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How amplification takes place in Avalanche photodiode (APD) used as optical detectors ?

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Working Principle:

• The RAPD is operated in the fully depleted mode. Photons enter the device through the p+ region and are mostly absorbed by the high resistivity intrinsic p type layer where electron hole pairs are created.
• The relatively weak electric field in this region forces or separates the carriers causing the electrons and holes to drift into the high electric field region.
• The electrons are drifted towards the p-n+ layer. Because of the high field intensity, electrons are imparted with high kinetic energy.
• The kinetic energy of electrons is greater than bandgap energy of the valence electrons, so the collision can free a bound electron.
• The free electron and hole so created acquire enough kinetic energy to cause further ionization. It results in avalanche with the number of carriers growing exponentially as the process continues.