Oxide semiconductor device and method for manufacturing same

Abstract

An object is to provide a technology for enabling prevention of deterioration of characteristics of an oxide semiconductor device. The oxide semiconductor device includes an n-type gallium oxide epitaxial layer, a p-type oxide semiconductor layer, and an oxide layer. The p-type oxide semiconductor layer is disposed above the n-type gallium oxide epitaxial layer, contains an element different from gallium as a main component, and has p-type conductivity. The oxide layer is disposed between the n-type gallium oxide epitaxial layer and the p-type oxide semiconductor layer, and is made of a material different from gallium oxide and different at least partly from a material of the p-type oxide semiconductor layer.

Claims

1. An oxide semiconductor device, comprising: an n-type gallium oxide layer; a p-type oxide semiconductor layer disposed above the n-type gallium oxide layer, the p-type oxide semiconductor layer containing, as a main component, an element different from gallium and having p-type conductivity; a first electrode electrically bonded to the p-type oxide semiconductor layer; and an oxide layer disposed between the n-type gallium oxide layer and the p-type oxide semiconductor layer, the oxide layer being made of a material different from gallium oxide and different at least partly from a material of the p-type oxide semiconductor layer.

2. The oxide semiconductor device according to claim 1, further comprising a second electrode electrically bonded to a lower surface of the n-type gallium oxide layer, wherein the first electrode is also electrically connected to an upper surface of the n-type gallium oxide layer.

3. The oxide semiconductor device according to claim 1, wherein a plurality of the p-type oxide semiconductor layers with spacings are embedded in an upper surface of the n-type gallium oxide layer.

4. The oxide semiconductor device according to claim 1, wherein the p-type oxide semiconductor layer is made of a metal oxide containing Cu.

5. The oxide semiconductor device according to claim 1, wherein the oxide layer is made of an oxide containing Al.sub.2O.sub.3 as a main component.

6. The oxide semiconductor device according to claim 1, wherein the oxide layer is made of a metal oxide containing Cu and Al.

7. The oxide semiconductor device according to claim 1, wherein the oxide layer is thicker than or equal to 3 nm.

8. A method for manufacturing the oxide semiconductor device according to claim 1, the method comprising forming the oxide layer at a temperature higher than or equal to 400° C.

9. The oxide semiconductor device according to claim 1, further comprising: a source electrode bonded to a portion adjacent to the p-type oxide semiconductor layer in the n-type gallium oxide layer; and a drain electrode bonded to a lower surface of the n-type gallium oxide layer, wherein the first electrode is a gate electrode.

10. The oxide semiconductor device according to claim 1, further comprising: a source electrode bonded to a first portion adjacent to the p-type oxide semiconductor layer in the n-type gallium oxide layer; and a drain electrode bonded to a second portion adjacent to the p-type oxide semiconductor layer in the n-type gallium oxide epitaxial layer on an opposite side of the first portion, wherein the first electrode is a gate electrode disposed between the source electrode and the drain electrode.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view schematically exemplifying a structure of an oxide semiconductor device according to Embodiment 1.

(2) FIG. 2 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(3) FIG. 3 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(4) FIG. 4 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(5) FIG. 5 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(6) FIG. 6 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(7) FIG. 7 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(8) FIG. 8 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(9) FIG. 9 is a cross-sectional view for describing a manufacturing process of the oxide semiconductor device according to Embodiment 1.

(10) FIG. 10 is a cross-sectional view schematically exemplifying a structure of an oxide semiconductor device according to Embodiment 2.

(11) FIG. 11 is a cross-sectional view schematically exemplifying a structure of an oxide semiconductor device according to Embodiment 3.

(12) FIG. 12 is a cross-sectional view schematically exemplifying a structure of an oxide semiconductor device according to Embodiment 4.

(13) FIG. 13 is a cross-sectional view schematically exemplifying a structure of an oxide semiconductor device according to Embodiment 5.

