Planar type heterojunction avalanche photodiode

The planar type heterojunction avalanche photodiode comprises a light absorbing layer (3) of a first conduction type provided on a substrate (1) of the first conduction type, an avalanche multiplication layer (5) of the first conduction type provided on the side of the light absorbing layer (3) which is opposite to the substrate (1), and a conduction region (7) of a second conduction type in contact with the avalanche multi­plication layer (5). A high reflection film (8a, 9) is provided on a side of the conduction region (7) which is opposite to the avalanche multiplication layer (5). An anti-reflection film (8) to which light is applied is provided on a side of the substrate (1) which is opposite to the light absorbing layer (3). The light is absorbed in the light absorbing layer (3) substantially, and the remaining light which is not absorbed in the light absorbing layer is reflected by the high reflection layer (8a, 9). The reflected light is absorbed in the lightabsorbing layer (3). Consequently, both a wide bandwidth and a high sensitivity are obtained.