Patent Publication Number: US-2022231174-A1

Title: Semiconductor element and semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of International Patent Application No. PCT/JP2020/037521 (Filed on Oct. 2, 2020), which claims the benefit of priority from Japanese Patent Application No. 2019-182969 (filed on Oct. 3, 2019). 
     The entire contents of the above applications, which the present application is based on, are incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present disclosure relates to a semiconductor element that is useful as, e.g., a power device, and a semiconductor device and a semiconductor system using the semiconductor element. 
     DESCRIPTION OF THE RELATED ART 
     Gallium oxide (Ga 2 O 3 ) is a transparent semiconductor that has a wide bandgap of 4.8-5.3 eV at room temperature and that absorbs little visible and ultraviolet light. Therefore, in particular, gallium oxide is a promising material for use in optical/electronic devices that operate in a deep ultraviolet range and transparent electronics, and in recent years, development of photodetectors, light-emitting diodes (LED) and transistors based on gallium oxide (Ga 2 O 3 ) has been underway. 
     There are five, α, β, γ, σ and ε, crystal structures of gallium oxide (Ga 2 O 3 ), and generally, the most stable structure is β-Ga 2 O 3 . However, β-Ga 2 O 3  has a β-gallia structure, and unlike crystalline systems and the like generally used in electronic materials, is not necessarily suitable for use in semiconductor elements. Also, growth of a β-Ga 2 O 3  thin film needs a high substrate temperature and a high degree of vacuum, causing a problem of an increase in manufacturing cost. In addition, in the case of β-Ga 2 O 3 , even a high concentration (for example, no less than 1×10 19 /cm 3 ) of a dopant (Si) needs to be subjected to annealing treatment at a high temperature of 800° C. to 1100° C. after ion implantation, to use the dopant as a doner. 
     On the other hand, α-Ga 2 O 3  has a crystal structure that is the same as that of a sapphire substrate, which has already been widely used, and thus, is suitable for use in optical and electronic devices. Furthermore, α-Ga 2 O 3  has a bandgap that is wider than that of β-Ga 2 O 3 , and thus, is particularly useful for power devices. Therefore, semiconductor elements using α-Ga 2 O 3  as a semiconductor have been anticipated. 
     Semiconductor elements using β-Ga 2 O 3  as a semiconductor and using two layers of a Ti layer and an Au layer, three layers of a Ti layer, an Al layer and an Au layer or four layers of a Ti layer, an Al layer, an Ni layer and an Au layer as an electrode providing ohmic characteristics meeting the semiconductor have been known. 
     Also, semiconductor elements using β-Ga 2 O 3  as a semiconductor and using any of Au, Pt and a stack of Ni and Au as an electrode providing Schottky characteristics meeting the semiconductor have been known. 
     However, if any of the above-described electrodes is applied to a semiconductor element using α-Ga 2 O 3  as a semiconductor, there are problems such as failure of the electrode to function as a Schottky electrode or an ohmic electrode, failure of the electrode to be bonded to a film and impairment of semiconductor characteristics. Furthermore, the above-described electrode configurations each have provided no practically satisfactory semiconductor element because of, e.g., occurrence of leak current from an electrode end portion. 
     SUMMARY OF THE INVENTION 
     According to an example of the present disclosure, there is provided a semiconductor element including; at least a multilayer structure including a semiconductor layer, a first metal layer, a second metal layer and a third metal layer, the semiconductor layer including an oxide semiconductor film, the first metal layer, the second metal layer and the third metal layer being arranged on the semiconductor layer, the first metal layer, the second metal layer and the third metal layer respectively including one or two or more different metals, the first metal layer being in ohmic contact with the semiconductor layer, the second metal layer being disposed between the first metal layer and the third metal layer, and the second metal layer containing Pt or/and Pd. 
     Thus, in a semiconductor element of the present disclosure, the semiconductor element of the present disclosure curbs diffusion of, e.g., oxygen into an ohmic electrode by an oxide semiconductor and provides a favorable ohmic characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically illustrating a preferable mode of a semiconductor element of the present disclosure; 
         FIG. 2  is a diagram illustrating a mode of a preferable method of manufacturing the semiconductor element in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a mode of a preferable method of manufacturing the semiconductor element in  FIG. 1 ; 
         FIG. 4  is a diagram illustrating a mode of a preferable method of manufacturing the semiconductor element in  FIG. 1 ; 
         FIG. 5  is a diagram illustrating a mode of a preferable method of manufacturing the semiconductor element in  FIG. 1 ; 
         FIG. 6  is a sectional view schematically illustrating a preferable mode of the semiconductor element of the present disclosure; 
         FIG. 7  is a diagram illustrating a result of SEM-EDS where Ti/Ni/Au was used as an ohmic electrode; 
         FIG. 8  is a diagram schematically illustrating a preferable example of a power supply system; 
         FIG. 9  is a diagram schematically illustrating a preferable example of a system device; 
         FIG. 10  is a diagram schematically illustrating a preferable example of a power supply circuit diagram of a power supply device; 
         FIG. 11  is a diagram schematically illustrating a preferable example of a semiconductor device; 
         FIG. 12  is a diagram schematically illustrating a preferable example of a power card; 
         FIG. 13  includes diagrams each illustrating a porosity of a porous layer:  FIG. 13( a )  illustrates a porosity of a porous layer after bonding with normal annealing; and  FIG. 13( b )  illustrates a porosity where thermal compression bonding was further performed for one hour after annealing; 
         FIG. 14  includes photographs illustrating a partial outer appearance of an ohmic electrode after sintering in a test example:  FIG. 14( a )  is a photograph illustrating an outer appearance after sintering where Ti/Pt/Au was used as an ohmic electrode; and  FIG. 14( b )  is a photograph illustrating an outer appearance after sintering where Ti/Ni/Au was used as an ohmic electrode; 
         FIG. 15  includes graphs illustrating temperature (sintering temperature) dependance of a contact resistivity in a case where Ti/Pt/Au was used as an ohmic electrode and that in a case where Ti/Ni/Au was used as an ohmic electrode, respectively, in a test example; 
         FIG. 16  is a diagram schematically illustrating a multilayer structure that is a major portion of the semiconductor element of the present disclosure; and 
         FIG. 17  is a sectional view schematically illustrating a product of an embodiment of the semiconductor element of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The inventors of present disclosure found out that Ti may diffuse if ohmic electrode containing Ti/Au. The inventors of present disclosure found out that oxygen in an oxide semiconductor may diffuse in ohmic electrode if a Ti anti-diffusion film is provided between a Ti layer and a Au layer. 
     According to an example of the present disclosure, there is provided a semiconductor element including, at least a multilayer structure including a semiconductor layer, a first metal layer, a second metal layer and a third metal layer, the semiconductor layer including an oxide semiconductor film, the first metal layer, the second metal layer and the third metal layer being arranged on the semiconductor layer, the first metal layer, the second metal layer and the third metal layer respectively including one or two or more different metals, the first metal layer being in ohmic contact with the semiconductor layer, the second metal layer being disposed between the first metal layer and the third metal layer, and the second metal layer containing Pt or/and Pd. 
     Thus, in a semiconductor element of the present disclosure, the semiconductor element of the present disclosure curbs diffusion of, e.g., oxygen into an ohmic electrode by an oxide semiconductor and provides a favorable ohmic characteristic. 
