Avalanche photodiode
An avalanche photodiode (APD) is a semiconductor device in which charge carriers are generated and multiplied when exposed to light. Avalanche photodiodes are incorporated in many high performance optical communications, imaging and sensing applications because they enable high signal to noise ratio and high-speed operation. Modern optical acquisition and tracking applications often require the use of a plurality of photodetectors in order to steer the boresight of the optical system toward the target. Of various semiconductor photo detectors, an avalanche photo diode (APD) is a photo detector having characteristics of high sensitivity and wide band, and is widely employed in optical communications. In a typical fiber optic network, optical signals need to be converted to electronic signals and electrical signals need to be converted to optical signals. Avalanche photodiodes are used in optical receivers for converting an optical signal into an electrical signal. The electrical signal output from the APD is coupled to an amplifier for amplification. The avalanche photodiode performs amplification of a photocurrent by applying a reverse bias voltage to its pn junction to cause an avalanche multiplication under a high electric field. This develops a defect such that before the avalanche multiplication of the photocurrent occurs, electric fields center on the peripheral portion of the photo detecting region to cause a breakdown there. Avalanche photodiodes typically include an island or body of layers of appropriately doped crystalline silicon disposed between two electrodes electrically coupled to opposite surfaces of the APD body. Relatively high voltages are applied across the APD to generate the electric field necessary to cause the avalanche effect. Avalanche occurs when carriers generated by photon induced free carrier generation in a light receiving area of the device are introduced into a high electric field area formed in a highly reverse biased (near breakdown voltage) semiconductor pn junction. The introduced carriers collide with neutral atoms to release other carriers by impact ionization. This collision process is then repeated and repeated in an avalanche fashion to effectively amplify the limited number of carriers. Typically, an APD has an absorption layer where an optical signal is absorbed. The optical signal includes a number of photons. Each photon impinging the absorption layer generates an electron-hole pair or a carrier.
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