Semiconductor device and method of manufacturing the semiconductor device

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

• forming a first nitride semiconductor layer above a substrate;
• forming a second nitride semiconductor layer over the first nitride semiconductor layer;
• forming a third nitride semiconductor layer over the second nitride semiconductor layer;
• forming a fourth nitride semiconductor layer over the third nitride semiconductor layer;
• forming a trench that extends up to the middle of the third nitride semiconductor layer while penetrating through the fourth nitride semiconductor layer;
• forming a gate electrode in the trench with a gate insulating film in between;
• forming a first electrode and a second electrode above the fourth nitride semiconductor layer on both sides of the gate electrode;
• and
• forming a through hole that extends up to the first nitride semiconductor layer, and forming a coupling electrode including a conductive film within the through hole,
• wherein electron affinity of the second nitride semiconductor layer is equal to or larger than electron affinity of the first nitride semiconductor layer,
• wherein electron affinity of the third nitride semiconductor layer is equal to or larger than the electron affinity of the first nitride semiconductor layer,
• wherein electron affinity of the fourth nitride semiconductor layer is smaller than the electron affinity of the first nitride semiconductor layer,
• wherein the coupling electrode is formed in a region that is electrically isolated from the first electrode, and
• wherein the coupling electrode is electrically coupled to a voltage different from a voltage applied to the first electrode.