     Embodiments of the present disclosure will be described below with reference to the accompanying drawings. In the following description, the same parts and components are designated by the same reference numerals. The present embodiment includes, for example, the following disclosures. 
     [Structure 1] 
     A semiconductor element including; at least a multilayer structure including a semiconductor layer, a first metal layer, a second metal layer and a third metal layer, the semiconductor layer including an oxide semiconductor film, the first metal layer, the second metal layer and the third metal layer being arranged on the semiconductor layer, the first metal layer, the second metal layer and the third metal layer respectively including one or two or more different metals, the first metal layer being in ohmic contact with the semiconductor layer, the second metal layer being disposed between the first metal layer and the third metal layer, and the second metal layer containing Pt or/and Pd. 
     [Structure 2] 
     The semiconductor element according to [Structure 1], wherein the oxide semiconductor film has a corundum structure. 
     [Structure 3] 
     The semiconductor element according to [Structure 2], wherein a principal plane of the oxide semiconductor film is an m-plane. 
     [Structure 4] 
     The semiconductor element according to any of [Structure 1] to [Structure 3], wherein the oxide semiconductor film contains gallium oxide and/or iridium oxide. 
     [Structure 5] 
     The semiconductor element according to any of [Structure 1] to [Structure 4], wherein the oxide semiconductor film contains gallium oxide. 
     [Structure 6] 
     The semiconductor element according to any of [Structure 1] to [Structure 5], wherein the oxide semiconductor film contains a dopant. 
     [Structure 7] 
     The semiconductor element according to any of [Structure 1] to [Structure 6], wherein the first metal layer is a Ti layer or an In layer. 
     [Structure 8] 
     The semiconductor element according to any of [Structure 1] to [Structure 7], wherein the third metal layer is a metal layer including at least one or two or more metals selected from Au, Ag and Cu. 
     [Structure 9] 
     The semiconductor element according to any of [Structure 1] to [Structure 8], wherein a porous layer is further disposed in contact with the third metal layer. 
     [Structure 10] 
     The semiconductor element according to [Structure 9], wherein a porosity of the porous layer is no more than 10%. 
     [Structure 11] 
     The semiconductor element according to [Structure 9] or [Structure 10], wherein the porous layer contains a precious metal. 
     [Structure 12] 
     The semiconductor element according to any of [Structure 9] to [Structure 11], wherein a substrate is further disposed on the porous layer. 
     [Structure 13] 
     The semiconductor element according to any of [Structure 1] to [Structure 12], further comprising a Schottky electrode. 
     [Structure 14] 
     The semiconductor element according to [Structure 13], wherein the Schottky electrode contains Mo and/or Co. 
     [Structure 15] 
     The semiconductor element according to [Structure 13] or [Structure 14], wherein the Schottky electrode is disposed on a first surface side of the oxide semiconductor film and an ohmic electrode is disposed on a second surface side of the oxide semiconductor film, and wherein the second surface side is opposite from the first surface side. 
     [Structure 16] 
     The semiconductor element according to any of [Structure 13] to [Structure 15], wherein the Schottky electrode includes at least a first metal layer, a second metal layer and a third metal layer, the first metal layer of the Schottky electrode, the second metal layer of the Schottky electrode and the third metal layer of the Schottky electrode respectively include different metals, the second metal layer of the Schottky electrode is disposed between the first metal layer of the Schottky electrode and the third metal layer of the Schottky electrode, and the first metal layer of the Schottky electrode is disposed on the semiconductor layer side relative to the third metal layer of the Schottky electrode. 
     [Structure 17] 
     The semiconductor element according to [Structure 16], wherein the first metal layer of the Schottky electrode is a Co layer or an Mo layer. 
     [Structure 18] 
     The semiconductor element according to [Structure 16] or [Structure 17], wherein the second metal layer of the Schottky electrode is a Ti layer. 
     [Structure 19] 
     The semiconductor element according to any of [Structure 16] to [Structure 18], wherein the third metal layer of the Schottky electrode is an Al layer. 
     [Structure 20] 
     A semiconductor element including; at least a semiconductor layer including an oxide semiconductor film, a Schottky electrode, and an ohmic electrode, the Schottky electrode including a Co layer or an Mo layer, and the ohmic electrode including at least a first metal layer, a second metal layer and a third metal layer, the first metal layer being a Ti layer or an In layer, the second metal layer being a Pt layer or a Pd layer, and the third metal layer being a metal layer including at least one or two or more metals selected from Au, Ag and Cu. 
     [Structure 21] 
     The semiconductor element according to any of [Structure 1] to [Structure 20], wherein the semiconductor layer is an n-type oxide semiconductor layer. 
     [Structure 22] 
     The semiconductor element according to any of [Structure 1] to [Structure 21], wherein the semiconductor element is a vertical device. 
     [Structure 23] 
     The semiconductor element according to any of [Structure 1] to [Structure 22], wherein the semiconductor element is a power device. 
     [Structure 24] 
     A semiconductor device including at least a semiconductor element bonded to a lead frame, a circuit substrate or a heat dissipation substrate with a bonding member, wherein the semiconductor element is the semiconductor element according to any of [Structure 1] to [Structure 23]. 
     [Structure 25] 
     The semiconductor device according to [Structure 24], wherein the semiconductor device is a power module, an inverter or a converter. 
     [Structure 26] 
     The semiconductor device according to [Structure 24] or [Structure 25], wherein the semiconductor device is a power card. 
     [Structure 27] 
     A semiconductor system including a semiconductor element or a semiconductor device, wherein the semiconductor element is the semiconductor element according to claim 1 and the semiconductor device is the semiconductor device according to any of [Structure 24] to [Structure 26]. 
     The semiconductor element of the present disclosure is a semiconductor element including at least a multilayer structure including a semiconductor layer, a first metal layer, a second metal layer and a third metal layer, the semiconductor layer including an oxide semiconductor film, the first metal layer, the second metal layer and the third metal layer being arranged on the semiconductor layer, the first metal layer, the second metal layer and the third metal layer including respective one or two more different metals, the first metal layer being in ohmic contact with the semiconductor layer, the second metal layer being disposed between the first metal layer and the third metal layer, and the second metal layer containing Pt or/and Pd. The multilayer structure is not specifically limited as long as, for example, as illustrated in  FIG. 16 , a first metal layer  102   a,  a second metal layer  102   b  and a third metal layer  102   c  are arranged on a semiconductor layer  101  including an oxide semiconductor film. 
