Heterojunction transistor having bipolar characteristics

A semiconductor device comprising a first semiconductor layer having a low impurity concentration and having a first electron affinity and a first energy sum of electron affinity and energy band gap at an upper surface thereof, an N-type second semiconductor layer provided above said first semiconductor layer and having a second electron affinity and a second energy sum of electron affinity and energy band gap, an intermediate layer interposed between said first and second semiconductor layers and having at a lower surface thereof a third electron affinity smaller than said first electron affinity to form a two-dimensional electron gas layer at a surface of said first semiconductor layer in response to charge in said second semiconductor layer and having at an upper surface thereof a third energy sum of electron affinity and energy band gap larger than said second energy sum, said intermediate layer being of sufficient thickness such that holes cannot penetrate therethrough due to tunnelling effect, a P-type third semiconductor layer forming a PN-junction with said second semiconductor layer, a control electrode connected to said third semiconductor layer so as to form an ohmic contact so that there will be no Schottky barrier with respect to said second semiconductor layer, and a common electrode and an output electrode formed on the opposite sides of said control electrode for electrical coupling to said two-dimensional electron gas layer.