Semiconductor device

1. A semiconductor device comprising:

a back barrier layer formed of a compound semiconductor over a substrate;

a first electron transit layer formed of a compound semiconductor in such a manner that a bottom surface of the first electron transit layer is physically in contact with an upper surface of the back barrier layer;

an opening formed in the first electron transit layer and the back barrier layer;

a second electron transit layer formed of a compound semiconductor over the first electron transit layer, a side surface of the first electron transit layer at a side surface within the opening, a side surface of the back barrier layer at a side surface within the opening, and a surface of the back barrier layer at a bottom surface within the opening;

an electron supply layer formed of a compound semiconductor over the second electron transit layer;

a source electrode formed over the electron supply layer in a region in which the first electron transit layer is formed;

a drain electrode formed over the electron supply layer within the opening; and

a gate electrode formed to cover a side surface of the electron supply layer at a side surface within the opening from an edge part of the opening,

the first electron transit layer and the second electron transit layer between the source electrode and the gate electrode have a combined thickness greater than a thickness of the second electron transit layer between the gate electrode and the drain electrode,

the side surface of the first electron transit layer at the side surface within the opening is covered entirely by the second electron transit layer.

2. The semiconductor device according to claim 1 , wherein the back barrier layer, the first electron transit layer, the second electron transit layer, and the electron supply layer are formed of a nitride semiconductor, and wherein the side surface of the first electron transit layer and the side surface of the second electron transit layer are on an m-plane.

3. The semiconductor device according to claim 2 , wherein, in the first electron transit layer, a surface parallel to a surface of the substrate is on a c-plane.

4. The semiconductor device according to claim 1 , wherein, in the second electron transit layer, a two-dimensional electron gas is generated in a vicinity of a boundary with the electron supply layer at a surface parallel to a surface of the substrate.

5. The semiconductor device according to claim 1 , wherein the first electron transit layer and the second electron transit layer are formed of a material that includes GaN, and wherein the electron supply layer is formed of a material that includes AlGaN.

6. The semiconductor device according to claim 5 , wherein the back barrier layer is formed of a material that includes AlGaN or AlN.

7. The semiconductor device according to claim 5 , wherein the back barrier layer is formed of a material that includes p-GaN or p-AlGaN.

8. The semiconductor device according to claim 1 , wherein an insulating film is formed between the electron supply layer and the gate electrode.

9. A power supply device comprising:

a semiconductor device, wherein the semiconductor device includes a back barrier layer formed of a compound semiconductor on a substrate, a first electron transit layer formed of a compound semiconductor in such a manner that a bottom surface of the first electron transit layer is physically in contact with an upper surface of the back barrier layer, an opening formed in the first electron transit layer and the back barrier layer, a second electron transit layer formed of a compound semiconductor over the first electron transit layer, a side surface of the first electron transit layer at a side surface within the opening, a side surface of the back barrier layer at a side surface within the opening, and a surface of the back barrier layer at a bottom surface within the opening, an electron supply layer formed of a compound semiconductor over the second electron transit layer, a source electrode formed over the electron supply layer in a region in which the first electron transit layer is formed, a drain electrode formed over the electron supply layer within the opening, and a gate electrode formed to cover a side surface of the electron supply layer at a side surface within the opening from an edge part of the opening, and wherein the first electron transit layer and the second electron transit layer between the source electrode and the gate electrode have a combined thickness greater than a thickness of the second electron transit layer between the gate electrode and the drain electrode,

the side surface of the first electron transit layer at the side surface within the opening is covered entirely by the second electron transit layer.