Semiconductor device and method of manufacturing the same

A method of manufacturing a semiconductor device comprising:

• forming a first nitride semiconductor layer, and forming a second nitride semiconductor layer having a band gap larger than that of the first nitride semiconductor layer over the first nitride semiconductor layer, thereby forming a stack;
• etching the stack thereby forming a trench;
• forming a gate insulation film in the inside of the trench;
• and
• forming a gate electrode over the gate insulation film,
• wherein the etching of the stack comprises forming the trench passing through the second nitride semiconductor layer and into the first nitride semiconductor layer, such that a corner of the trench between a side wall of the trench and a bottom of the trench comprises a rounded shape, and the forming of the gate insulation film comprises forming a corner of the gate insulation film in contact with the corner of the trench to have a rounded shape.