HETEROJUNCTION BIPOLAR TRANSISTOR

PURPOSE:To prevent electrons, which are minority carriers, from being subjected to scattering by a P-type impurity in a base layer by a method wherein the base layer at least has a periodicity in the direction parallel to a junction surface and is provided with a superlattice structure consisting of impurity-doped first semiconductors and high-purity second semiconductors having a forbidden band width narrower than those of the first semiconductors. CONSTITUTION:A base layer 12a has a superlattice structure constituted of AlAs parts 21 and high-purity GaAs parts 22. An impurity is doped to the parts 21 and the parts 21 are constituted so as to have a P-type conductivity. Periodic quantum wells are respectively formed in conduction bands C.B. and valence bands V.B. The P-type AlAs parts 21 and the parts 22 are very thin and electrons can freely come and go from one GaAs part to its adjacent GaAs part. The electrons can not exist at any place excluding an energy level higher than the bottoms of the conduction bands C.B. of the parts 22. A band gap Eg can be changed by adjusting the composition ratio of the parts 21 and 22 forming the layer 12a and a period.