Semiconductor device and method for manufacturing the same

A semiconductor device comprising:

• a first nitride semiconductor layer;
• a second nitride semiconductor layer formed on the first nitride semiconductor layer;
• a third nitride semiconductor layer formed on the second nitride semiconductor layer;
• a fourth nitride semiconductor layer of a mesa type formed on the third nitride semiconductor layer;
• a source electrode formed in a part on the third nitride semiconductor layer, the part being closer to one side of the fourth nitride semiconductor layer;
• a drain electrode formed in a part on the third nitride semiconductor layer, the part being closer to the other side of the fourth nitride semiconductor layer;
• a gate insulating film covering the fourth nitride semiconductor layer;
• and
• a gate electrode formed on the gate insulating film,
• wherein an electron affinity of the second nitride semiconductor layer is equal to or larger than an electron affinity of the first nitride semiconductor layer,
• an electron affinity of the third nitride semiconductor layer is smaller than an electron affinity of the first nitride semiconductor layer,
• an electron affinity of the fourth nitride semiconductor layer is equal to or larger than an electron affinity of the second nitride semiconductor layer,
• the fourth nitride semiconductor layer and the gate electrode are separated from each other by the gate insulating film, and
• a film thickness of the gate insulating film is larger than a film thickness of the fourth nitride semiconductor layer.

A method for manufacturing a semiconductor device comprising the steps of:

• (a) forming a second nitride semiconductor layer on a first nitride semiconductor layer;
• (b) forming a third nitride semiconductor layer on the second nitride semiconductor layer;
• (c) forming a fourth nitride semiconductor layer on the third nitride semiconductor layer;
• (d) processing the fourth nitride semiconductor layer as a mesa type;
• and
• (e) forming a gate electrode on the fourth nitride semiconductor layer of the mesa type via a gate insulating film,
• wherein the step (d) further includes the steps of:
• (d1) etching the fourth nitride semiconductor layer;
• and
• (d2) after the step (d1), performing a heating process to the fourth nitride semiconductor layer, and,
• by the step (d2), a side surface of the fourth nitride semiconductor layer is formed into a tapered shape.