A photodiode has an optical absorption layer composed of a depleted first semiconductor optical absorption layer with a layer width WD and a p-type neutral second semiconductor optical absorption layer with a layer width WA. The ratio between WA and WD is set such that the total carrier transit time τtot becomes minimum in the optical absorption layer. The photodiode can further include a depleted semiconductor optical transmission layer with a bandgap greater than that of the first semiconductor optical absorption layer, between the first semiconductor optical absorption layer and an n-type semiconductor electrode layer.