Time response modelling in submicron avalanche photodiodes

Conventional models of the time-dependent current response to an impulse of injected carriers in an avalanche photodiode assume that carriers travel always at their saturated drift velocities. A Monte Carlo model is used to show that, while this assumption is reasonable for a 1.0 μm avalanche region in GaAs, it significantly underestimates the avalanche speed in a submicron device at high electric fields. The disparity is identified as due to an enhancement in the velocities of carriers to ionisation, defined as the path length to ionisation divided by the time to traverse it. Using the mean values of these velocities in the conventional model produces good agreement with the Monte Carlo results. Comparison with a local ionisation model also suggests that thin APDs will operate more quickly than expected.