In a semiconductor device having, at least, a first semiconductor layer which contains substantially no impurity, a second semiconductor layer which has a band gap greater than that of the first semiconductor layer and which contains an impurity, an interface between the first and second semiconductor layers forming a heterojunction, at least one pair of electrodes which are electronically connected with the first semiconductor layer, and means to control carriers developing at the heterojunction interface; a semiconductor device characterized in that the first semiconductor layer is a Ge layer, while the second semiconductor layer is a group III-V compound semiconductor layer.