A semiconductor device includes: a first semiconductor layer of a nitride semiconductor formed on a substrate; a second semiconductor layer of a nitride semiconductor formed on the first semiconductor layer; and a gate electrode, a source electrode, a drain electrode, and a hole extraction electrode, each of which is formed on the second semiconductor layer, wherein between the source electrode and the hole extraction electrode or in a region right under the source electrode, the first semiconductor layer and the second semiconductor layer form a vertical interface approximately perpendicular to a surface of the substrate, and a surface of the first semiconductor layer configured to form the vertical interface is an N-polar surface.