A semiconductor device according to an embodiment includes: a first GaN based semiconductor layer; a second GaN based semiconductor layer disposed on the first GaN based semiconductor layer and having a bandgap larger than that of the first GaN based semiconductor layer; a source electrode disposed on the second GaN based semiconductor layer; a drain electrode disposed on the second GaN based semiconductor layer; a p-type third GaN based semiconductor layer disposed between the source electrode and the drain electrode on the second GaN based semiconductor layer; a gate electrode disposed on the third GaN based semiconductor layer; and a p-type fourth GaN based semiconductor layer disposed between the gate electrode and the drain electrode on the second GaN based semiconductor layer and disposed separated from the third GaN based semiconductor layer.