A semiconductor device according to the present embodiment includes a semiconductor layer. A gate dielectric film is provided on a surface of the semiconductor layer. A gate electrode is provided on the semiconductor layer via the gate dielectric film. A drain layer of a first conductivity type is provided in a part of the semiconductor layer on a side of a first end of the gate electrode. A source layer of a second conductivity type is provided in a part of the semiconductor layer on a side of a second end of the gate electrode and below the gate electrode. The source layer has a substantially uniform impurity concentration at the part of the semiconductor layer below the gate electrode. Voltages of a same polarity are applied to the gate electrode and the drain layer.