Nitride semiconductor device

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.