PURPOSE: To increase the range of a voltage between an emitter and a base which cause a resonance by forming the fourth and fifth layers of semiconductor crystal between the first and second layers and between the second and third layers, and forming the second layer in a multiple quantum well structure with the second layer as a hetero junction.

CONSTITUTION: N-type GaAs layers 11, 12 having 101cm-3 of impurity density respectively have thicknesses of 300 and 200μm to form emitter and collector regions. N-type GaAs layers 12, 12' having 1017cm-3 have 50 of thickness to form a base region. Undoped AlxGa1-xAs layers 14, 15, 16 have 0.3 of molar ratio, 50 of thickness, and are interposed between the layers 11 and 12, be tween the layers 12' and 15, and between the layers 12 and 12'. Thus, the height of the barrier can be reduced, lattice constant difference of the hetero junctions is alleviated, and the range of voltage between the emitter and the base flowed with a switching current can be doubled.