A semiconductor device has: a buffer layer formed on a conductive substrate and made of AlxGa1−xN with a high resistance; an element-forming layer formed on the buffer layer, having a channel layer, and made of undoped GaN and N-type AlyGa1−N; and a source electrode, a drain electrode and a gate electrode which are selectively formed on the element-forming layer. The source electrode is filled in a through hole provided in the buffer layer and the element-forming layer, and is thus electrically connected to the conductive substrate.