A semiconductor device has a semiconductor base of a first conductivity type; a hetero semiconductor region in contact with the semiconductor base; a gate electrode adjacent to a portion of a junction between the hetero semiconductor region and the semiconductor base across a gate insulating film; a source electrode connected to the hetero semiconductor region; and a drain electrode connected to the semiconductor base. The hetero semiconductor region has a band gap different from that of the semiconductor base. The hetero semiconductor region includes a first hetero semiconductor region and a second hetero semiconductor region. The first hetero semiconductor region is formed before the gate insulating film is formed. The second hetero semiconductor region is formed after the gate insulating film is formed.