Field-effect transistor with a heat absorber in contact with a surface of the gate electrode on its back side

A semiconductor device may include a semiconductor substrate, an insulator film covering a part of an upper surface of the substrate, and a gate electrode opposing the upper surface via the insulator film. In the semiconductor substrate, a drift layer extending through a body layer to the upper surface opposes the gate electrode via the insulator film. The insulator film extends from the upper surface of the semiconductor substrate to an upper surface of the gate electrode by passing between the gate electrode and an upper electrode, and defines an opening at the upper surface of the gate electrode. A side surface of the opening of the insulator film is entirely located outside a volume space consisting of all straight lines that passes through the opposing surface of the drift layer at angle of 45 degrees to the opposing surface.