Patent ID: 8530933

Claim:
A photo-transistor comprising: a collector layer formed of a first semiconductor layer with a first conductivity type; a base layer formed of a second semiconductor layer with a second conductivity type opposite to the first conductivity type and disposed on the first semiconductor layer; an emitter layer formed of a third semiconductor layer and disposed on the second semiconductor layer, the emitter layer having the first conductivity type and a band gap wider than that of the second semiconductor layer; a wide band gap buffer layer with the first conductivity type formed under the first semiconductor layer, and having a band gap wider than that of the first semiconductor layer; and a substrate located under the wide band gap buffer layer; wherein the first, second, and third semiconductor layers stacked together have a mesa structure with an island shape by mesa etching, the mesa structure having a heterojunction bipolar phototransistor structure with a mesa side surface exposed to outside; and wherein the photo-transistor further comprises: a diffusion region having a ring shape in a top view, and converted into the second conductivity type which is a same conductivity type as the second semiconductor layer, the diffusion region having a predetermined width in a horizontal direction from the exposed sidewall of said mesa structure to an inside thereof; another diffusion region formed of the second conductivity type, located at a part of the emitter surface parallel to a substrate principal plane, and extending from the sidewall to an inside thereof with a predetermined width in the horizontal direction; and a further-diffusion region formed of the second conductivity type, located at a part of the wide band gap buffer layer surface parallel to the substrate principal plane, and extending from a region contacting the mesa structure toward outside in a predetermined width in the horizontal direction.