Effect on the performance of staircase APDs of electron impact ionization within the graded-gap region

The performance of a staircase avalanche photodiode is usually analyzed assuming that electrons ionize at the steps only while the holes ionize within the graded-gap regions. Here, the analysis is extended in order to consider the effect of carrier (electron and hole) impact ionizations both in the graded-gap regions and at the steps. The modeling procedure adopted represents an expedient simplification, selected to render tractable a complex subject, and appending this to potential physical device models leads to consideration of three extreme cases. The results indicate that electron ionization in the graded region can lead to improved sensitivity when residual hole ionization in both regions is kept to a minimum