Nitride semiconductor device

A nitride semiconductor device comprising:

• a first nitride semiconductor layer with an n-type conductivity;
• a second nitride semiconductor layer with a p-type conductivity disposed above the first nitride semiconductor layer;
• a source electrode in direct contact with the second nitride semiconductor layer;
• and
• a mesa structure with two or more steps disposed in a termination region between an active region of the nitride semiconductor device and an inactive region surrounding the active region,
• wherein the mesa structure includes a mesa at a first step in which a lower end of a side wall of the mesa is located in the second nitride semiconductor layer, and each of mesas at a second and subsequent steps in which a lower end of a side wall of the mesa reaches the first nitride semiconductor layer, and
• the mesa at the first step has a depth that is half or more of a film thickness of the second nitride semiconductor layer.