Avalanche photodiode are used to obtain the large gain, i.e. large output because Conventional photodiodes and PIN photodiodes obtain the limited gain.
CONSTRUCTION: APD’s are usually variation of PIN diodes. The materials used and thus the spectral ranges are the same. One form of APD, a reach through diode, is shown in figure.
It consists of p+ -π-p-n + layers. The p+ and n+ layers are highly doped, low resistance regions having a very small voltage drop. The π region is lightly doped nearly intrinsic. Most of the photons are absorbed in this layer creating electron hole pairs. When the APD is reverse biased, most of the voltage appears across the p-n + junction because of negligibly small photocurrent.
OPERATION:
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 band gap 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.