A third semiconductor layer (105) including a third nitride semiconductor is provided between an electrode (110) and a second semiconductor layer (104) including a second nitride semiconductor. The band gap of the second nitride semiconductor is set such that the carrier movement between a first semiconductor layer (103) and the third semiconductor layer (105) via the second semiconductor layer (104) is rate-determined by a diffusion process. The thickness of the second semiconductor layer (104) is set such that the carrier movement between the first semiconductor layer (103) and the third semiconductor layer (105) via the second semiconductor layer (104) is rate-determined by the diffusion process. The carrier movement between the first semiconductor layer (103) and the third semiconductor layer (105) via the second semiconductor layer (104) is rate-determined by a field emission process.