Group III nitride semiconductor device which can be used as a power transistor

1. A semiconductor device comprising:

a substrate;

a first nitride semiconductor layer formed on the substrate;

a p-type nitride semiconductor layer formed on the first nitride semiconductor layer;

a recess having a bottom portion which reaches the first nitride semiconductor layer through a part of the p-type nitride semiconductor layer;

a third nitride semiconductor layer formed to cover the bottom portion of the recess, a side portion of the recess, and a part of an upper surface of the p-type nitride semiconductor layer;

a fourth nitride semiconductor layer formed on an upper surface of the third nitride semiconductor layer;

a first electrode formed on a rear surface of the substrate;

a gate electrode formed on the upper surface of the p-type nitride semiconductor layer; and

a second electrode that is in contact with the third nitride semiconductor layer or the fourth nitride semiconductor layer, wherein

the third nitride semiconductor layer has a bandgap different from a bandgap of the fourth nitride semiconductor layer.

2. The semiconductor device according to claim 1 , wherein the fourth nitride semiconductor layer has the bandgap greater than the bandgap of the third nitride semiconductor layer.

3. The semiconductor device according to claim 2 , wherein a thickness of the fourth nitride semiconductor layer is greater in a portion of the fourth nitride semiconductor layer formed along the bottom portion of the recess and the upper surface of the p-type nitride semiconductor layer than in a portion of the fourth nitride semiconductor layer formed along the side portion of the recess.

4. The semiconductor device according to claim 2 , wherein a thickness of the third nitride semiconductor layer is greater in a portion of the third nitride semiconductor layer formed along the bottom portion of the recess and the upper surface of the p-type nitride semiconductor layer than in a portion of the third nitride semiconductor layer formed along the side portion of the recess.

5. The semiconductor device according to claim 2 , wherein the side portion of the recess has a tapered angle that falls within a range from 40 degrees to 80 degrees both inclusive.

6. The semiconductor device according to claim 2 , further comprising a fifth nitride semiconductor layer between the first nitride semiconductor layer and the p-type nitride semiconductor layer,

wherein a bottom portion of the fifth nitride semiconductor layer is located above the bottom portion of the recess.

7. The semiconductor device according to claim 1 , wherein the third nitride semiconductor layer has the bandgap greater than the bandgap of the fourth nitride semiconductor layer.

8. The semiconductor device according to claim 7 , wherein a thickness of the third nitride semiconductor layer is greater in a portion of the third nitride semiconductor layer formed along the bottom portion of the recess and the upper surface of the p-type nitride semiconductor layer than in a portion of the third nitride semiconductor layer formed along the side portion of the recess.

9. The semiconductor device according to claim 7 , wherein the side portion of the recess has a tapered angle that is between 40 degrees and 80 degrees both inclusive.

10. The semiconductor device according to claim 7 , further comprising a fifth nitride semiconductor layer between the first nitride semiconductor layer and the p-type nitride semiconductor layer,

wherein a bottom portion of the fifth nitride semiconductor layer is located above the bottom portion of the recess.