Abstract: An optical semiconductor includes a photo diode integrated with a transistor built on first and second epitaxial layers grown on a substrate, the first epitaxial layer is grown on the substrate from intrinsic material. The second epitaxial layer is grown doped on the first epitaxial layer. A separating area divides the optical semiconductor into first and second isolated islands. The separating area is made up of a lower separating area, a middle separating area and an upper separating area united to form a single separating area. The lower separating area is diffused at least upward from an interface between the substrate and the first epitaxial area. The middle separating area is diffused both downward and upward from an interface between the first and second epitaxial layers. The upper separating area is diffused downward from the surface of the second epitaxial layer. The photo diode is formed in the first island area, and the transistor is formed in the second island area.

Abstract: An optical semiconductor is integrated with a transistor by epitaxially growing a lightly doped epitaxial layer on a substrate. One isolated island area of the epitaxial layer contains a diffusion area on its surface to form the optical semiconductor. A second isolated island area has its conductivity type inverted by a buried layer that is diffused upward into contact with a surface layer that is diffused downward. The upward-diffused and downward-diffused layers unite to form a collector of the transistor. A base area in the surface of the collector contains an emitter in its surface. The emitter and the diffusion area are formed of the same material, and in the same process steps.