A semiconductor device includes an undoped GaN layer (103) formed on a substrate (101), an undoped AlGaN layer (104) formed on the undoped GaN layer (103) and having a band gap energy larger than that of the undoped GaN layer (103), a p-type AlGaN layer (105) and a high-concentration p-type GaN layer (106) formed on the undoped AlGaN layer (104), and an n-type AlGaN layer (107) formed on the high-concentration p-type GaN layer (106). A gate electrode (112) which makes ohmic contact with the high-concentration p-type GaN layer (106) is formed on the high-concentration p-type GaN layer (106) in a region thereof exposed through an opening (107a) formed in the n-type AlGaN layer (107).