1  of the first oxide semiconductor layer  152   a  is formed using a rare gas such as Ar as a sputtering gas, and the second area  152   a   2  is formed using a rare gas such as Ar and an oxygen gas as a sputtering gas. Increasing the oxygen partial pressure at the time of forming the second area  152   a   2  with respect to the first area  152   a   1 , the donor-defect of the second area  152   a   2  can be reduced, and the crystallinity rate can be improved. As a result, the carrier concentration of the second area  152   a   2  can be made lower than that of the first area  152   a   1 , and the specific resistance value can be made higher accordingly. 
     The first oxide semiconductor layer  152   a  may be combined so as to have the same composition of the first area  152   a   1  and the second area  152   a   2  and have different crystallinity rates. In addition, in the first oxide semiconductor layer  152   a , the same type of metal oxide may be used in the first area  152   a   1  and the second area  152   a   2  and combined to have different compositions. Further, metal oxides having different compositions may be combined in the first area  152   a   1  and the second area  152   a   2 . The carrier concentration can be made different, and the specific resistance value can be made different by applying such a combination to the first area  152   a   1  and the second area  152   a   2 . 
     As shown in  FIG.  21 A , even when an oxide semiconductor target material (e.g., InGaSnZnO x ) having the same composition ratio is used, the crystallinity rate can be changed by changing the O 2 / (Ar+O 2 ) oxygen partial pressure of the sputtering gas, and it is possible to change the carrier concentration and the bandgap of each area. For example, the crystallinity rate of the second area  152   a   2  of the first oxide semiconductor layer  152   a , which is a microcrystal, is higher than that of the first area  152   a   1  of the first oxide semiconductor layer  152   a , which is amorphous, the carrier concentration is lower, the bandgap is larger, and the work function value is smaller. Forming in this manner makes it possible to improve the problem of reduction reaction due to hydrogen radical generated when the third insulating layer  122  is formed on the second area  152   a   2  of the first oxide semiconductor layer  152   a . A P—SiO 2  film formed by a plasma CVD method using SiH 4  gas and N 2 O gas as a raw material is used for the third insulating layer  122 . There is a problem in that hydrogen present in the raw material SiH 4  becomes hydrogen radical and reduces the surface of the second area  152   a   2  of the first oxide semiconductor layer  152   a . Decreasing the carrier concentration of the second area  152   a   2  of the first oxide semiconductor layer  152   a  and increasing the crystallinity rate makes it possible to prevent the reduction by hydrogen radical from occurring and widen the process margin when forming a P—SiO 2  film by the plasma CVD method. Further, as shown in  FIG.  22   , the threshold voltage (Vth) of a thin film transistor element can be precisely controlled by changing the oxygen partial pressure at the time of a deposition of the second area  152   a   2 . In order to simplify the circuit system and reduce the cost, the sub-threshold voltage of the thin film transistor element must be moved closer to the positive side than 0 V. According to  FIG.  22   , it can be understood that the partial pressure of oxygen at the time of the deposition of the second area  152   a   2  needs to be about 5%. If the thickness of the second area  152   a   2  is increased, it is also possible to shift the sub-threshold voltage to the positive side, and the optimum thickness may be selected. 
     As shown in  FIG.  21 B , for example, the same effect as in  FIG.  21 A  can be obtained by using the amorphous InGaSnO x  film in the first area  152   a   1  of the oxide semiconductor layer  152   a  and the amorphous GaO x  film in the second area  152   a   2  of the first oxide semiconductor layer  152   a . Even if the surface of the amorphous GaO x  film is subjected to hydrogen (H 2 ) plasma treatment, the carrier concentration of the amorphous GaO x  is increased from 10 13  level to 10 15  level and does not become conductive. Amorphous GaO x  is less likely to cause a reduction reaction due to hydrogen radicals under the normal P—SiO 2  deposition condition, the P—SiO 2  film can be deposited by increasing the substrate temperature to 250° C. or higher, and a highly reliable thin film transistor element can be manufactured. Even in the case where the amorphous GaO x  is used in the second area  152   a   2 , the threshold voltage (Vth) and the sub-threshold voltage of the thin film transistor element can be shifted to the positive side by increasing the oxygen partial pressure at the time of the sputtering deposition or increasing the thickness of the amorphous GaO x . 