DESCRIPTION OF EMBODIMENTS

(14) Hereinafter, Embodiments of the present invention will be described with reference to the accompanying drawings. The drawings are drawn in schematic form, and the structures are appropriately omitted or simplified for convenience in description. The mutual relationships in size and position between the structures in the different drawings are not necessarily accurate but may be changed when needed.

(15) In the following description, the same reference numerals are assigned to the same constituent elements, and their names and functions are the same. Therefore, detailed description of such constituent elements may be omitted to avoid redundant description.

Embodiment 1

(16) An oxide semiconductor device and a method for manufacturing the oxide semiconductor device according to Embodiment 1 will be described hereinafter. First, a structure of the oxide semiconductor device according to Embodiment 1 will be described. In the following description, the oxide semiconductor device may be simply referred to as a “semiconductor device”.

(17) FIG. 1 is a cross-sectional view schematically exemplifying a structure for materializing the semiconductor device according to Embodiment 1. In the semiconductor device according to Embodiment 1, a termination structure of a Schottky barrier diode (SBD) includes p-type oxide semiconductor layers 6.

(18) The semiconductor device according to Embodiment 1 including an electrode on the upper side of a substrate as an anode electrode 1 and an electrode on the lower side of the substrate as a cathode electrode 2 will be described. However, the semiconductor device according to Embodiment 1 is not limited to the SBD but may be another power device element such as a switching element.

(19) The semiconductor device exemplified in FIG. 1 includes an n-type gallium oxide layer. Although an example of the n-type gallium oxide layer including an n-type single-crystal gallium oxide substrate 3 and an n-type gallium oxide epitaxial layer 4 will be described, the n-type gallium oxide layer is not limited to this example.

(20) The n-type single-crystal gallium oxide substrate 3 is an n-type oxide semiconductor including an upper surface (a first principal surface) and a lower surface opposite to the upper surface (a second principal surface). The n-type gallium oxide epitaxial layer 4 is an epitaxial layer disposed on the upper surface of the n-type single-crystal gallium oxide substrate 3.

(21) The semiconductor device exemplified in FIG. 1 includes, on the upper surface of the n-type gallium oxide epitaxial layer 4, the anode electrode 1 that is the first electrode and forms an electrical Schottky junction with the n-type gallium oxide epitaxial layer 4. Furthermore, the semiconductor device exemplified in FIG. 1 includes, on the lower surface of the n-type single-crystal gallium oxide substrate 3, the cathode electrode 2 that is the second electrode and forms an electrical Ohmic junction with the lower surface of the n-type single-crystal gallium oxide substrate 3.

(22) The semiconductor device exemplified in FIG. 1 includes the p-type oxide semiconductor layers 6 containing, as a main component, an element different from gallium and having p-type conductivity. Although the plurality of p-type oxide semiconductor layers 6 with spacings are embedded from the upper surface to the inside of the n-type gallium oxide epitaxial layer 4 according to Embodiment 1, they have only to be disposed above the n-type gallium oxide epitaxial layer 4. The anode electrode 1 forms an electrical Ohmic junction with the p-type oxide semiconductor layers 6.

(23) The semiconductor device exemplified in FIG. 1 includes oxide layers 7 which are disposed between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6 to separate these. The oxide layers 7 are made of a material different from gallium oxide and different at least partly from that of the p-type oxide semiconductor layers 6. The oxide layers 7 being made of a material different at least partly from that of the p-type oxide semiconductor layers 6 involve, for example: (i) compounds of the oxide layers 7 being different from those of the p-type oxide semiconductor layers 6; (ii) a part of the plurality of compounds of the oxide layers 7 being identical to those of the p-type oxide semiconductor layers 6 but the remaining part of the compounds being different from those of the p-type oxide semiconductor layers 6; and (iii) a part of the plurality of compounds of the p-type oxide semiconductor layers 6 being identical to those of the oxide layers 7 but the remaining part of the compounds being different from those of the oxide layers 7.

(24) Such oxide layers 7 can inhibit chemical reactions in a p-n interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6 and maintain the normal interface. As a result, the deterioration of the characteristics of the oxide semiconductor device such as heat resistance and voltage resistance can be prevented.

