Manufacturing method of semiconductor device

A manufacturing method of a semiconductor device, comprising:

• (a) forming a second nitride semiconductor layer over a first nitride semiconductor layer;
• (b) forming a third nitride semiconductor layer over the second nitride semiconductor layer;
• (c) forming a fourth nitride semiconductor layer over the third nitride semiconductor layer;
• (d) forming a first film over a first region of the fourth nitride semiconductor layer;
• (e) forming a silicon nitride film over the fourth nitride semiconductor layer and the first film;
• and
• (f) removing the silicon nitride film,
• wherein the electron affinity of the second nitride semiconductor layer is equal to or larger than the electron affinity of the first nitride semiconductor layer,
• wherein the electron affinity of the third nitride semiconductor layer is smaller than the electron affinity of the first nitride semiconductor layer,
• wherein the electron affinity of the fourth nitride semiconductor layer is larger than the electron affinity of the first nitride semiconductor layer, and
• wherein the first film comprises a laminate of a gate insulating film and a gate electrode.