     In order to control the threshold voltage (Vth) and the sub-threshold voltage of the thin film transistor element described above, the thickness of the third insulating layer  122  must be thinner than the thickness of the first insulating layer  104  in the transistor configuration of  FIGS.  20 A and  20 B . Specifically, the thickness of the third insulating layer  122  is preferably 150 nm to 250 nm and the thickness of the first insulating layer  104  is preferably 400 nm to 600 nm, which is twice or more of that thickness. That is, the electric field acting on the first oxide semiconductor  152   a  is preferred to be stronger on the side of the second gate electrode  156  than on the side of the first gate electrode  154 . With such a configuration, at an interface where the first area  152   a   1  and the second area  152   a   2  of  FIG.  21 A  and  FIG.  21 B  are in contact with each other, a bandgap of about 0.3 eV is generated at the conduction band side. When the bottom energy (Ec) of the conduction band in the second area  152   a   2  is higher than that in the first area  152   a   1  and a positive gate voltage is applied, carriers (electrons) gather at this interface and become conductive. That is, the interface between the third insulating layer  122  and the second area  152   a   2  of the first oxide semiconductor layer  152   a  does not become conductive, but carriers (electrons) concentrate at the interface between the first area  152   a   1  and the second area  152   a   2 , and a current flows. For this reason, a transistor element operation of a buried-channel structure is performed. The reliability of the buried-channel transistor element is very high, and almost no threshold voltage (Vth) shift occurs even in a PBTS evaluation test. In the double-gate, source/drain bottom contact type TFT used in an embodiment of the present invention, it is very critical to make the thickness of the third insulating layer  122 , which is the gate insulating film of the top gate side, thinner than the thickness of the first insulating layer  104 , which is the gate insulating film of the bottom gate side. The thickness of the third insulating layer  122 , which is the gate insulating film of the top gate side, is preferably about ½ of the thickness of the first insulating layer  104 , which is the gate insulating film of the bottom gate side. 
     The select transistor  136  has a configuration similar to that of the drive transistor  138 . That is, the select transistor  136  is configured to include the second oxide semiconductor layer  152   b , the first gate electrode  154   b , and the second gate electrode  156   b . In the present embodiment, the second oxide semiconductor layer  152   b  has a structure in which a first area  152   b   1  and a second area  152   b   2  are stacked from the substrate  100 . 
     The capacitive element  140  is formed in an area where the second electrode  108   d  overlaps with the capacitance electrode  162  via the first insulating layer  104 . The capacitance electrode  162  is also formed as the common potential line  144 . 
     The EL element  200   h  is electrically connected to the drive transistor  138 . In the EL element  200   h , an area corresponding to the second electrode  108  is contiguously formed from the drive transistor  138 . With such a configuration, the routing of wirings is simplified, and the opening ratio of the pixel  302  (the ratio of the area where the EL element actually emits light with respect to the area occupied by one pixel) can be increased. 
     In the EL element  200   h , an area corresponding to the first electron transport layer  106   a  is arranged with the same structure as that of the drive transistor  138 . As shown in  FIG.  20 A , the first electron transport layer  106   a  may be arranged contiguously from the area of the drive transistor  138 . In the present embodiment, the first electron transport layer  106   a  has a structure in which a first area  106   a   1  and a second area  106   a   2  are stacked from the substrate  100 . In the first electron transport layer  106   a  has different carrier concentrations (concentrations of majority carriers) of the first area  106   a   1  and the second area  106   a   2 . The carrier concentration of the second area  106   a   2  is smaller than the carrier concentration of the first area  106   a   1 . 
     As described above, the display device according to the present embodiment can be manufactured through the same manufacturing process as elements such as the drive transistor and the select transistor manufactured using the oxide semiconductor layer by using the oxide semiconductor layer as the electron transport layer for forming the EL element. In addition, in the display device according to the present embodiment, an electrode for controlling the amount of carrier injection is arranged in the EL element, and an electron transport layer with a high carrier (electron) mobility is arranged with respect to the electrode for controlling the amount of carrier injection via an insulating layer, so that the emission intensity in the pixel plane can be made uniform and high definition can be achieved. 
