Semiconductor device and manufacturing method thereof

A semiconductor device, comprising:

• a first nitride semiconductor layer formed over a substrate;
• a second nitride semiconductor layer formed over the first nitride semiconductor layer;
• a third nitride semiconductor layer formed over the second nitride semiconductor layer;
• a mesa-shaped fourth nitride semiconductor layer formed over the third nitride semiconductor layer;
• a gate electrode disposed over the fourth nitride semiconductor layer via a gate insulating film;
• a first electrode and a second electrode which are formed over the third nitride semiconductor layer located on both sides of the gate electrode;
• and
• a coupling portion coupling a potential fixing electrode to the first nitride semiconductor layer,
• wherein an electron affinity of the second nitride semiconductor layer is larger than an electron affinity of the third nitride semiconductor layer,
• wherein the first nitride semiconductor layer has an impurity having a first conductivity type and an impurity having a second conductivity type,
• wherein, in the first nitride semiconductor layer, a concentration of the second-conductivity-type impurity is higher than a concentration of the first-conductivity-type impurity,
• wherein, of the first nitride semiconductor layer, a region closer to the first electrode is a semiconductor region having the first conductivity type and a region closer to the second electrode is a semiconductor region having the second conductivity type opposite to the first conductivity type, and
• wherein the coupling portion couples the potential fixing electrode to the first-conductivity-type semiconductor region.

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

• (a) forming a first nitride semiconductor layer having a p-type impurity and an n-type impurity over a substrate;
• (b) forming a second nitride semiconductor layer over the first nitride semiconductor layer;
• (c) forming a third nitride semiconductor layer over the second nitride semiconductor layer;
• (d) forming a gate electrode over the second nitride semiconductor layer;
• (e) introducing hydrogen into the first nitride semiconductor layer in a first region located on one side of the gate electrode;
• and
• (f) forming a first electrode over the third nitride semiconductor layer in the first region located on the one side of the gate electrode, forming a second electrode over the third nitride semiconductor layer in a second region located on the other side of the gate electrode, and forming a coupled portion to be coupled to the first nitride semiconductor layer,
• wherein an electron affinity of the second nitride semiconductor layer is larger than an electron affinity of the third nitride semiconductor layer.