Avalanche Photodiode

Definition: Avalanche photodiodes utilize the avalanche multiplication process to achieve higher electric output. The avalanche photodiode resembles a PIN photodiode except for the insertion of an additional p-type multiplying region. Most of the incident photons are absorbed in the intrinsic (‘I’) region and electron-hole pairs are first generated in this region as in the PIN photodiode. The holes drift toward the p-type region (‘P’) and the electrons toward the n-type (‘N’). The electrons, before reaching the n-type region, pass through a p-type multiplying region. If the electrons have undergone sufficient acceleration on reaching the multiplying region, new electron-hole pairs are generated by the collision- ionization process. The newly generated electron-hole pairs themselves generate more electron-hole pairs. The drawbacks to avalanche photodiodes are the high bias voltage needed, and the temperature compensation necessary for stable operation.

Alternative definition: An avalanche photodiode (APD) is a type of semiconductor photodetector used to detect low levels of light in applications such as fNIRS. It is based on the principle of avalanche multiplication, which occurs when an electron in the photodiode collides with an atom and generates a cascade of secondary electrons, resulting in a much larger current than would be caused by a single photon.


References: E. Hui Pan. Fiber Optics Weekly Update. Information Gatekeepers Inc.

Related terms: Photomultiplier tube, Detector, Sensor, Silicon Photomultiplier  

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