(25) The semiconductor device exemplified in FIG. 1 includes, in the termination structure, a field-plate insulating material layer 5 disposed between the n-type gallium oxide epitaxial layer 4 and the anode electrode 1. A laminated portion of the field-plate insulating material layer 5 and the anode electrode 1 forms a field plate structure. This increases the breakdown voltage of the semiconductor device when a reverse voltage is applied to the semiconductor device.

(26) Next, the aforementioned constituent elements will be further described in detail.

(27) The n-type single-crystal gallium oxide substrate 3 is an n-type oxide semiconductor made of single crystal Ga.sub.2O.sub.3, and is more preferably an n-type oxide semiconductor made of single crystal β-Ga.sub.2O.sub.3. Since the n-type single-crystal gallium oxide substrate 3 exhibits n-type conductivity due to oxygen deficiency in crystals, it does not have to contain n-type impurities. However, the n-type single-crystal gallium oxide substrate 3 may contain n-type impurities such as silicon (Si) or tin (Sn). In other words, the n-type single-crystal gallium oxide substrate 3 may be any of the following substrates: a substrate that exhibits n-type conductivity due to only oxygen deficiency; a substrate that exhibits n-type conductivity due to only n-type impurities; and a substrate that exhibits n-type conductivity due to both oxygen deficiency and n-type impurities.

(28) The electron carrier concentration of the n-type single-crystal gallium oxide substrate 3 containing n-type impurities is a total density calculated from oxygen deficiency and the n-type impurities. The electron carrier concentration of the n-type single-crystal gallium oxide substrate 3 may be, for example, higher than or equal to 1×10.sup.17 cm.sup.−3 and lower than or equal to 1×10.sup.19 cm.sup.−3. The impurity concentration may be set higher than the numerical range to reduce contact resistance between the n-type single-crystal gallium oxide substrate 3 and the cathode electrode 2.

(29) The n-type gallium oxide epitaxial layer 4 is disposed on the upper surface of the n-type single-crystal gallium oxide substrate 3. The n-type gallium oxide epitaxial layer 4 is an n-type oxide semiconductor made of single crystal Ga.sub.2O.sub.3, and is more preferably an n-type oxide semiconductor made of single crystal β-Ga.sub.2O.sub.3. An n-type carrier density of the n-type gallium oxide epitaxial layer 4 is preferably lower than that of the n-type single-crystal gallium oxide substrate 3, and may be, for example, higher than or equal to 1×10.sup.15 cm.sup.−3 and lower than or equal to 1×10.sup.17 cm.sup.−3.

(30) The cathode electrode 2 is disposed on the lower surface of the n-type single-crystal gallium oxide substrate 3. Since the cathode electrode 2 forms an Ohmic junction with the n-type single-crystal gallium oxide substrate 3, the cathode electrode 2 is preferably made of a metal whose work function is smaller than that of the n-type single-crystal gallium oxide substrate 3. Furthermore, the cathode electrode 2 is preferably made of a metal that reduces contact resistance between the n-type single-crystal gallium oxide substrate 3 and the cathode electrode 2 through heat treatment performed after forming the cathode electrode 2 on the lower surface of the n-type single-crystal gallium oxide substrate 3.

(31) Such metal may be, for example, titanium (Ti). Furthermore, the cathode electrode 2 may be formed by laminating a plurality of metals. For example, when a metal easily oxidized comes in contact with the lower surface of the n-type single-crystal gallium oxide substrate 3, a metal hardly oxidized may be formed on the lower surface of the metal easily oxidized to obtain the cathode electrode 2 with a laminated structure. For example, the cathode electrode 2 may be formed by forming a first layer made of Ti which comes in contact with the n-type single-crystal gallium oxide substrate 3 and then forming a second layer made of gold (Au) or silver (Ag) on the lower surface of the first layer. Furthermore, the cathode electrode 2 may be disposed on the entire lower surface or a part of the lower surface of the n-type single-crystal gallium oxide substrate 3.

