OPTICAL SEMICONDUCTOR DEVICE

An optical semiconductor device includes a light-receiving element on a semiconductor substrate of a first conductivity type, the light-receiving element including a light-receiving portion for converting incident light to an electrical current signal and performing a current amplification. The light-receiving portion includes: a semiconductor layer formed on the semiconductor substrate and having an impurity concentration substantially equal to or less than that of the semiconductor substrate; a first semiconductor region of a second conductivity type formed on the semiconductor layer and having an impurity concentration higher than that of the semiconductor layer; and a second semiconductor region of the first conductivity type selectively formed between the semiconductor substrate and the semiconductor layer and having an impurity concentration higher than those of the semiconductor substrate and the semiconductor layer.