Saturation of multiplication mechanism in silicon avalanche photodiodes used for single electron detection

We have examined the multiplication factor of silicon avalanche photodiodes designed for fast timing detection of monoenergetic electrons. The multiplication mechanism was found to be saturated. The highly concentrated space charge generated by a primary electron causes the saturation. This was confirmed by a Monte Carlo simulation of electron trajectories in silicon and an experiment exploiting a subpicosecond laser pulse with a variable illuminating spot size to simulate an electron bombardment.