Patent ID: 6909161

Claim:
A photodiode including an n-type semiconductor electrode layer, an optical absorption layer having a first semiconductor optical absorption layer and a second semiconductor optical absorption layer having a p-type, and a p-type semiconductor electrode layer, which are stacked sequentially, said photodiode further including an n-type electrode formed on said n-type semiconductor electrode layer, and a p-type electrode formed on said p-type semiconductor electrode layer, wherein when a specified reverse-bias is applied across said n-type electrode and said p-type electrode, said first semiconductor optical absorption layer is depleted, and said second semiconductor optical absorption layer remains neutral in a region other than a region near an interface with said first semiconductor optical absorption layer, and wherein in said optical absorption layer with a fixed layer width equal to a sum (W=W D +W A ) of a layer width (W D ) of said first semiconductor optical absorption layer and a layer width (W A ) of said second semiconductor optical absorption layer, a ratio between W D and W A is determined such that |Δτ A |=|−Δτ D | holds, where −Δτ D is a decrease of a hole transit time through said first semiconductor optical absorption layer and Δτ A is an increase of an electron transit time through said second semiconductor optical absorption layer, when the layer width of said first semiconductor optical absorption layer is made W D −ΔW, and the layer width of said second semiconductor optical absorption layer is made W A +ΔW.