A semiconductor device includes: substrate region; a gate electrode, a source electrode, and a drain electrode which are placed on a first surface of the substrate regions; an active area between gate and source placed between the gate electrode and the source electrode; an active area between gate and drain placed between the gate electrode and the drain electrode; an active area placed on the substrate region of the underneath part of the gate electrode, the source electrode, and the drain electrode; and a non-active area placed adjoining the active area, the active area between gate and source, and the active area between gate and drain. Furthermore, width WA1 of the active area between gate and source is wider than width WA2 of the active area between gate and drain. Channel resistance of an active area between source and gate placed between a gate electrode and a source electrode is reduced, and high-frequency performance is provided.