To use the effect of built-in electric field for the"allowable value" of the value of a change in composition when an inclined base structure using a mixed-crystal semiconductor.

The reference numeral 1 designates a half-insulated Inp substrate; 2, a sub-collector layer of n+-InP; 3, a collector layer of n--GaxIn1-xAs; 4, a base layer of p+-GaxIn1-xAs; 5, an emitter layer of -InP; 6, an emitter cap layer of n+-GaxIn1-xAs; 7, a collector electrode; 8, a base electrode; 9, an emitter electrode; 41, a base layer region A; and 42, a base layer region B. These regions are provided within the base layer 4 and intensity of the built-in electric field thereof is different depending on the places, and is higher as it goes to the area nearer to the emitter end. In this example, the GaxIn1-xAs is used as the base material and amount of increase in the GaAs composition ratio is excessively increased only on the region near to the emitter end.