Abstract

A large difference in the ionization rates for electrons and holes is essential for a low-noise avalanche photodiode (APD). Enhancement of electron impact ionization in a heterojunction superlattice has been experimentally demonstrated since 1982. However, the trapped carriers in heterostructures can occasionally contribute to impact ionization, and charging or discharging delay, therefore, degrades the originally expected performance. We propose to use a sharp p-n junction to improve the k-ratio. Such a scheme can be applied to both the heterojunction and homojunction and is free of the miscibility problem and trapping effect. The length of the sharp p-n junction is much shorter than its carrier energy relaxation mean free path. Impact ionization happens in the region right after the junction. Because the carrier distribution function is a nonlinear function of energy, an equal amount of injection level from the junction affects electrons and holes differently when their ionization thresholds are different. Calculations based on a lucky-drift model¹ show that the k-ratio improvement increases with the height of the injection level and becomes extremely significant when the level approaches the lower one of the thresholds.

© 1988 Optical Society of America