Patent ID: 7557387

Claim:
An avalanche photodiode comprising: a stacked layer body in which an n-type electrode layer, an avalanche multiplication layer, an electric field control layer, a graded bandgap layer, a light absorbing layer with a layer thickness of W A , and a p-type electrode layer are stacked sequentially, the light absorbing layer has a junction of a p-type layer with a layer thickness of W AN located on the side of the p-type electrode layer and a low concentration layer with a layer thickness of W AD located on the side of the graded bandgap inclined layer, an each doping profile of the p-type and the low concentration layers is determined under device operating conditions so that a p-type neutral state is maintained for the p-type layer except a region in a vicinity of an interface of the junction with the low concentration layer while the low concentration layer is depleted, and a ratio between the layer thickness W AN of the p-type layer and the layer thickness W AD of the low concentration layer is determined so as to satisfy a next formula in a case where t total is a delay time of element response accompanying a transit of carriers generated in the light absorbing layer by light absorption, t N2 is a delay time caused by the p-type layer, t D1 is a delay time caused by the low concentration layer, and t D is a delay time when an entire region of the light absorbing layer is the low concentration layer, under a condition that a layer thickness W A (=W AN +W AD ) of the light absorbing layer is constant t D >t total =( W AD ×t D1 +W AN ×t N2 )/ W A [Formula 1].