Characteristics of a semiconductor device are improved. A semiconductor device includes a voltage clamp layer, a channel base layer, a channel layer, and a barrier layer on a substrate. A trench extends to a certain depth of the channel layer through the barrier layer. A gate electrode is disposed on a gate insulating film within the trench. A source electrode and a drain electrode are provided on the two respective sides of the gate electrode. A coupling within a through-hole that extends to the voltage clamp layer electrically couples the voltage clamp layer to the source electrode. An impurity region containing an impurity having an acceptor level deeper than that of a p-type impurity is provided under the through-hole. The voltage clamp layer decreases variations in characteristics such as threshold voltage and on resistance. The contact resistance is reduced through hopping conduction due to the impurity in the impurity region.