Properties of a semiconductor device are improved. A semiconductor device is configured so as to include a voltage clamp layer, a channel underlayer, a channel layer, and a barrier layer, which are formed in order above a substrate, a trench that extends up to the middle of the channel layer while penetrating through the barrier layer, a gate electrode disposed within the trench with a gate insulating film in between, a source electrode and a drain electrode formed above the barrier layer on both sides of the gate electrode, and a fourth electrode electrically coupled to the voltage clamp layer. The fourth electrode is electrically isolated from the source electrode, and a voltage applied to the fourth electrode is different from a voltage applied to the source electrode. Consequently, threshold control can be performed. For example, a threshold of a MISFET can be increased.