(32) The anode electrode 1 is disposed on the upper surface of the n-type gallium oxide epitaxial layer 4. Since the anode electrode 1 forms a Schottky junction with the n-type gallium oxide epitaxial layer 4, the anode electrode 1 is preferably made of a metal whose work function is larger than that of the n-type gallium oxide epitaxial layer 4. Since the anode electrode 1 also forms an Ohmic junction with the p-type oxide semiconductor layers 6, the anode electrode 1 is more preferably made of a metal whose work function is smaller than that of the p-type oxide semiconductor layers 6 (a p-type oxide semiconductor material).

(33) Examples of the metal may include platinum (Pt), nickel (Ni), gold (Au), and palladium (Pd). The anode electrode 1 may have a laminated structure similarly to the cathode electrode 2. For example, the anode electrode 1 may be formed by forming a first layer made of a metal suited for the Schottky junction with the n-type gallium oxide epitaxial layer 4, in contact with the n-type gallium oxide epitaxial layer 4 and then forming a second layer made of another metal on the upper surface of the first layer.

(34) The p-type oxide semiconductor layers 6 are embedded from the upper surface to the inside of the n-type gallium oxide epitaxial layer 4. The p-type oxide semiconductor layers 6 are made of a p-type oxide semiconductor exhibiting p-type conductivity without being doped with p-type impurities, such as copper oxide (Cu.sub.2O), silver oxide (Ag.sub.2O), nickel oxide (NiO), or tin oxide (SnO). For example, Cu.sub.2O, which is a metal oxide, exhibits p-type conductivity because the 3d orbital of Cu forms the valence band maximum that undertakes hole conduction, and holes appear due to Cu deficiency. When Cu.sub.2O transforms into CuO due to oxidation, the 3d orbital of Cu does not form the valence band maximum, and the p-type conductivity is lost. The p-type oxide semiconductor layers 6 are made of a p-type oxide semiconductor that is a metal oxide with such properties. The described p-type oxide semiconductor such as Cu.sub.2O typically exhibits p-type conductivity without being doped with p-type impurities.

(35) Although being made of a p-type oxide semiconductor exhibiting p-type conductivity without being doped with p-type impurities, the p-type oxide semiconductor layers 6 may be doped with p-type impurities. For example, when the p-type oxide semiconductor layers 6 are made of Cu.sub.2O, nitrogen (N) may be used as the p-type impurities. When the p-type oxide semiconductor layers 6 are not doped with p-type impurities, the p-type carrier density is a density of metal atom deficiency in the p-type oxide semiconductor. When the p-type oxide semiconductor layers 6 are doped with p-type impurities, the p-type carrier density is a total density calculated from the metal atom deficiency and the p-type impurities in the p-type oxide semiconductor.

(36) When the p-type oxide semiconductor layers 6 are doped with p-type impurities, even after the metal oxide of the p-type oxide semiconductor is oxidized and loses p-type conductivity, the entire p-type oxide semiconductor sometimes exhibits the p-type conductivity with the p-type impurities. When the metal oxide of the p-type oxide semiconductor is oxidized and loses the p-type conductivity corresponding to the oxidation, the p-type conductivity of the entire p-type oxide semiconductor decreases. Thus, it is preferred not to oxidize the metal oxide of the p-type oxide semiconductor.

(37) The field-plate insulating material layer 5 is made of a material, for example, silicon dioxide (SiO.sub.2) or aluminum oxide (Al.sub.2O.sub.3). Such a material has higher breakdown field strength than that of Ga.sub.2O.sub.3 contained in the n-type gallium oxide epitaxial layer 4. The field-plate insulating material layer 5 may be approximately several hundred nanometers thick, for example, thicker than or equal to 100 nm and thinner than or equal to 200 nm.

