PHOTOTRANSISTOR

PURPOSE:To increase the product of the speed of response of a photodiode and the gain band width of a transistor, and to obtain the phototransistor operating at high speed by making the width of a depletion layer just under an emitter region narrower than a depletion layer under a base region as a light receiving section. CONSTITUTION:An n type layer 2 is grown onto an n type Si substrate 1 in epitaxial form, an n type region 10 reaching the substrate 1 from the surface is shaped to a predetermined region through diffusion or ion implantation an n type layer is grown to the whole surface containing the region 10 again and unified with the layer 2 previously formed, and the thickness of the layer 2 is brought to prescribed one. The p type base region 3 is diffused and shaped to the surface layer section, and the small n type emitter region 4 is formed to a region corresponding to the region 10 in the region 3. Accordingly, only the width of the depletion layer just under the region 4 is partially made narrower than the depletion layer under the region 3 as the light receiving section, and the speed of response is improved remarkably.