Nitride semiconductor device and manufacturing method thereof

A method of manufacturing a nitride semiconductor device, comprising:

• forming a first nitride semiconductor layer as an electron transit layer;
• forming a second nitride semiconductor layer as an electron supply layer on the first nitride semiconductor layer, the second nitride semiconductor layer having a band gap greater than that of the first nitride semiconductor layer;
• and
• forming a third nitride semiconductor layer on the second nitride semiconductor layer, the third nitride semiconductor layer having a band gap greater than that of the first nitride semiconductor layer and smaller than that of the second nitride semiconductor layer;
• and
• forming a fourth nitride semiconductor layer including an acceptor type impurity on the third nitride semiconductor layer;
• forming a gate electrode film on the fourth nitride semiconductor layer;
• and
• forming a gate part including the fourth nitride semiconductor layer and a gate electrode on the third nitride semiconductor layer by selectively etching the gate electrode film, the fourth nitride semiconductor layer, and the third nitride semiconductor layer.