     The oxide semiconductor film (hereinafter, also simply referred to as a “semiconductor layer” or a “semiconductor film”) is not specifically limited as long as the oxide semiconductor film is a semiconductor film containing an oxide; however, in the present disclosure, the oxide semiconductor film is preferably a semiconductor film containing a metal oxide, more preferably a semiconductor film containing a crystalline oxide semiconductor, and most preferably a semiconductor film containing a crystalline oxide semiconductor as a major component. Also, in the present disclosure, the crystalline oxide semiconductor contains preferably one or two or more metals selected from group 9 (for example, cobalt, rhodium or iridium) and group 13 (for example, aluminum, gallium or indium) of the periodic table, more preferably at least one metal selected from aluminum, indium, gallium and iridium, and most preferably at least gallium or iridium. A crystal structure of the crystalline oxide semiconductor is not specifically limited. Examples of the crystal structure of the crystalline oxide semiconductor include, e.g., a corundum structure, a β-gallia structure and a hexagonal crystal structure (for example, an ε-structure). In the present disclosure, it is preferable that the crystalline oxide semiconductor have a corundum structure and it is more preferable that the crystalline oxide semiconductor have a corundum structure and a principal plane is an m-plane because such crystalline oxide semiconductor is capable of curbing diffusion of, e.g., oxygen and have more excellent electrical characteristics. Also, the crystalline oxide semiconductor may have an off angle. In the present disclosure, the semiconductor film preferably contains gallium oxide and/or iridium oxide and more preferably contains α-Ga 2 O 3  and/or α-Ir 2 O 3 . Here, the “major component” means that the crystalline oxide semiconductor is contained at an atom ratio of preferably no less than 50%, more preferably no less than 70%, still more preferably no less than 90% to all components of the semiconductor layer and means that the atom ratio may be 100%. Also, a thickness of the semiconductor layer is not specifically limited and may be no more than 1 μm or may be no less than 1 μm; however, in the present disclosure, the thickness of the semiconductor layer is preferably no less than 1 μm, and more preferably no less than 10 μm. A surface area of the semiconductor film is not specifically limited and may be no less than 1 mm 2  or may be no more than 1 mm 2 , but is preferably 10 mm 2  to 300 cm 2 , and more preferably 100 mm 2  to 100 cm 2 . Also, the semiconductor film is preferably a single crystal film but may be a polycrystalline film or a crystal film containing a polycrystal. Also, the oxide semiconductor film is a semiconductor layer of a multi-layer film containing at least a first oxide semiconductor film and a second oxide semiconductor film, and where a Schottky electrode is provided on the first oxide semiconductor film, it is preferable that the multi-layer film is a multi-layer film in which a carrier density of the first oxide semiconductor film is smaller than a carrier density of the second oxide semiconductor film. In this case, at least the second oxide semiconductor film normally contains a dopant and it is possible to arbitrarily set a carrier density of the semiconductor layer by adjusting a doping amount. 
     The semiconductor layer preferably contains a dopant. The dopant is not specifically limited and may be a publicly known one. Example of the dopant include, e.g., n-type dopants such as tin, germanium, silicon, titanium, zirconium, vanadium and niobium and p-type dopants such as magnesium, calcium and zinc. In the present disclosure, the semiconductor layer preferably contains an n-type dopant and is more preferably an n-type oxide semiconductor layer. Also, in the present disclosure, it is preferable that the n-type dopant be Sn, Ge or Si. An amount of the dopant contained is preferably no less than 0.00001 atom %, more preferably 0.00001 atom % to 20 atom %, and most preferably 0.00001 atom % to 10 atom % in a composition of the semiconductor layer. More specifically, a concentration of the dopant may normally approximately 1×10 16 /cm 3  to 1×10 22 /cm 3  or the concentration of the dopant may be set to be a low concentration of, for example, approximately no more than 1×10 17 /cm 3 . Also, according to an aspect of the present disclosure, the dopant may be contained in a high concentration of approximately 1×10 20 /cm 3 . Also, a concentration of fixed charge in the semiconductor layer is not specifically limited; however, in the present disclosure, it is preferable that the concentration of fixed charge in the semiconductor layer be no more than 1×10 17 /cm 3  because such concentration enables more favorably forming a depletion layer with the semiconductor layer. 
     The semiconductor layer may be formed using a publicly known method. Examples of a method of forming the semiconductor layer include, e.g., a CVD method, an MOCVD method, an MOVPE method, a mist CVD method, a mist epitaxy method, an MBE method, an HVPE method, a pulse growth method and an ALD method. In the present disclosure, it is preferable that the method of forming the semiconductor layer be the mist CVD method or the mist epitaxy method. In the mist CVD method or the mist epitaxy method, the semiconductor layer is formed by, for example, atomizing a raw material solution (atomization step) to make droplets be suspended, and after the atomization, carrying the resulting atomized droplets above a base with a carrier gas (carrying step), and subsequently stacking a semiconductor film containing a crystalline oxide semiconductor as a major component on the base through thermal reaction of the atomized droplets in the vicinity of the base (film forming step). 
     (Atomization Step) 
     In the atomization step, the raw material solution is atomized. A method of atomization of the raw material solution is not specifically limited as long as such method enables atomization of the raw material solution and may be a publicly known method; however, in the present disclosure, an atomization method using ultrasound is preferable. Atomized droplets obtained using ultrasound are preferable because of having an initial velocity of zero and being suspended in air. The atomized droplets (including mist) are very preferable because the atomized droplets are not, for example, those sprayed with a sprayer but are suspended in space and are carried as gas, and thus, are not damaged by collision energy. A size of each of the droplets is not specifically limited and may be around several millimeters, but preferably no more than 50 μm, more preferably 100 nm to 10 μm. 
     (Raw Material Solution) 
     The raw material solution is not specifically limited as long as atomization of the raw material solution is possible and the raw material solution contains a raw material that enables forming of a semiconductor film, and may be an inorganic material or may be an organic material. In the present disclosure, the raw material is preferably a metal or a metal compound, more preferably one or two or more metals selected from aluminum, gallium, indium, iron, chromium, vanadium, titanium, rhodium, nickel, cobalt and iridium. 
     In the present disclosure, as the raw material solution, one obtained by dissolving or dispersing the metals in an organic solvent or water in the form of a complex or a salt may be preferably used. Examples of the form of a complex include, e.g., acetylacetonate complexes, carbonyl complexes, ammine complexes and hydride complexes. Examples of the form of a salt include, e.g., organic metal salts (for example, metal acetates, metal oxalates, metal citrates, etc.), metal sulfide salts, metal nitrate salts, metal phosphate salts and metal halide salts (for example, metal chloride salts, metal bromide salts, metal iodide salts, etc.). 
     Also, it is preferable that additive agents such as a hydrohalic acid and an oxidizing agent be mixed in the raw material solution. Examples of the hydrohalic acid include, e.g., a hydrobromic acid, a hydrochloric acid and a hydroiodic acid, and among others, a hydrobromic acid or a hydroiodic acid is preferable because of being capable of more efficiently curbing generation of abnormal grains. Examples of the oxidizing agent include, e.g., peroxides such as hydrogen peroxide (H 2 O 2 ), sodium peroxide (Na 2 O 2 ), barium peroxide (BaO2) and benzoyl peroxide (C 6 H 5 CO) 2 O 2  and organic peroxides such as a hypochlorous acid (HClO), a perchloric acid, a nitric acid, ozone water, acetyl hydroperoxide and nitrobenzene. 
     The raw material solution may contain a dopant. A dopant being contained in the raw material solution enables doping to be performed favorably. The dopant is not specifically limited as long as such dopant does not hinder the present disclosure. Examples of the dopant include, e.g., n-type dopants such as tin, germanium, silicon, titanium, zirconium, vanadium and niobium and p-type dopants such as Mg, H, Li, Na, K, Rb, Cs, Fr, Be, Ca, Sr, Ba, Ra, Mn, Fe, Co, Ni, Pd, Cu, Ag, Au, Zn, Cd, Hg, Ti, Pb, N and P. An amount of the dopant contained is appropriately set using a calibration curve indicating a relationship of a concentration of a dopant in a raw material with a desired carrier density. 
