Constant-Current APD Bias Method Automatically Optimizes Optical Comms Performance (.PDF Download)
An avalanche photodiode (APD) delivers a useful electron-multiplier gain and generally improves signal-to-noise ratio (SNR) in optical communication systems. An APD is similar to a PIN photodiode with the addition of a thin, high electric field region next to the depletion region where photons generate electron-hole pairs. Photo-generated carriers rapidly drift into the high field region and acquire energy from the applied field.
When photo-generated carriers receive sufficient energy, they knock other electrons out of the semiconductor lattice to create secondary electron-hole carrier pairs. The secondary carriers in turn receive energy from the applied electric field and create additional carriers as they all drift through the thin, high electric field region. The process continues until the carriers pass out of the high field region.
Avalanche multiplier gain is nonlinearly proportional to the bias potential voltage applied to the APD. Gain increases to hundreds or thousands as bias potential approaches reverse breakdown, where an APD behaves like a Zener diode.
There is a large field of study1,2 regarding gain and noise mechanisms of various APD structures. Noise increases rapidly at higher avalanche gain, so there will always be an optimum gain and bias setting, depending on many parameters. For our purposes here, it’s sufficient to say that APD gain isn’t noise-free and does degrade the SNR as it appears at the input to the photodiode.
However, Johnson (resistor) noise in the following preamplifier dominates the noise out of PIN photodiode detectors by perhaps 100 times, so any avalanche gain initially improves performance no matter how noisy the APD. Silicon APDs generally have much lower noise than III-V APDs and deliver very much more usable gain.
Substitution of an APD for a PIN photodiode significantly boosts performance, but there are constraints and penalties.
Below optimal gain, performance is less than it can be. Above optimal gain, APDs generate a lot of noise that rapidly degrades performance.