Non-equilibrium modelling of avalanche photodiode speed

Avalanche photodiodes (APDs) are particularly suited to detecting weak optical signals. However, in general, they suffer from a bandwidth limitation imposed by carrier feedback within the avalanche process. Although faster response times can be obtained by reducing the length of the avalanche region, dead space increasingly degrades the improvement relative to predictions from a purely local ionization model using the same velocities for the carriers. Conventionally these velocities are chosen to be the carriers´ saturated drift velocities, v_s. However, our recent Monte Carlo (MC) modelling showed that an enhancement in the mean velocities of carriers to ionization in short (<0.3 μm) APDs produces a much faster avalanche speed than a model with similar spatial ionization using saturated drift velocities. This velocity enhancement promises to compensate for the dead space degradation although the extent is not clear. For example, if the velocity enhancement overcompensates, APD bandwidth will be greater than expected from a local ionization model using v_s. Since the latter (conventional) model is particularly popular for APD bandwidth a study of its accuracy in a non-equilibrium regime is desirable. The results obtained suggest that APDs with short avalanche regions can be expected to operate more quickly than conventional model predictions, particularly so in very short devices.