     A solvent of the raw material solution is not specifically limited and may be an inorganic solvent such as water or may be an organic solvent such as alcohol or may be a mixed solvent of an inorganic solvent and an organic solvent. In the present disclosure, the solvent preferably contains water, and more preferably is a mixed solvent of water and alcohol. 
     (Carrying Step) 
     In the carrying step, the atomized droplets are carried into a film forming chamber by the carrier gas. The carrier gas is not specifically limited as long as the carrier gas does not hinder the present disclosure, and preferable examples of the carrier gas include, e.g., inert gases such as oxygen, ozone, nitrogen and argon and reducing gases such as hydrogen gas and forming gas. Also, for a type of the carrier gas, a single type or two or more types of the carrier gas may be used, and, e.g., a dilute gas with a flow rate lowered (for example, a 10-fold diluted gas) may further be used as a second carrier gas. Also, the number of locations for supply of the carrier gas is not limited to one but may be two or more. A flow rate of the carrier gas is not specifically limited but is preferably 0.01 to 20 L/minute, and more preferably 1 to 10 L/minute. In the case of a dilute gas, a flow rate of the dilute gas is preferably 0.001 to 2 L/minute, and more preferably 0.1 to 1 L/minute. 
     (Film Forming Step) 
     In the film forming step, the semiconductor film is formed on the base through thermal reaction of the atomized droplets in the vicinity of the base. It is only necessary that the atomized droplets react with heat, and conditions, etc., of the thermal reaction are not specifically limited as long as such conditions, etc., do not hinder the present disclosure. In the present step, the thermal reaction is normally performed at a temperature that is equal to or exceeds an evaporation temperature of the solvent but preferably no more than a temperature that is not too high (for example, 1000° C.), more preferably no more than 650° C., and most preferably no more than 300° C. to 650° C. Also, the thermal reaction may be performed under any atmosphere of vacuum, a non-oxygen atmosphere (for example, an inert gas atmosphere, etc.), a reducing gas atmosphere and an oxygen atmosphere as long as such atmosphere does not hinder the present disclosure; however, it is preferable that the thermal reaction be performed under an inert gas atmosphere or an oxygen atmosphere. Also, the thermal reaction may be performed under any condition of atmospheric pressure, increased pressure and reduced pressure; however, in the present disclosure, it is preferable that the thermal reaction be performed under atmospheric pressure. Note that it is possible to set a film thickness of the semiconductor film by adjusting film forming time. 
     (Base) 
     The base is not specifically limited as long as the base is capable of supporting the semiconductor film. A material of the base is also not specifically limited as long as the material does not hinder the present disclosure, and may be a publicly known base, and may be an organic compound or may be an inorganic compound. A shape of the base may be any shape and the base is effective in any and all shapes including, for example, plate-like shapes such as a flat plate and a circular plate, a fibrous shape, a rod-like shapes, a columnar shape, a prism shape, a tubular shape, a helical shape, a spherical shape and a ring-like shape; however, in the present disclosure, a substrate is preferable. A thickness of the substrate is not specifically limited in the present disclosure. 
     The substrate is not specifically limited as long as the substrate has a plate-like shape and serves as a support for the semiconductor film. The substrate may be an insulator substrate, may be a semiconductor substrate or may be a metal substrate or an electrically conductive substrate; however, the substrate is preferably an insulator substrate and is also preferably a substrate including a metal film on a surface. Examples of the substrate include, e.g., a base substrate containing a substrate material having a corundum structure as a major component, a base substrate containing a substrate material having a β-gallia structure as a major component and a base substrate containing a substrate material having a hexagonal crystal structure as a major component. Here, the “major component” means that any of the substrate materials each having a particular crystal structure is contained at an atom ratio of preferably no less than 50%, more preferably no less than 70%, still more preferably no less than 90% to all components of the substrate material, and the atom ratio may be 100%. 
     The substrate material is not specifically limited as long as the substrate material does not hinder the present disclosure, and may be a publicly known one. Preferable examples of the substrate material having a corundum structure include α-Al 2 O 3  (sapphire substrate) and α-Ga 2 O 3 , and more preferable examples of the same include, e.g., an a-plane sapphire substrate, an m-plane sapphire substrate, an r-plane sapphire substrate, a c-plane sapphire substrate and an α-gallium oxide substrate (a-plane, m-plane or r-plane). Examples of the base substrate containing a substrate material having a β-gallia structure as a major component include, e.g., a β-Ga 2 O 3  substrate and a mixed crystal substrate containing Ga 2 O3 and Al 2 O 3  in which Al 2 O 3  is contained at no less than 0 wt % and no more than 60 wt %. Also, examples of the base substrate containing a substrate material having a hexagonal crystal structure as a major component include, e.g., an SiC substrate, a ZnO substrate and a GaN substrate. 
     In the present disclosure, after the film forming step, annealing treatment may be performed. A temperature of the annealing treatment is not specifically limited as long as such temperature does not hinder the present disclosure, and is normally 300° C. to 650° C., and preferably 350° C. to 550° C. Also, a length of time of the annealing treatment is normally 1 minute to 48 hours, preferably 10 minutes to 24 hours, and more preferably 30 minutes to 12 hours. Note that the annealing treatment may be performed under any atmosphere as long as such atmosphere does not hinder the present disclosure. The atmosphere may be a non-oxygen atmosphere or an oxygen atmosphere. Examples of the non-oxygen atmosphere include, e.g., inert gas atmospheres (for example, a nitrogen atmosphere) and reducing gas atmospheres, and in the present disclosure, an inert gas atmosphere is preferable and a nitrogen atmosphere is more preferable. 
     Also, in the present disclosure, the semiconductor film may be provided directly on the base or the semiconductor film may be provided on the base with other layers in between such as a stress relaxation layer (for example, a buffer layer or an ELO layer) and a removal sacrificial layer. Methods for forming the respective layers are not specifically limited and may be publicly known methods, but in the present disclosure, a mist CVD method is preferable. 
     In the present disclosure, the semiconductor film may be used in a semiconductor element as the semiconductor layer after use of a publicly known method of, e.g., removal of the semiconductor film from, e.g., the base or may be used as it is in a semiconductor element as the semiconductor layer. 
     The first metal layer, the second metal layer and the third metal layer in the multilayer structure are not specifically limited as long as the first metal layer, the second metal layer and the third metal layer include respective one or two or more different metals, the second metal layer is disposed between the first metal layer and the third metal layer, the second metal layer contains Pt or/and Pd and the first metal layer is in ohmic contact with the semiconductor layer. Component metals of the respective metal layers of the multilayer structure are not specifically limited and examples of the component metals include, e.g., transition metals and base metals. In the present disclosure, it is preferable that the semiconductor element include at least the first metal layer, the second metal layer and the third metal layer as an ohmic electrode. Note that the ohmic electrode may include a multi-layered electrically conductive layer further including a fourth metal layer and a fifth metal layer as long as such multi-layered electrically conductive layer does not hinder the present disclosure, and may include, for example, 1 to 100 electrically conductive layers. 
