a bipolar integrated circuit requiring less silicon area is provided by the use of a three layer epitaxy on top of a substrate . the first epitaxial layer is of the same conductivity type as the substrate and adds additional height to the substrate surrounding the buried layer . the buried layer serves as a collector and it is surrounded by an isolation area . the top two epitaxial layers are of a conductivity type opposite to that of the substrate with the upper most epitaxial layer having a higher dopant density than does the middle epitaxial layer . a master mask is used to provide self - alignment between the isolation area , a collector plug which makes contact to the buried layer , and a base region .