The photodiode is a semiconductor device composed of a PN junction. The depletion layer of the PN junction is only a few microns, and most of the irradiated light is absorbed by the neutral region, so the photoelectric conversion efficiency is low and the response speed is slow. In order to improve the characteristics of the device, a layer of intrinsic semiconductor (called |) is arranged in the middle of the PN junction. This structure is a commonly used PIN photodiode.

　　PIN photodiode improves the time response of PN junction photodiode, but the sensitivity of the device is not improved much. The avalanche photodiode is a diode that uses the avalanche effect generated by the PN junction under high reverse voltage. It can improve the sensitivity of the photodiode. Application: High-speed optical communication, high-speed optical detection, remote control, phototransistor, writing pen, fax.