Nitride semiconductor device

1. A nitride semiconductor device, comprising:

a substrate having a first main surface and a second main surface on a reverse side of the first main surface;

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

a block layer above the first nitride semiconductor layer;

a first opening penetrating through the block layer and reaching the first nitride semiconductor layer;

an electron transit layer and an electron supply layer provided sequentially above the block layer and along an inner surface of the first opening, the electron transit layer being closer to the substrate than the electron supply layer;

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

a second opening in a location away from the gate electrode, the second opening penetrating through the electron supply layer and the electron transit layer and reaching the block layer;

a source electrode provided in the second opening and connected to the block layer; and

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

wherein when the first main surface is seen in a plan view,

(i) the first opening and the source electrode extend in a same, longitudinal direction and are longer in the longitudinal direction than in a lateral direction perpendicular to the longitudinal direction, and

(ii) at least part of an outline of a first end of the first opening in the longitudinal direction either:

(a) is an arc or an elliptical arc; or

(b) is part of a polygon which has vertices, with interior vertex angles at all of the vertices being greater than 120°.

2. The nitride semiconductor device according to claim 1 ,

wherein the gate electrode includes:

a metal film; and

a semiconductor layer of a second conductivity type sandwiched between the metal film and the electron supply layer, the second conductivity type having a polarity different from a polarity of the first conductivity type.

3. The nitride semiconductor device according to claim 1 ,

wherein at least part of an outline of a second end opposite to the first end of the first opening in the longitudinal direction follows an arc or an elliptical arc.

4. The nitride semiconductor device according to claim 1 ,

wherein the nitride semiconductor device includes the source electrode including two or more source electrodes and the first opening including two or more first openings, and

portions of the two or more first openings extending in the longitudinal direction and the two or more source electrodes are alternately aligned in a direction intersecting perpendicular to the longitudinal direction.

5. The nitride semiconductor device according to claim 1 ,

wherein the block layer includes a second nitride semiconductor layer of a second conductivity type having a polarity different from a polarity of the first conductivity type.

6. The nitride semiconductor device according to claim 1 ,

wherein the first opening in the plan view has a pair of parallel straight sides extending in the longitudinal direction, the pair of parallel straight sides being connected to each other by the at least part of the outline of the first end of the first opening.

7. The nitride semiconductor device according to claim 6 ,

wherein the pair of parallel straight sides of the first opening is longer than a length of the source electrode in the longitudinal direction.