In a semiconductor device, a first-layer includes a group-III nitride semiconductor of a first conduction type. A second-layer includes a group-III nitride semiconductor of a second conduction type on a first surface of the first layer. A third-layer includes an Al-containing group-III nitride semiconductor on a first region of a surface of the second layer. A gate electrode has one end above a surface of the third-layer and has the other end within the first-layer via the second-layer. The gate electrode is insulated from the first- to third-layers. A first electrode is connected to the third-layer. A second electrode is connected to a second region of the surface of the second-layer. A third electrode is provided above a second surface of the first layer. The second surface is opposite to the first surface of the first layer.