Nitride semiconductor device

1. A nitride semiconductor device, comprising:

a substrate;

a first nitride semiconductor layer of a first conductivity disposed above the substrate;

a second nitride semiconductor layer of a second conductivity different from the first conductivity, disposed above the first nitride semiconductor layer;

a first opening penetrating through the second nitride semiconductor layer;

an electron transport layer and an electron supply layer disposed along inner surfaces of the first opening, the electron transport layer and electron supply layer being arranged in sequence from a substrate side such that the electron transport layer is closer to the substrate side and electron supply layer is further away from the substrate side;

a gate electrode disposed above the electron supply layer to cover the first opening;

a source electrode connected to the electron supply layer and the electron transport layer, at a position separated from the gate electrode; and

a drain electrode disposed on a surface of the substrate which is opposite to a surface on which the first nitride semiconductor layer is disposed,

wherein at least part of the second nitride semiconductor layer is fixed to a first potential that is different from a second potential applied to the source electrode.

2. The nitride semiconductor device according to claim 1 ,

wherein at least part of the second nitride semiconductor layer is fixed to the first potential and a same potential is applied to the gate electrode.

3. The nitride semiconductor device according to claim 2 , further comprising:

a first high-resistance layer disposed above the second nitride semiconductor layer;

a second opening penetrating through the electron supply layer, the electron transport layer, and the first high-resistance layer, and extending to the second nitride semiconductor layer; and

a fixed-potential electrode provided in a bottom surface of the second opening and contacting the second nitride semiconductor layer,

wherein the first opening penetrates through the first high-resistance layer and the second nitride semiconductor layer, and

the fixed-potential electrode is electrically connected to the gate electrode.

4. The nitride semiconductor device according to claim 3 , further comprising:

a third opening penetrating through the electron supply layer and reaching up extending to the electron transport layer,

wherein the source electrode is provided along part of inner surfaces of the third opening, and

in a plan view of the substrate, the second opening is disposed at a position separated from the source electrode, inside the third opening.

5. The nitride semiconductor device according to claim 2 , further comprising:

a third nitride semiconductor layer of the second conductivity disposed between the gate electrode and the electron supply layer.

6. The nitride semiconductor device according to claim 2 , further comprising:

a second high-resistance layer disposed between the first nitride semiconductor layer and the second nitride semiconductor layer.

7. The nitride semiconductor device according to claim 2 ,

wherein the electron transport layer includes:

a flat portion disposed on a top surface of the first high-resistance layer; and

a sloped portion disposed along a side surface of the first opening,

wherein the sloped portion has a length in a direction parallel to the substrate longer than a thickness of the flat portion in a normal direction to the substrate.

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

a second opening at a position separated from the first opening, penetrating through the electron supply layer and the electron transport layer, and extending to the second nitride semiconductor layer; and

a high-resistance layer separating the second nitride semiconductor layer into a first portion near the first opening and a second portion near the second opening,

wherein the source electrode is disposed in the second opening,

the second portion is fixed to a potential that is the same as the second potential applied to the source electrode, and

the first portion is fixed to the potential that is different from the second potential applied to the source electrode.

9. The nitride semiconductor device according to claim 8 ,

wherein the high-resistance layer is a nitride semiconductor layer containing iron.

10. The nitride semiconductor device according to claim 8 ,

wherein the first portion is fixed to a potential that is the same potential applied to the gate electrode.

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

a third nitride semiconductor layer of the second conductivity disposed between the gate electrode and the electron supply layer.

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

a third opening provided in a bottom surface of the second opening, penetrating through the second nitride semiconductor layer, and extending to the first nitride semiconductor layer,

wherein the source electrode is further disposed in the third opening and is connected to the first nitride semiconductor layer.

13. The nitride semiconductor device according to claim 12 ,

wherein the third opening comprises a plurality of third openings provided in the bottom surface of the second opening.

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

a fourth nitride semiconductor layer of the second conductivity disposed between the first nitride semiconductor layer and the second nitride semiconductor layer and including a fourth opening exposing at least part of the first nitride semiconductor layer; and

a fifth nitride semiconductor layer of the first conductivity disposed between the fourth nitride semiconductor layer and the second nitride semiconductor layer and along inner surfaces of the fourth opening,

wherein the first opening exposes a part of the fifth nitride semiconductor layer,

the fourth nitride semiconductor layer is fixed to a potential that is the same as the second potential applied to the source electrode, and

the second nitride semiconductor layer is fixed to the first potential which is the same potential applied to the gate electrode.

15. The nitride semiconductor device according to claim 14 , further comprising:

a high-resistance layer disposed between the fifth nitride semiconductor layer and the second nitride semiconductor layer and having a resistance higher than a resistance of one of the fifth nitride semiconductor layer and the second nitride semiconductor layer.

16. The nitride semiconductor device according to claim 14 ,

wherein the fifth nitride semiconductor layer has an effective carrier concentration higher than an effective carrier concentration of the first nitride semiconductor layer.

17. The nitride semiconductor device according to claim 14 ,

wherein the fourth opening has an opening width shorter than an opening width of the first opening.

18. The nitride semiconductor device according to claim 17 ,

wherein the fourth opening includes a plurality of fourth openings in the fourth nitride semiconductor layer.

19. The nitride semiconductor device according to claim 14 , further comprising:

a Schottky barrier diode disposed at a position separated from the first opening in a plan view,

wherein the Schottky barrier diode includes an anode electrode disposed on the fifth nitride semiconductor layer,

the Schottky barrier diode includes a cathode electrode that is a part of the drain electrode, and

the fourth nitride semiconductor layer further includes, between the anode electrode and the cathode electrode, a fifth opening exposing part of the first nitride semiconductor layer.

20. The nitride semiconductor device according to claim 19 ,

wherein the anode electrode is electrically connected to the fourth nitride semiconductor layer.

21. The nitride semiconductor device according to claim 19 ,

wherein the fifth opening includes a plurality of fifth openings in the fourth nitride semiconductor layer.