A semiconductor device includes a substrate which includes a first face. The device also includes a buffer layer, a semiconductor layer, source and drain electrodes, and a gate electrode. A trench is formed on the semiconductor layer so that the trench surrounds the source electrode, the drain electrode, and the gate electrode in a plan view, the trench passes through the semiconductor layer and the buffer layer, and a bottom of the trench reaches at least an inside of the substrate. A distance from the first face of the substrate to the bottom of the trench is 100 nm or more in a thickness direction of the substrate.