(38) The oxide layers 7 are made of, for example, a mixed crystal of Cu.sub.2O and Al.sub.2O.sub.3. The oxide layers 7 are disposed at the entire interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6 to separate these. Particularly, when the metal oxide contained in the p-type oxide semiconductor layers 6 is included in the mixed crystal of the oxide layers 7, each of the oxide layers 7 is preferably thicker than or equal to 3 nm, for example, thicker than or equal to 3 nm and thinner than or equal to 200 nm.

(39) For example, when the oxide layers 7 are made of the mixed crystal of Cu.sub.2O and Al.sub.2O.sub.3 and the p-type oxide semiconductor layers 6 are made of Ag.sub.2O, the compounds of the oxide layers 7 are different from those of the p-type oxide semiconductor layers 6. For example, when the oxide layers 7 are made of the mixed crystal of Cu.sub.2O and Al.sub.2O.sub.3 and the p-type oxide semiconductor layers 6 are made of Cu.sub.2O, a part (Cu.sub.2O) of the compounds of the oxide layers 7 is identical to that (Cu.sub.2O) of the p-type oxide semiconductor layers 6 and the remaining part (Al.sub.2O.sub.3) of the compounds is different from that (Cu.sub.2O) of the p-type oxide semiconductor layers 6.

(40) [Method for Manufacturing Oxide Semiconductor Device]

(41) Next, a method for manufacturing the semiconductor device according to Embodiment 1 will be described.

(42) First, the n-type single-crystal gallium oxide substrate 3 is prepared as illustrated in FIG. 2. The n-type single-crystal gallium oxide substrate 3 can be a substrate obtained by cutting, into substrates, a single-crystal bulk β-Ga.sub.2O.sub.3 produced by a melt growth process.

(43) Then, the n-type gallium oxide epitaxial layer 4 is deposited on the upper surface of the n-type single-crystal gallium oxide substrate 3 through epitaxial growth as illustrated in FIG. 3. The n-type gallium oxide epitaxial layer 4 can be formed on the upper surface of the n-type single-crystal gallium oxide substrate 3 by a method such as metal organic chemical vapor deposition (i.e., MOCVD), molecular beam epitaxy (i.e., MBE), or halide vapor phase epitaxy (i.e., HVPE).

(44) Next, trenches 4a that are ditches are formed on the upper surface of the n-type gallium oxide epitaxial layer 4, using dry etching gas such as boron trichloride (BCl.sub.3) as illustrated in FIG. 4. The method for forming the trenches 4a is not particularly limited, but can be an existing formation method such as dry etching or wet etching.

(45) Next, a metal for the cathode electrode 2 is deposited by vapor deposition or sputtering on the lower surface of the n-type single-crystal gallium oxide substrate 3 as illustrated in FIG. 5. For example, a Ti layer of a thickness of 100 nm is deposited on the lower surface of the n-type single-crystal gallium oxide substrate 3 by electron beam evaporation (EB evaporation). Then, an Ag layer of a thickness of 300 nm is deposited on the Ti layer by electron beam evaporation to form the cathode electrode 2 having a two-layer structure. Thereafter, for example, heat treatment is performed at 550° C. for five minutes under a nitrogen or oxygen atmosphere. As a result, the cathode electrode 2 that forms an Ohmic junction with the n-type single-crystal gallium oxide substrate 3 is formed on the lower surface of the n-type single-crystal gallium oxide substrate 3. To reduce contact resistance between the n-type single-crystal gallium oxide substrate 3 and the cathode electrode 2, an RIE process using gas such as BCl.sub.3 is performed on the lower surface of the n-type single-crystal gallium oxide substrate 3 before forming the cathode electrode 2.

(46) Next, the oxide layers 7 are formed to cover the trenches 4a. There are the following two methods for forming the oxide layers 7.

(47) The first method is a method for directly forming the oxide layers 7 with desired physical properties on the trenches 4a, using a method such as co-sputtering or Pulse Laser Deposition (i.e., PLD) as illustrated in FIG. 6. In this method, the p-type oxide semiconductor layers 6 are subsequently formed on the oxide layers 7 to fill the trenches 4a with the p-type oxide semiconductor layers 6 as illustrated in FIG. 7.

