Semiconductor device having metal oxide film

1. A semiconductor device comprising:

a nitride group semiconductor functional layer including a second nitride group semiconductor region on a first nitride group semiconductor region where a two-dimensional carrier gas layer is made, the second nitride group semiconductor region functioning as a barrier layer;

a first main electrode electrically connected to one end of the two-dimensional carrier gas layer;

a second main electrode electrically connected to the other end of the two-dimensional carrier gas layer; and

a metal oxide film placed between the first and second main electrodes buried in a recess extending from a surface of the nitride group semiconductor functional layer to the two-dimensional carrier gas layer, and electrically connected to the first main electrode,

wherein the first main electrode extends over an upper surface of the metal oxide film, the metal oxide film is separated from the second main electrode to convey a substantially same voltage between the first main electrode and the metal oxide film during a reverse biasing.

2. The semiconductor device defined in claim 1 , wherein the metal oxide film is constituted by at least one of nickel oxide, iron oxide, cobalt oxide, manganese oxide and copper oxide.

3. The semiconductor device defined in claim 1 , wherein the metal oxide film surrounds the first main electrode intermittently or continuously.

4. The semiconductor device defined in claim 1 , wherein the first main electrode extends to the upper surface of the metal oxide film via an insulating film.

5. The semiconductor device defined in claim 1 , wherein the metal oxide film is placed on the two-dimensional carrier gas layer via a part of the nitride group semiconductor functional layer, and the metal oxide film reduces a carrier density of the two-dimensional carrier gas layer.

6. The semiconductor device defined in claim 5 , wherein the metal oxide film is constituted by at least one of nickel oxide, iron oxide, cobalt oxide, manganese oxide and copper oxide.

7. The semiconductor device defined in claim 5 , wherein the metal oxide film surrounds the first main electrode intermittently or continuously.

8. The semiconductor device defined in claim 5 , wherein the first main electrode extends to the upper surface of the metal oxide film via an insulating film.

9. A semiconductor device comprising:

a nitride group semiconductor functional layer including a second nitride group semiconductor region on a first nitride group semiconductor region where a two-dimensional carrier gas layer is made, the second nitride group semiconductor region functioning as a barrier layer;

a first main electrode electrically connected to one end of the two-dimensional carrier gas layer;

a second main electrode electrically connected to the other end of the two-dimensional carrier gas layer; and

a metal oxide film placed between the first and second main electrodes so as to extend over an upper surface of the first main electrode, and electrically connected to the first main electrode, the metal oxide film is separated from the second main electrode to convey a substantially same voltage between the first main electrode and the metal oxide film during a reverse biasing.

10. The semiconductor device defined in claim 9 , wherein the metal oxide film is constituted by at least one of nickel oxide, iron oxide, cobalt oxide, manganese oxide and copper oxide.

11. The semiconductor device defined in claim 9 , wherein the metal oxide film surrounds the first main electrode intermittently or continuously.

12. The semiconductor device defined in claim 9 , wherein the metal oxide film is buried in a recess extending from a surface of the nitride group semiconductor functional layer to the two-dimensional carrier gas layer.