According to one embodiment, a semiconductor device, including a semiconductor layer including a first region and a second region isolated from the first region, a source in a surface of the first region, a drain in a surface of the second region, a back-gate in the surface of the source, a gate insulator on a surface of the first region, an end of a drain side of the back-gate being located closer to the drain side than an end of the drain side of the source, a gate insulator on a surface of the semiconductor layer between the first region and the second region, a gate electrode on the gate insulator, a source electrode being contacted to both the source and the back-gate, and a drain electrode being contacted to the drain area.