     The ohmic electrode is not specifically limited as long as the ohmic electrode includes at least a first metal layer that is in ohmic contact with the semiconductor layer, a second metal layer and a third metal layer, the first metal layer, the second metal layer and the third metal layer include respective one or two or more different metals, the second metal layer is disposed between the first metal layer and the third metal layer and the second metal layer is a Pt layer or a Pd layer. In the present disclosure, it is preferable that the first metal layer of the ohmic electrode be a Ti layer or an In layer. Also, it is preferable that the third metal layer of the ohmic electrode be a metal layer including at least one or two or more metals selected from an Au layer, an Ag layer and a Cu layer. A thickness of each of the metal layers of the ohmic electrode is not specifically limited but is preferably 0.1 nm to 10 μm, and more preferably 1 nm to 1000 nm. 
     A method of forming the ohmic electrode is not specifically limited and may be a publicly known method. Specific examples of the method of forming the ohmic electrode include, e.g., a dry method and a wet method. Examples of the dry method include, e.g., sputtering, vacuum vapor deposition and CVD. Examples of the wet method include, e.g., screen printing and die coating. 
     Also, the semiconductor element may include a Schottky electrode or may include no Schottky electrode. In the present disclosure, as a preferable form, it is preferable that the semiconductor element be a Schottky barrier diode. The Schottky electrode (hereinafter, simply referred to as an “electrode layer”) is not specifically limited as long as the Schottky electrode has electrical conductivity and is usable as a Schottky electrode and does not hinder the present disclosure. A component material of the electrode layer may be an electrically conductive inorganic material or may be an electrically conductive organic material. In the present disclosure, it is preferable that the material of the electrode be a metal. Preferable examples of the metal include, e.g., at least one metal selected from groups 4 to 10 of the periodic table. Examples of a metal in group 4 of the periodic table include, e.g., titanium (Ti), zirconium (Zr) and hafnium (Hf). Examples of a metal in group 5 of the periodic table include, e.g., vanadium (V), niobium (Nb) and tantalum (Ta). Examples of a metal in group 6 of the periodic table include, e.g., chromium (Cr), molybdenum (Mo) and tungsten (W). Examples of a metal in group 7 of the periodic table include, e.g., manganese (Mn), technetium (Tc) and rhenium (Re). Examples of a metal in group 8 of the periodic table include, e.g., iron (Fe), ruthenium (Ru) and osmium (Os). Examples of a metal in group 9 of the periodic table include, e.g., cobalt (Co), rhodium (Rh) and iridium (Ir). Examples of a metal in group 10 of the periodic table include, e.g., nickel (Ni), palladium (Pd) and platinum (Pt). In the present disclosure, the electrode layer preferably contains at least one metal selected from groups 4, 6 and 9 of the periodic table, more preferably contains at least one metal selected from groups 6 and 9 of the metal periodic table, and more preferably contains Mo and/or Co. A layer thickness of the electrode layer is not specifically limited, but is preferably 0.1 nm to 10 μm, more preferably 5 nm to 500 nm, and most preferably 10 nm to 200 nm. Also, in the present disclosure, it is preferable that the electrode layer be one including two or more layers having different compositions. The electrode layer having such preferable configuration enables not only providing a semiconductor element having more excellent Schottky characteristics but also more favorably exerting a leak current curbing effect. 
     If the electrode layer includes two or more layers including a first electrode layer and a second electrode layer, it is preferable that the second electrode layer have an electrical conductivity that is higher than that of the first electrode layer. A component material of the second electrode layer may be an electrically conductive inorganic material or may be an electrically conductive organic material. In the present disclosure, it is preferable that the material of the second electrode be a metal. In the present disclosure, it is preferable that the material of the second electrode be a metal. Preferable examples of the metal include, e.g., at least one metal selected from groups 8 to 13 of the periodic table. Examples of a metal in groups 8 to 10 of the periodic table include, e.g., the metals indicated by example as metals in groups 8 to 10 of the periodic table in the above-described description of the electrode layer. Examples of a metal in group 11 of the periodic table include, e.g., copper (Cu), silver (Ag) and gold (Au). Examples of a metal in group 12 of the periodic table include, e.g., zinc (Zn) and cadmium (Cd). Also, examples of a metal in group 13 of the periodic table include, e.g., aluminum (Al), gallium (Ga) and indium (In). In the present disclosure, the second electrode layer preferably contains at least one metal selected from groups 11 and 13 of the periodic table and more preferably contains at least one metal selected from silver, copper, gold and aluminum. Note that a layer thickness of the second electrode layer is not specifically limited but is preferably 1 nm to 500 μm, more preferably 10 nm to 100 μm, and most preferably 0.5 μm to 10 μm. Note that in the present disclosure, it is preferable that a film thickness of a part, under an outer end portion of the electrode layer, of the insulator film be larger than a film thickness of a part, in a range of 1 μm from the opening portion, of the insulator film because such larger film thickness enables the semiconductor element having a more excellent voltage resistance property. 
     Also, in the present disclosure, it is preferable that the Schottky electrode include at least a first metal layer, a second metal layer and a third metal layer, the first metal layer, the second metal layer and the third metal layer include respective different metals, the second metal layer be disposed between the first metal layer and the third metal layer and the first metal layer be located on the semiconductor layer side relative to the third metal layer. Note that if the Schottky electrode includes a first metal layer, a second metal layer and a third metal layer, it is preferable that the first metal layer be a metal layer containing a metal in group 6 of the periodic table or a metal layer containing a metal in group 9 of the periodic table, the second metal layer be a metal layer containing a metal in group 4 of the periodic table and the third metal layer be a metal layer containing a metal in group 13 of the periodic table, and it is more preferable that the first metal layer be a Co layer or an Mo layer, the second metal layer be a Ti layer and the third metal layer be an Al layer. 
     A method of forming the electrode layer is not specifically limited and may be a publicly known method. Specific examples of the method of forming the electrode layer include, e.g., a dry method and a wet method. Example of the dry method include, e.g., sputtering, vacuum vapor deposition and CVD. Examples of the wet method include, e.g., screen printing and die coating. 
     Also, in an aspect of the present disclosure, it is preferable that the Schottky electrode have a structure in which a film thickness decreases toward the outer side of the semiconductor element. In this case, the Schottky electrode may include a tapered region in each side surface and the Schottky electrode may include two or more layers including a first electrode layer and a second electrode layer, and an outer end portion of the first electrode layer may be located outside of an outer end portion of the second electrode layer. In an aspect of the present disclosure, if the Schottky electrode includes a tapered region, a taper angle of the tapered region is not specifically limited as long as the taper angle does not hinder the present disclosure, but is preferably no more than 80°, more preferably no more than 60°, and most preferably no more than 40°. A lower limit of the taper angle is not also specifically determined, but is preferably 0.2°, and more preferably 1°. Also, in an aspect of the present disclosure, if the outer end portion of the first electrode layer of the Schottky electrode is located outside of the outer end portion of the second electrode layer, it is preferable that a distance between the outer end portion of the first electrode layer and the outer end portion of the second electrode layer be no less than 1 μm because such distance enables more curbing leak current. Also, in an aspect of the present disclosure, a part of the first electrode layer of the Schottky electrode, the part extending outward of the outer end portion of the second electrode layer (hereinafter also referred to an “extension part”), at least partially has a structure in which a film thickness decreases toward the outer side of the semiconductor element is preferable because such configuration enables providing more excellent voltage resistance of the semiconductor element. Also, combination of such preferable electrode configuration and the above-described preferable component materials of the semiconductor layer enables provision of a lower-loss semiconductor element with leak current more favorably curbed. 
