Semiconductor device and method for manufacturing semiconductor device

A method for manufacturing a semiconductor device, comprising:

• forming a first insulating layer on a first nitride semiconductor layer having a first principal surface;
• forming, on the first insulating layer, a mask including a first mask opening through which a portion of the first insulating layer is exposed;
• forming a first opening in the first insulating layer through the first mask opening, to expose a portion of the first nitride semiconductor layer;
• forming a second nitride semiconductor layer on the first nitride semiconductor layer inside the first opening, through the first mask opening;
• forming a second insulating layer covering a boundary line between the second nitride semiconductor layer and the first insulating layer, through the first mask opening;
• removing the mask after the forming the second insulating layer;
• forming a second opening in the second insulating layer, to expose a portion of the second nitride semiconductor layer;
• forming a first electrode on the second insulating layer, making contact with the second nitride semiconductor layer through the second opening;
• and
• forming a gate electrode above the first nitride semiconductor layer, at a position separated from the second insulating layer in a plan view perpendicular to the first principal surface.