Abstract: An avalanche photodiode sensitive to a wavelength range of from 1.2 to 1.65 micrometers is provided with a light absorbing layer, a middle layer and an active layer grown in order, on a substrate. All the layers contain impurities with the same conductivity but the impurity concentration is higher in the middle layer than in either of the light absorbing layer and the active layer. A p-n junction having a flat bottom and either a gradually inclined side or a step-shaped side is produced in the active layer, so that the breakdown voltage is made much less in the area facing the flat bottom of the p-n junction than in the area facing a side which has the aforementioned irregular shape. As a result, the side acts as a guard ring without being accompanied by a large amount of tunnel current flowing through the light absorbing layer in response to the intensity of the electric field.

Abstract: An avalanche photodiode sensitive to wavelengths ranging from 1.2 to 1.65 micrometers and comprising a light absorbing layer and first and second window layers of a semiconductor having a band gap larger than the semiconductor of the light-absorbing layer respectively grown on a substrate, is disclosed. All the layers are doped with the same conductivity type impurity but the doping concentration is higher for the second window layer than for the first window layer. A p-n junction for avalanche multiplication is formed in the second window layer and another deep p-n junction providing a guard ring effect is formed surrounding the avalanche p-n junction to reach the first window layer but remaining a specific distance above the heterojunction between the first window layer and the light absorbing layer.