     It is preferable that the semiconductor element include an oxide semiconductor layer and a dielectric film covering at least side surfaces of the oxide semiconductor layer. Such configuration enables curbing occurrence of a problem in semiconductor characteristic of the oxide semiconductor film due to, e.g., moisture absorption and oxygen in air. Note that in an aspect of the present disclosure, furthermore, tapering side surfaces of the semiconductor layer enables not only enhancement of, e.g., close contact with the dielectric film but also more favorable stress relaxation and thus enables further enhancement of reliability, etc. 
     The dielectric film is formed on the semiconductor layer and normally includes an opening portion, but is not specifically limited in terms of, e.g., relative permittivity and may be a publicly known dielectric film. In an aspect of the present disclosure, a dielectric film formed in such a manner as to extend at least no less than 1 μm from the opening portion and have a relative permittivity of no more than 5 is preferable. The “relative permittivity” is a ratio between a permittivity of a film and a permittivity of vacuum. In the present disclosure, it is preferable that the dielectric film be a film containing Si. Preferable examples of the film containing Si include a silicon oxide-based film. Examples of the silicon oxide-based film include, e.g., an SiO 2  film, a phosphorus-doped SiO 2  (PSG) film, a boron-doped SiO 2  film, a phosphorus-boron-doped SiO 2  film (BPSG film), an SiOC film and an SiOF film. A method of forming the dielectric film is not specifically limited but examples of the method include, e.g., a CVD method, atmospheric CVD method, a plasma CVD method, a mist CVD method and a thermal oxidation method. In the present disclosure, it is preferable that the method of forming the dielectric film be a mist CVD method or an atmospheric CVD method. 
     Also, in the semiconductor element in an aspect of the present disclosure, it is preferable that a porous layer be further disposed in contact with the third metal layer of the ohmic electrode. The porous layer is not specifically limited, but preferably has electrical conductivity and more preferably contains a precious metal. In an aspect of the present disclosure, it is preferable that a porosity of the porous layer be no more than 10%. This preferable porosity enables relaxation of, e.g., warpage and concentration of thermal stress without impairing semiconductor characteristics. Note that a method of making the porosity of the porous layer 10% is not specifically limited and may be a publicly known method. It is possible to easily set the porosity of the porous layer to 10% by appropriately setting sintering conditions such as sintering time, pressure and a sintering temperature. Examples of the method include, e.g., a method in which the porosity is adjusted to no more than 10% with compression bonding under heat (thermal compression bonding). More specific examples of the method include, e.g., sintering for sintering time that is longer than normal sintering time, under a fixed pressure.  FIG. 13( a )  illustrates a porosity where a porous layer formed of Ag was bonded by normal annealing as a test example. As illustrated in  FIG. 13( a ) , the porosity of the porous layer normally exceeds 10%, but as illustrated in  FIG. 13( b ) , compression bonding under pressure of, for example, 0.2 to 10 MPa with heating at, for example, 300° C. to 500° C. for another hour makes the porosity no more than 10%, and use of such porous layer having a porosity of no more than 10% for a semiconductor element enables relaxing, of e.g., warpage and concentration of thermal stress without imparting semiconductor characteristics. Here, the “porosity” refers to a proportion of a volume of space generated by voids in a volume of a porous layer (volume including the voids). It is possible to obtain a porosity of a porous layer based on, for example, a sectional photograph taken using a scanning electron microscope (SEM). More specifically, a sectional photograph (SEM image) of a porous layer is taken from a plurality of positions. Next, the taken SEM images are binarized using commercially available image analysis software to obtain a proportion of parts (for example, black parts) corresponding to holes (voids) in each of the SEM images. The proportions of the black parts obtained from the SEM images taken from the plurality of positions are averaged to determine the resulting proportion as the porosity of the porous layer. Note that the “porous layer” includes not only one in the form of a porous film, which is a continuous film-like structure, but also one in the form of a porous aggregate. 
     Also, in the semiconductor element of the present disclosure, it is further preferable that a substrate is disposed on the porous layer. Note that the substrate may be directly stacked on the porous layer or the substrate may be stacked on the porous layer with other layers in between such as one or two or more metal layers (for example, metals indicated by example as illustrated above). 
     In an aspect of the present disclosure, the semiconductor element is not specifically limited in terms of, e.g., a direction in which current flows, it is preferable that a Schottky electrode be disposed on a first surface side of the oxide semiconductor film and an ohmic electrode be disposed on a second surface side opposite from the first surface side, and it is more preferable that the semiconductor element be a vertical device. 
     Embodiment 
     Preferable embodiments of the present disclosure will be described in more detail below with reference to the drawing, but the present disclosure is not limited to these embodiments. 
       FIG. 1  illustrates a major part of a Schottky barrier diode (SBD) as a semiconductor element that is a preferable embodiment of the present disclosure. The SBD in  FIG. 1  includes an ohmic electrode  102 , a semiconductor layer  101 , a Schottky electrode  103  and a dielectric film  104 . The ohmic electrode  102  includes a metal layer  102   a,  a metal layer  102   b  and a metal layer  102   c.  The semiconductor layer  101  includes a first semiconductor layer  101   a  and a second semiconductor layer  101   b.  The Schottky electrode  103  includes a metal layer  103   a,  a metal layer  103   b  and a metal layer  103   c.  The first semiconductor layer  101   a  includes, for example, an n−-type semiconductor layer and the second semiconductor layer  101   b  is, for example, an n+-type semiconductor layer. Also, the dielectric film  104  (hereinafter may be referred to as an “insulator film”) covers side surfaces of the semiconductor layer  101  (side surfaces of the first semiconductor layer  101   a  and side surfaces of the second semiconductor layer  101   b ) and includes an opening portion located on an upper surface of the semiconductor layer  101  (first semiconductor layer  101   a ). The opening portion is provided between a part of the first semiconductor layer  101   a  and the metal layer  103   c  of the Schottky electrode  103 . The dielectric film  104  may be provided in such a manner as to cover the side surfaces of the semiconductor layer  101  and cover a part of the upper surface of the semiconductor layer  101  (first semiconductor layer  101   a ). In the semiconductor element in  FIG. 1 , the dielectric film  104  enables improvement of a crystal defect at an end portion, more favorable formation of a depletion layer, further enhancement of electric field relaxation and more favorable curbing of leak current. Note that  FIG. 17  illustrates a preferable example of an SBD in which a porous layer  108  and a substrate  109  are disposed. 
       FIG. 6  illustrates a major portion of a Schottky barrier diode (SBD) as a semiconductor element, which is a preferable embodiment of the present disclosure. The SBD in  FIG. 6  is different from the SBD in  FIG. 1  in including a tapered region in each side surface of the Schottky electrode  103 . In the semiconductor element in  FIG. 6 , outer end portions of a metal layer  103   b  and/or a metal layer  103   c,  which correspond to a first metal layer, are located outside of an outer end portion of a metal layer  103   a,  which corresponds to a second metal layer, enabling more favorably curbing leak current. Furthermore, parts of the metal layer  103   b  and/or the metal layer  103   c,  the parts extending outside of the outer end portion of the metal layer  103   a,  each include a tapered region having a film thickness decreasing toward the outer side of the semiconductor element, providing a configuration having more excellent voltage resistance. 
