HETEROJUNCTION TRANSISTOR HAVING BIPOLAR CHARACTERISTICS

A heterojunction transistor has a first semiconductor layer of a semi-insulating or a low impurity concentration, a second semiconductor layer formed on the first semiconductor layer and made of such a semiconductor material that, in cooperation with the first semiconductor layer, a first energy recess for electrons and a second energy recess for holes are respectively formed at the bottom of the conduction band and at the top of the valence band to constitute a conductive channel, a third semiconductor layer formed on the second semiconductor layer and forming a PN-junction with the upper surface of the second semiconductor layer to inject carriers into the conductive channel, a control electrode for applying an input signal to the third semiconductor layer, and a ground and an output electrode formed on the second semiconductor layer on the opposite sides of the third semiconductor layer.