A stochastic noise model based excess noise factor expressions for staircase avalanche photodiodes
Multistep staircase avalanche photodiodes (APDs) are the solid-state analogue of photomultiplier tubes, owing to their deterministic amplification with twofold stepwise gain via impact ionization. Yet, the stepwise impact ionization irregularities worsen with increasing step counts, which are a major source of internal noise in these APDs. Some noise models for staircase APDs have been previously reported, where the excess noise factor expressions are based on Friis' noise factor formula for cascade networks, erroneously considering the power gains as the gains. Excess noise factor being a key component in staircase APDs' noise models, we formulate generalized excess noise factor expressions for multilayer graded-bandgap APDs in terms of their layer-wise ionization probabilities, applicable for all operating biases, which include the sub-threshold, staircase, and tunnelling breakdown regimes. We further derive simplified expressions for staircase APDs and prove that these expressions match Bangera's corrections to Friis' noise factor formulas for cascade networks.