Nitride semiconductor device

1. A nitride semiconductor device, comprising:

a substrate having a first main surface and a second main surface which face in opposite directions;

a first nitride semiconductor layer of a first conductivity type provided above the first main surface;

a second nitride semiconductor layer of a second conductivity type provided above the first nitride semiconductor layer, the second conductivity type being different from the first conductivity type;

a first opening which penetrates through the second nitride semiconductor layer to the first nitride semiconductor layer;

an electron transport layer provided above the second nitride semiconductor layer and on an inner surface of the first opening;

a gate electrode provided above the electron transport layer and covering the first opening;

a source electrode connected to the second nitride semiconductor layer;

a drain electrode provided on a second main surface-side of the substrate;

a high-resistance layer provided between the second nitride semiconductor layer and the electron transport layer in the first opening, the high-resistance layer including a nitride semiconductor and having a higher resistance value than the second nitride semiconductor layer; and

wherein the high resistance layer has a third opening at a bottom surface of the first opening.

2. The nitride semiconductor device according to claim 1 ,

wherein the high-resistance layer contains iron.

3. The nitride semiconductor device according to claim 1 ,

wherein the high-resistance layer contains carbon.

4. The nitride semiconductor device according to claim 3 ,

wherein a carbon concentration of the high-resistance layer is higher than a carbon concentration of the electron transport layer.

5. The nitride semiconductor device according to claim 1 ,

wherein the first opening includes a bottom part and a side wall part, and

the high-resistance layer extends from the side wall part to a part of the bottom part.

6. The nitride semiconductor device according to claim 1 , further comprising:

a third nitride semiconductor layer provided between the gate electrode and the electron transport layer and having the second conductivity type.

7. The nitride semiconductor device according to claim 1 ,

wherein when the substrate is viewed in plan view, an end part of the gate electrode is located at a position closer to the source electrode than an end part of the first opening.

8. The nitride semiconductor device according to claim 1 , further comprising:

a second opening at a position distanced from the gate electrode, the second opening penetrating through the electron transport layer to the second nitride semiconductor layer,

wherein the source electrode is provided in the second opening.