     Examples of a material of the metal layer  103   a  include, e.g., the metals illustrated above by example. Also, examples of component materials of the metal layer  103   b  and the metal layer  103   c  include, e.g., the metals illustrated above by example. A method of forming each of the layers in  FIG. 1  is not specifically limited as long as such method does not hinder the present disclosure, and may be a publicly known method. Examples of the method include, e.g., a vacuum vapor deposition method, a CVD method, a sputtering method, a method in which a film is formed by any of various coating techniques and then patterned in a photolithography method and a method in which patterning is directly performed using, e.g., a printing technique. 
     A preferable process of manufacturing the SBD in  FIG. 17  will be described below; however, the present disclosure is not limited to these preferable manufacturing methods.  FIGS. 2 to 5  are diagram illustrating a mode of a preferable method of manufacturing a semiconductor element according to the present disclosure.  FIG. 2  provides illustration from a start with a stack  100   a  to obtainment of a stack  100   c.    FIG. 3  provides illustration from the stack  100   c  to obtainment of a stack  100   d.    FIG. 4  provides illustration from the stack  100   d  to obtainment of a stack  100   f.    FIG. 5  provides illustration from the stack  100   f  to obtainment of a stack  100   g.  The stack  100   a  illustrated in  FIG. 2  is formed by stacking a first semiconductor layer  101   a  and a second semiconductor layer  101   b  above a crystal growth substrate (sapphire substrate)  110  with a stress relaxation layer  111  in between using the mist CVD method. A stack  100   b  is obtained by forming a metal layer  102   a,  a metal layer  102   b  and a metal layer  102   c  as an ohmic electrode above the second semiconductor layer  101   b  of the stack  100   a  using the dry method or the wet method. The first semiconductor layer  101   a  is, for example, an n−-type semiconductor layer and the second semiconductor layer  101   b  is, for example, an n+-type semiconductor layer  101   b.  Also, the stack  100   c  illustrated in  FIG. 2  is obtained by stacking a substrate  109  on the stack  100   b  with a porous layer  108  formed of a precious metal in between. Then, as illustrated in  FIG. 3 , the crystal growth substrate  110  and the stress relaxation layer  111  of the stack  100   c  are removed using a known removal method to obtain the stack  100   d.  Then, as illustrated in  FIG. 4 , side surfaces of the semiconductor layers of the stack  100   d  are tapered by etching to obtain a stack  100   e,  and then, a dielectric film  104  is stacked on the tapered side surfaces and an upper surface, except a part corresponding to an opening portion, of the semiconductor layer to obtain the stack  100   f.  Next, as illustrated in  FIG. 5 , metal layers  103   a,    103   b  and  103   c  are formed as a Schottky electrode on a part of the upper surface of the semiconductor layer of the stack  100   f,  the part corresponding to the opening portion, using the dry method or the wet method to obtain the stack  100   g.  The semiconductor element obtained as described above has a configuration that enables favorable curbing of diffusion of, e.g., oxygen in the semiconductor layer, provision of excellent ohmic characteristics and improvement of a crystal defect at an end portion, more favorable forming of a depletion layer, further enhancement of electric field relaxation and more favorable curbing of leak current. 
     Note that although in the present embodiment, Pt or Pd is used as the metal layer  102   b,  where Ni is used as the metal layer  102   b,  for example, as illustrated in  FIG. 14 , a problem of discoloration occurs.  FIG. 14( a )  is a photograph showing an outer appearance after sintering where a Ti layer/Pt layer/Au layer (Ti/Pt/Au) was used as an ohmic electrode and  FIG. 14( b )  is a photograph showing an outer appearance after sintering where a Ti layer/Ni layer/Au layer (Ti/Ni/Au) was used as an ohmic electrode. As is clear from  FIG. 14 , where Ti/Ni/Au is used as an ohmic electrode in  FIG. 14( b ) , there is a problem of discoloration. Also,  FIG. 7  illustrates a result of SEM-EDS where Ti/Ni/Au was used as an ohmic electrode in  FIG. 14( b ) . As illustrated in  FIG. 7 , it is understandable that where Ti/Ni/Au is used as an ohmic electrode, diffusion of oxygen occurs. Note that  FIG. 7  is a diagram illustrating a result of SEM-EDS where Ti/Ni/Au was used as an ohmic electrode, and indicates respective distributions of Ti, Ni and O. As illustrated in the upper portion of  FIG. 7 , it is understandable that diffusion of Ti was curbed by the Ni layer, and as illustrated in the lower portion of  FIG. 7 , it is understandable that diffusion of oxygen occurred. Also, respective temperature (sintering temperature) dependances of contact resistivity of a case where Ti/Pt/Au was used as an ohmic electrode and contact resistivity of a case where Ti/Ni/Au was used as an ohmic electrode were evaluated.  FIG. 15  illustrates results. In the case where Ti/Pt/Au was used as an ohmic electrode in  FIG. 15( a ) , as the sintering temperature increased, the contact resistivity was favorably lowered. On the other hand, in the case where Ti/Ni/Au was used as an ohmic electrode in  FIG. 15( b ) , a problem such as the contact resistivity increasing as the sintering temperature increases, occurred. Note that when a power cycle test of a product of the present embodiment was conducted for performance evaluation, after completion of 3000 cycles in five minutes, a favorable evaluation result was obtained. 
     Also, it is preferable that the semiconductor element be a vertical device, and among others, the semiconductor element is useful for a power device. Examples of the semiconductor element include, e.g., diodes (for example, a P-N diode, a Schottky barrier diode and a junction barrier Schottky diode) and transistors (for example, a MOSFET and a MESFET), and among others, diodes are preferable and a Schottky barrier diode (SBD) is more preferable. 
     In addition to the above-described matters, the semiconductor element of the present disclosure is suitable for use as a semiconductor device by being bonded to a lead frame, a circuit substrate or a heat dissipation substrate with a bonding member based on a conventional method, and in particular, the semiconductor element is suitable for use as a power module, an inverter or a converter, and furthermore, is suitable for use in, e.g., a semiconductor system using a power supply device.  FIG. 11  illustrates a preferable example of the semiconductor device. In the semiconductor device in  FIG. 11 , each of opposite surfaces of a semiconductor element  500  is bonded to a lead frame, circuit substrate or heat dissipation substrate  502  with a solder  501 . Such configuration enables provision of a semiconductor device having excellent heat dissipation performance. Note that in the present disclosure, it is preferable that the peripheries of the bonding members such as solders be encapsulated by resin. 
