| 摘要 |
PURPOSE: To obtain a heterojunction bipolar transistor in which the ballast resistance required for uniform operation can be decreased by providing one or more layer, having impurity concentration lower than a specified value, between an emitter layer and an emitter contact layer. CONSTITUTION: A collector layer 4, a base layer 3, an emitter layer 2 and an emitter contact layer 1 are formed sequentially on a compound semiconductor substrate 6. At least one layer 1', having impurity concentration of 1×10<17> cm<-3> or less, is also provided between the emitter layer 2 and the emitter contact layer 1. The base layer 3 is composed of a material having coefficient of thermal expansion higher than that of the compound semiconductor substrate 6. For example, an n-GaAs subcollector layer 5, an n-GaAs collector layer 4, a p-GaAs base layer 3, an n-AlGaAs emitter layer 2, an n-GaAs emitter ballast layer 1', and an n-GaAs emitter contact layer 1 are formed on an n type Si substrate.
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