Impact ionization in semiconductor heterojunctions

A theoretical study is made of impact ionization in heterojunction systems. It is found that impact ionization by electrons near the conduction band minimum of the larger band gap material can occur in two ways: the electron can transmit into the smaller gap material and undergo essentially bulk impact ionization; the electron can impact ionize at the interface. The investigation is concentrated on the second process, which leads to a probability for interface impact ionization. For a simple model an expression is found for the interface impact ionization probability which demonstrates the dependences of this probability on the incident electron wave vector component normal to the interface, the conduction band offsets, and the energy gap of the smaller gap material.