Photodiode

A photodiode has an optical absorption layer composed of a depleted first semiconductor optical absorption layer with a layer width W_D and a p-type neutral second semiconductor optical absorption layer with a layer width W_A. The ratio between W_A and W_D is set such that the total carrier transit time _tot 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.