Reach through avalanche photodiode (RAPD)

Avalanche photodiodes are required in fiber optic communication at the receiving end. Photodiode detects the light signal and converts it into electrical form. There exist many forms of avalanche photodiodes. Reach through avalanche photodiode (RAPD) is a promising candidate in this category. Consider a positive-intrinsic-negative semiconductor photodiode operating in reverse biased mode. When the applied reverse voltage exceeds threshold value, photoelectrons generated as a result of its exposure to light, gets accelerated through the junction, collides with other atoms to produce secondary electron-hole pairs. Carrier concentration increases exponentially with the electric field intensity. This phenomenon is known as impact ionization or avalanche effect. The reach through avalanche photodiode consists of a high resistivity p-type material deposited on an epitaxial layer with an extremely high order of impurity concentration. Commonly used doping atoms to achieve this are Boron and Phosphorous. Operation of RAPD is always in fully depleted mode.