In a high electron mobility transistor, with a normally-off operation maintained, on-resistance can be sufficiently reduced, so that the performance of a semiconductor device including the high electron mobility transistor is improved. Between a channel layer and an electron supply layer, a spacer layer whose band gap is larger than the band gap of the electron supply layer is provided. Thereby, due to the fact that the band gap of the spacer layer is large, a high potential barrier (electron barrier) is formed in the vicinity of an interface between the channel and the electron supply layer.