A semiconductor device includes: a compound semiconductor substrate; a buffer layer, a channel layer, and a Schottky junction forming layer sequentially disposed on the compound semiconductor substrate, the buffer layer, the channel layer, and the Schottky junction forming layer each being compound semiconductor materials; a source electrode and a drain electrode located on the Schottky junction forming layer; and a gate electrode disposed between the source and drain electrodes and forming a Schottky junction with the Schottky junction forming layer. The dopant impurity concentration in the channel layer is inversely proportional to the third power of depth into the channel layer from a top surface of the channel layer. The gate electrode has a gate length in a range from 0.2 μm to 0.6 μm.