(48) The second method is a method for forming the oxide layers 7 through heat treatment performed after forming the p-type oxide semiconductor layers 6, which is not illustrated. For example, when the p-type oxide semiconductor layers 6 are made of Cu.sub.2O, Al.sub.2O.sub.3 can be selected as a material for the oxide layers 7. Here, forming an Al.sub.2O.sub.3 film on the trenches 4a, forming the p-type oxide semiconductor layers 6 made of Cu.sub.2O on the Al.sub.2O.sub.3 film, and then performing heat treatment thereon forms a mixed crystal oxide of the Al.sub.2O.sub.3 film and Cu.sub.2O. As a result, the oxide layers 7 made of a mixed crystal of Cu.sub.2O on the Al.sub.2O.sub.3 can be formed. Here, the Al.sub.2O.sub.3 film is preferably thicker than or equal to 3 nm. Furthermore, the heat treatment, that is, formation of the oxide layers 7 is preferably performed at a temperature higher than or equal to 400° C., and more preferably at a temperature higher than or equal to 400° C. and lower than or equal to 1200° C.

(49) Next, the field-plate insulating material layer 5 is formed on the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6 in the termination structure as illustrated in FIG. 8. The method for forming the field-plate insulating material layer 5 is not particularly limited, but can be, for example, Spin-on Glass (i.e., SOG).

(50) Lastly, the anode electrode 1 is formed on the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6 to be exposed from the field-plate insulating material layer 5 as illustrated in FIG. 9 to complete the semiconductor device according to Embodiment 1.

Conclusion of Embodiment 1

(51) In the oxide semiconductor device according to Embodiment 1, the oxide layers 7, which are made of a material different from gallium oxide and different at least partly from that of the p-type oxide semiconductor layers 6, are disposed between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6. Since this structure can inhibit chemical reactions at the p-n interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6, the deterioration of the characteristics of the oxide semiconductor device such as heat resistance and voltage resistance can be prevented.

Embodiment 2

(52) FIG. 10 is a cross-sectional view schematically exemplifying a structure of a semiconductor device according to Embodiment 2 of the present invention. The method for manufacturing the semiconductor device according to Embodiment 2 is identical to that according to Embodiment 1.

(53) In the semiconductor device according to Embodiment 1 (FIG. 1), the plurality of p-type oxide semiconductor layers 6 are disposed in the termination structure as guard rings. In the semiconductor device according to Embodiment 2 (FIG. 10), the plurality of p-type oxide semiconductor layers 6 are disposed not only in the termination structure but also in the element portions. Specifically, the plurality of p-type oxide semiconductor layers 6 with spacings are embedded in the upper surface of the n-type gallium oxide epitaxial layer 4 that is exposed from the field-plate insulating material layer 5. Thus, a Merged PiN/Schottky (MPS) structure that is a combination of a p-n junction and a Schottky junction is disposed according to Embodiment 2.

(54) In the MPS structure, a bipolar operation of a PND enables a larger surge current exceeding the rating to flow with a small voltage drop than by a single SBD. Thus, the oxide semiconductor device with the MPS structure according to Embodiment 2 improves the forward surge tolerance. This can suppress increase in the forward voltage drop, and materialize a semiconductor device with a rectification function and the high forward surge tolerance.

Embodiment 3

(55) FIG. 11 is a cross-sectional view schematically exemplifying a structure of a semiconductor device according to Embodiment 3 of the present invention. The method for manufacturing the semiconductor device according to Embodiment 3 is identical to that according to Embodiment 1.

(56) In the semiconductor device exemplified in FIG. 11, the bottom of each of the trenches 4a, or the bottom of each of the p-type oxide semiconductor layers 6 and the oxide layers 7 is different from that in the semiconductor device (FIG. 10) exemplified in Embodiment 2. Specifically, the bottom of each of the p-type oxide semiconductor layers 6 and the oxide layers 7 is bent and is almost arc-shaped as exemplified in FIG. 11 according to Embodiment 3. Since concentration of the electric field can be reduced when a reverse voltage is applied to the oxide semiconductor device according to Embodiment 3, increase in the reverse breakdown voltage can be expected.