      In the present embodiment, the first oxide semiconductor layer  152   a  of the drive transistor  138  is composed of the first area  152   a   1  and the second area  152   a   2 . Also, the carrier concentration of the second area  152   a   2  is configured to be lower than that of the first area  152   a   1 . As a result, the drive transistor  138  is structured such that a channel is formed in the first area  152   a   1  of the first oxide semiconductor layer  152   a  away from the third insulating layer  122 . The drive transistor  138  according to the present embodiment can improve field-effect mobility by arranging the second area  152   a   2  between the first area  152   a   1  of the first oxide semiconductor layer  152   a  and the third insulating layer  122 . In addition, the fluctuation of the threshold voltage of the drive transistor  138  can be suppressed, and the reliability can be improved by the stable electrical characteristics. Further, since the drive transistor  138  has a double gate structure, the current drive capability is improved. Therefore, sufficient current can be supplied even if the voltage of the third electrode  118 , which serves as an anode, is reduced when driving the EL element. Even if the operation point of the EL element fluctuates, constant current driving can be performed according to the fluctuation of the operation point. Since the drive transistor  138  has a double-gate structure, it is possible to reduce the power consumption, and therefore, it is possible to solve the heat generation problem that becomes apparent when the EL display device is enlarged, and the life of the EL element can be prolonged. 
     The present invention is not limited to the above-described embodiments, and can be appropriately modified without departing from the spirit thereof. In addition, the embodiments can be combined as appropriate. 
       FIG.  35    shows a cross-sectional structure of an EL element  200   i  according to a modification of the present invention. Since the EL element  200   i  shown in  FIG.  35    is the same as the EL element  200   a  shown in  FIG.  1    except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
       FIG.  36    shows a cross-sectional structure of an EL element  200   j  according to a modification of the present invention. Since the EL element  200   j  shown in  FIG.  36    is the same as the EL element  200   b  shown in  FIG.  2    except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
       FIG.  37    shows a cross-sectional view of an EL element  200   k  according to a modification of the present invention. Since the EL element  200   k  shown in  FIG.  37    is the same as the EL element  200   f  shown in  FIG.  16    except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
       FIGS.  38 A and  38 B  shows a cross-sectional view of an EL element  200   l  according to a modification of the present invention. Since the EL element  200   l  shown in  FIGS.  38 A and  38 B  is the same as the EL element  200   g  shown in  FIGS.  17 A and  17 B  except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
       FIG.  39    shows a cross-sectional view of an EL element  200   m  according to a modification of the present invention. Since the EL element  200   m  shown in  FIG.  39    is the same as the EL element  200   c  shown in  FIG.  13    except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
       FIG.  40    shows a cross-sectional view of an EL element  200   n  according to a modification of the present invention. Since the EL element  200   n  shown in  FIG.  40    is the same as the EL element shown in  FIG.  29    except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
       FIG.  41    shows a cross-sectional view of an EL element  200   o  according to a modification of the present invention. Since the EL element  200   o  shown in  FIG.  41    is the same as the EL element shown in  FIG.  30    except that the light emitting layer  112  is arranged on substantially the entire surface, descriptions of common portions will be omitted. 
     The EL elements  200   i ,  200   j ,  200   k ,  200   l ,  200   m ,  200   n ,  200   o  according to  FIG.  35    to  FIG.  41    can be applied when the light emitting layer  112  is common to all pixels in the display device. For example, in the case where the light emitting layer  112  emits white light, the configurations shown in  FIG.  35    to  FIG.  41    can be applied. Since the light emitting layer  112  is common, they can be formed in the same process. That is, it is not necessary to paint the light emitting layer  112  for each predetermined color (material). Although the light emitting layer  112  of  FIG.  35    to  FIG.  41    is shown as a single layer, the light emitting layer  112  may be formed by stacking a plurality of layers having different emission wavelengths.