     Also, it is possible to fabricate the power supply device from the semiconductor device or as the semiconductor device by connecting the semiconductor device to, e.g., a wiring pattern using a publicly known method. In  FIG. 8 , a power supply system  170  is configured using a plurality of the power supply devices  171 ,  172  and a control circuit  173 . As illustrated in  FIG. 9 , the power supply system is usable for a system device  180  in combination of an electronic circuit  181  and a power supply system  182 . Note that  FIG. 10  illustrates an example of a power supply circuit diagram of a power supply device.  FIG. 10  illustrates a power supply circuit of a power supply device formed of a power circuit and a control circuit, and a DC voltage is converted into an AC voltage by being switched at a high frequency by an inverter  192  (formed of MOSFETs A to D) and then, the AC voltage is subjected to insulation and transformation with a transformer  193  and rectified by rectifying MOSFETs  194  (A to B′), and then smoothed by a DCL  195  (smoothing coils L1, L2) and a capacitor to output a direct-current voltage. At this time, the output voltage is compared with a reference voltage in a voltage comparator  197  and a PWM control circuit  196  controls the inverter  192  and the rectifying MOSFETs  194  so that the output voltage becomes a desired output voltage. 
     In an aspect of the present disclosure, the semiconductor device is preferably a power card, more preferably includes coolers and insulating members, the coolers being provided on opposite sides of the semiconductor layer with at least the insulating members in between, respectively, and most preferably includes heat dissipation layers provided on the opposite sides of the semiconductor layer, respectively, the coolers being provided on respective outer sides of the heat dissipation layers with the insulating members in between, respectively.  FIG. 12  illustrates a power card, which is a preferable embodiment of the present disclosure. The power card in  FIG. 12  is a double-sided cooling-type power card  201  and includes refrigerant tubes  202 , spacers  203 , insulating plates (insulating spacers)  208 , a resin encapsulating portion  209 , a semiconductor chip  301   a  including a semiconductor element, a metal heat transfer plate (projecting terminal portion)  302   b,  a heatsink and electrode  303 , a metal heat transfer plate (projecting terminal portion)  303   b,  solder layers  304 , a control electrode terminal  305  and a bonding wire  308 . A section in a thickness direction of the refrigerant tube  202  includes a multitude of flow channels  222  defined by a multitude of partitioning walls  221  extending in a flow channel direction in such a manner as to be spaced a predetermined distance from one another. Such preferable power card enables provision of higher heat dissipation performance and enables provision of higher reliability. 
     The semiconductor chip  301   a  is bonded to a principal surface on the inner side of the metal heat transfer plate  302   b  with a solder layer  304  and the metal heat transfer plate (projecting terminal portion)  302   b  is bonded to a remaining principal surface of the semiconductor chip  301   a  with a solder layer  304 . Consequently, an anode electrode surface and a cathode electrode surface of a flywheel diode are connected in what is called inverse-parallel to a collector electrode surface and an emitter electrode surface of an IGBT. Examples of materials of the metal heat transfer plates (projecting terminal portions)  302   b  and  303   b  include, e.g., Mo and W. The metal heat transfer plates (projecting terminal portions)  302   b  and  303   b  each have thickness variations that absorb thickness variations of the semiconductor chip  301   a,  and consequently, each of respective outer surfaces of the metal heat transfer plates  302   b  and  303   b  is a flat surface. 
     The resin encapsulating portion  209  is formed of, for example, an epoxy resin and molded in such a manner as to cover side surfaces of the metal heat transfer plates  302   b  and  303   b,  and the semiconductor chip  301   a  is molded by the resin encapsulating portion  209 . However, outer principal surfaces, that is, contact/heat receiving surfaces, of the metal heat transfer plate  302   b  and  303   b  are completely exposed. The metal heat transfer plates (projecting terminal portions)  302   b  and  303   b  project rightward from the resin encapsulating portion  209  in  FIG. 9 , and the control electrode terminal  305 , which is what is called a lead frame terminal, connects, for example, a gate (control) electrode surface of the semiconductor chip  301   a  with the IGBT formed therein and the control electrode terminal  305 . 
     The insulating plate  208 , which is an insulating spacer, is formed of, for example, an aluminum nitride film but may be formed of another insulating film. The insulating plates  208  are in close contact with the metal heat transfer plates  302   b  and  303   b  in such a manner as to completely cover the metal heat transfer plates  302   b  and  303   b,  respectively; however, the insulating plates  208  and the metal heat transfer plates  302   b  and  303   b  may simply be in contact with each other, may be coated with a high thermally conductive material such as silicon grease or may be bonded by any of various methods, respectively. Also, insulating layers may be formed by ceramic spraying, for example, or the insulating plates  208  may be bonded to the metal heat transfer plates or may be bonded to or formed on refrigerant tubes, respectively. 
     The refrigerant tubes  202  are each fabricated by cutting a plate material into necessary lengths, the plate material being formed by molding an aluminum alloy by a pultrusion molding method or an extrusion molding method. A section in a thickness direction of each refrigerant tube  202  includes a multitude of flow channels  222  defined by a multitude of partitioning walls  221  extending in a flow channel direction in such a manner as to be spaced a predetermined distance from one another. Each of the spacers  203  may be, for example, a flexible metal plate of, e.g., a solder alloy or may be a film formed by, e.g., coating of contact surfaces of the metal heat transfer plates  302   b  and  303   b.  A surface of each flexible spacer  203  easily deforms, and fits in minute bumps and dips and warpage of the corresponding insulating plate  208  and minute bumps and dips and warpage of the corresponding refrigerant tube  202 , resulting in decrease in thermal resistance. Note that, e.g., known high thermally conductive grease may be applied to, e.g., the surfaces of the spacers  203  or the spacers  203  may be omitted. 
     The semiconductor element of the present disclosure is usable in various fields of semiconductors (for example, compound semiconductor electronic devices, etc.), electronic components and electrical equipment components, optical and electronic photography-related devices, industrial members, etc., and particularly, is useful for a power device. 
     The embodiments of the present invention are exemplified in all respects, and the scope of the present invention includes all modifications within the meaning and scope equivalent to the scope of claims. 
     REFERENCE SIGNS LIST 
     
         
           101  Semiconductor layer 
           101   a  First semiconductor layer 
           101   b  Second semiconductor layer 
           102  Ohmic electrode 
           102   a  Metal layer 
           102   b  Metal layer 
           102   c  Metal layer 
           103  Schottky electrode 
           103   a  Metal layer 
           103   b  Metal layer 
           103   c  Metal layer 
           104  Dielectric film 
           108  Porous layer 
           109  Substrate 
           110  Crystal growth substrate 
           170  Power supply system 
           171  Power supply device 
           172  Power supply device 
           173  Control circuit 
           180  System device 
           181  Electronic circuit 
           182  Power supply system 
           192  Inverter 
           193  Transformer 
           194  Rectifying MOSFET 
           195  DCL 
           196  PWM control circuit 
           197  Voltage comparator 
           201  Double-sided cooling-type power card 
           202  Refrigerant tube 
           203  Spacer 
           208  Insulating plate (insulating spacer) 
           209  Resin encapsulating portion 
           221  Partitioning wall 
           222  Flow channel 
           301   a  Semiconductor chip 
           302   b  Metal heat transfer plate (projecting terminal portion) 
           303  Heatsink and electrode 
           303   b  Metal heat transfer plate (projecting terminal portion) 
           304  Solder layer 
           305  Control electrode terminal 
           308  Bonding wire 
           500  Semiconductor element 
           501  Solder 
           502  Lead frame, circuit substrate or heat dissipation substrate