A semiconductor device according to an embodiment includes: a first nitride semiconductor layer having a first surface and a second surface; a first source electrode provided on the first surface; a first drain electrode provided on the first surface; a first gate electrode provided on the first surface between the first source electrode and the first drain electrode; a second nitride semiconductor layer having a third surface and a fourth surface, the third surface being provided on the second surface and facing the second surface, and the second nitride semiconductor layer having a smaller band gap than the first nitride semiconductor layer; and a first semiconductor device having a fifth surface provided on the fourth surface and facing the fourth surface with a size equal to or smaller than a size of the fourth surface, the first semiconductor device including a first semiconductor material having a smaller band gap than the second nitride semiconductor layer.