A high-electron-mobility transistor has a buffer layer, a channel layer, a barrier layer, a mesa-shaped cap layer, a source electrode formed on one side of the cap layer, a drain electrode formed on the other side, and a gate electrode formed over the cap layer via a gate insulating film. The semiconductor device has an element isolation region defining an active region in which the semiconductor device is provided. The gate electrode extends from over the active region to the over the element isolation region. In plan view, the active region has a projection part projected to the direction of the element isolation region in a region overlapped with the gate electrode. By providing the active region with a projection part, the channel length of a parasitic transistor can be increased, and turn-on of the parasitic transistor can be suppressed.