Nitride semiconductor device

A nitride semiconductor device comprising:

• a first semiconductor layer made of a nitride semiconductor of a first conductivity type;
• a second semiconductor layer provided on the first semiconductor layer, the second semiconductor layer being made of a nitride semiconductor of a second conductivity type;
• a third semiconductor layer provided on the second semiconductor layer, the third semiconductor layer being made of a nitride semiconductor with a forbidden band width wider than a forbidden band width of the second semiconductor layer;
• a first electrode electrically connected to the second semiconductor layer;
• a second electrode physically separated from the first electrode and electrically connected to the second semiconductor layer;
• and
• a control electrode provided between the first electrode and the second electrode, one end of the control electrode being disposed in the first semiconductor layer with an insulating film between the one end of the control electrode and the first semiconductor layer, the insulating film being provided in the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer, and the other end of the control electrode being disposed above a level of an interface between the first semiconductor layer and the second semiconductor layer.