SEMICONDUCTOR DEVICE AND ITS MANUFACTURE

PURPOSE:To form a substrate transistor by forming a fourth impurity diffusion layer of a second conductivity type on a semiconductor substrate adjoining to an epitaxial layer which becomes a base region for a first bipolar transistor. CONSTITUTION:A base region of a substrate pnp transistor 21 comprises an impurity diffusion layer 3 and an epitaxial layer 7 formed on a semiconductor substrate 1. The breakdown strength between collector-emitter of a substrate pnp transistor can be determined independently from the film thickness of the epitaxial layer by adjusting base concentration and width, so that the formation of the substrate pnp transistor having a sufficient breakdown strength can be achieved even though the epitaxial layer is less than 3mum without increasing the costs. By doing this, the substrate pnp transistor capable of coexisting with a high-speed npn transistor can be realized.