A semiconductor device includes a buffer layer, a channel layer, a barrier layer, and agate electrode over a substrate, the gate electrode being disposed in a first opening with agate insulating film in between, the first opening running up to the middle of the channel layer through the barrier layer. The concentration of two-dimensional electron gas in a first region on either side of a second opening that will have a channel is controlled to be lower than the concentration of two-dimensional electron gas in a second region between an end of the first region and a source or drain electrode. The concentration of the two-dimensional electron gas in the first region is thus decreased, thereby the conduction band-raising effect of polarization charge is prevented from being reduced. This prevents a decrease in threshold potential, and thus improves normally-off operability.