Semiconductor photo device

A semiconductor photo detector provided with a photo detecting portion, electrodes and a substrate, in which at least three semiconductor layers of the same conductivity type and different energy gaps are formed in the order of magnitude of the energy gap so as to be connected to one of said electrodes through said substrate, said three semiconductor layers being a first semiconductor layer (9), a second semiconductor buffer layer (10 and 11) and a third semiconductor layer (2) employed as a main light absorbing layer, and in which a fourth semiconductor layer (8) forms a p-n junction together with the first semiconductor layer and has the same composition as the semiconductor layer having the largest energy gap among the at least three semiconductor layers but is different in conductivity type from the first semiconductor layer and is in contact with the first semiconductor layer so as to be connected to the other of said electrodes, said at least three semiconductor layers and said fourth semiconductor layer defining said photo detecting portion, said second semiconductor layer having a larger energy gap than that of said third semiconductor layer for providing graded composition between said first semiconductor layer and said third semiconductor layer, said second semiconductor layer and said third semiconductor layer defining the light absorbing region, said third semiconductor layer having the largest thickness among said three semiconductor layers, said third semiconductor layer and said light absorbing region having carrier concentrations so that the end of a depletion layer can be extended to said third semiconductor layer, and said substrate having a larger forbidden bandgap energy than said main light absorbing layer.