A semiconductor device includes insulating substrate; a compound semiconductor layer provided in a first region of a surface of the insulating substrate; and a silicon layer provided in a second region, differing from the first region, of the surface of the insulating substrate. The semiconductor device further includes: a first gate electrode provided on a surface of the compound semiconductor layer; a pair of conductive members provided at positions on the surface of the compound semiconductor layer to sandwich the first gate electrode between the pair of conductive members; a second gate electrode provided on a surface of the silicon layer; and a pair of diffusion layers provided at positions in the silicon layer to sandwich the second gate electrode between the pair of diffusion layers. One of the conductive members is electrically connected to one of the diffusion layers.