photodetectors are the essential device in the front end of an optical receiver that converts the incoming optical signal into an electrical signal in communication systems, known as O/E convertor. Semiconductor photodetectors, commonly referred to as photodiodes, are the prevalent types of photodetectors used in optical communication systems because of their small size, fast detection speed, and high detection efficiency. Similar to the structures of laser diodes, photodiodes are also based on the PN junctions. However, unlike a laser diode in which the PN junction is forward biased, the PN junction of a photodetector is reversely biased so that only a very small reverse saturation current flows through the diode without an input optical signal.