Embodiment 4

(57) FIG. 12 is a cross-sectional view schematically exemplifying a structure of a semiconductor device according to Embodiment 4 of the present invention. Among the constituent elements according to Embodiment 4, the same reference numerals are assigned to the same constituent elements already described, and the different constituent elements will be mainly described in the following description.

(58) The semiconductor device exemplified in FIG. 12 is a field effect transistor whose current direction is the vertical direction in FIG. 12. The semiconductor device exemplified in FIG. 12 includes a source electrode 8 and gate electrodes 10 that are disposed above a substrate, and a drain electrode 9 that is disposed on the bottom of the substrate. The source electrode 8 is directly bonded to a portion adjacent to the p-type oxide semiconductor layers 6 in the n-type gallium oxide epitaxial layer 4. The drain electrode 9 is directly bonded to the lower surface of the n-type single-crystal gallium oxide substrate 3. The gate electrodes 10 that are the first electrodes are directly bonded to the p-type oxide semiconductor layers 6 without being bonded to the n-type gallium oxide epitaxial layer 4.

(59) In the semiconductor device according to Embodiment 4, the oxide layers 7 are inserted into the interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6, similarly to the structures described in Embodiments above. Thus, the chemical reactions at the p-n interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layers 6 can be inhibited, and the normal interface can be maintained also in the semiconductor device according to Embodiment 4. As a result, the deterioration of the characteristics of the oxide semiconductor device such as heat resistance and voltage resistance can be prevented.

Embodiment 5

(60) FIG. 13 is a cross-sectional view schematically exemplifying a structure of a semiconductor device according to Embodiment 5 of the present invention. Among the constituent elements according to Embodiment 5, the same reference numerals are assigned to the same constituent elements already described, and the different constituent elements will be mainly described in the following description.

(61) The semiconductor device exemplified in FIG. 13 is a field effect transistor whose current direction is the horizontal direction in FIG. 13. The semiconductor device exemplified in FIG. 13 includes the source electrode 8, the drain electrode 9, and the gate electrode 10 that are disposed above the substrate. The source electrode 8 is directly bonded to a first portion adjacent to the p-type oxide semiconductor layer 6 in the n-type gallium oxide epitaxial layer 4. The drain electrode 9 is directly bonded to a second portion adjacent to the p-type oxide semiconductor layer 6 in the n-type gallium oxide epitaxial layer 4 on the opposite side of the first portion. The gate electrode 10 that is the first electrode is directly bonded to the p-type oxide semiconductor layer 6 without being bonded to the n-type gallium oxide epitaxial layer 4, and is disposed between the source electrode 8 and the drain electrode 9.

(62) In the semiconductor device according to Embodiment 5, the oxide layer 7 is inserted into the interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layer 6, similarly to the structures described in Embodiments above. Thus, the chemical reactions at the p-n interface between the n-type gallium oxide epitaxial layer 4 and the p-type oxide semiconductor layer 6 can be inhibited, and the normal interface can be maintained also in the semiconductor device according to Embodiment 5. As a result, the deterioration of the characteristics of the oxide semiconductor device such as heat resistance and voltage resistance can be prevented.

(63) Embodiments and the modifications can be freely combined, and appropriately modified or omitted within the scope of the invention.

(64) Although this invention is described in detail, the description is in all aspects illustrative and does not restrict the invention. Therefore, numerous modifications and variations that have not yet been exemplified are devised without departing from the scope of the present invention.

EXPLANATION OF REFERENCE SIGNS

(65) 1 anode electrode, 2 cathode electrode, 3 n-type single-crystal gallium oxide substrate, 4 n-type gallium oxide epitaxial layer, 5 field-plate insulating material layer, 6 p-type oxide semiconductor layer, 7 oxide layer, 8 source electrode, 9 drain electrode, 10 gate electrode.