Patent ID: 7982276

Claim:
An optical semiconductor device provided with a light receiving element and a transistor on the same substrate, the optical semiconductor device comprising: an embedding layer of a first conductivity type having a high dopant concentration formed at an upper section of a semiconductor substrate of the first conductivity type; a first epitaxial layer of the first conductivity type having a low dopant concentration formed on the embedding layer; a second epitaxial layer of a second conductivity type having a low dopant concentration formed on the first epitaxial layer; a first diffusion layer of the first conductivity type having a low dopant concentration selectively formed on the second epitaxial layer; and a second diffusion layer of the second conductivity type having a high dopant concentration formed at an upper section of the first diffusion layer, wherein: the first and second diffusion layers constitute the light receiving element, and the transistor is formed in the second epitaxial layer, and the first diffusion layer has its dopant concentration peak in an interface between the first and second epitaxial layers.