Nitride-based semiconductor device and method of manufacturing the same

A nitride-based semiconductor device comprising:

• a first semiconductor layer including GaN;
• a second semiconductor layer formed on and directly contacting the first semiconductor layer, the second semiconductor layer including AlYGa1-YN (0<Y≤1) having a lattice constant smaller than a lattice constant of the first semiconductor layer;
• a gate insulating film formed so as to cover a recess structure and the second semiconductor layer, a bottom of the recess structure being positioned below a boundary between the first semiconductor layer and the second semiconductor layer, the gate insulating film including a first region and a second region, the first region covering a side face of the recess structure, the first region contacting the first semiconductor layer and the second semiconductor layer, and the second region covering the bottom of the recess structure, the gate insulating film including a first insulating film and a second insulating film, the first insulating film formed so as to cover the second semiconductor layer, the second insulating film formed so as to cover the first insulating film and a recess structure;
• a gate electrode formed on the gate insulating film in the recess structure, the gate electrode being insulated from the first semiconductor layer and the second semiconductor layer;
• and
• a source electrode and a drain electrode formed on the second semiconductor layer, wherein the gate electrode is positioned between the source electrode and the drain electrode, and the nitride-based semiconductor device is a normally off-type device.