Equivalent circuit and fundamental limit of multi-stage infrared photodetectors

Abstract

Based on an equivalent circuit model, a signal current in a multi-stage infrared (IR) photodetector is derived and used to discover a meaningful relation among quantum efficiency, collection efficiency, and particle conversion efficiency. Based on such a relation, it is demonstrated theoretically that the ultimate detectivities in multi-stage interband cascade infrared photodetectors (ICIPs) with identical discrete absorbers are the same as that in a conventional single-absorber detector in the limit of perfect collection (i.e., independent of the number of cascade stages) but higher than that in the single-stage detector with a finite diffusion length. Detailed derivations and calculations, along with relevant discussion, are provided to show how ICIPs are optimized for maximizing the detectivity and to understand the underlying physics. Multi-stage ICIPs with identical discrete absorbers are robust and durable against structural variations without being restricted by current matching and, therefore, are of more significance for practical applications such as those that require high-speed response or circumventing the diffusion length limitation. The results obtained for ICIPs with identical discrete absorbers can also be applied for quantum cascade detectors and photovoltaic quantum well infrared photodetectors. The results and insights gained from this work will further improve the understanding of multi-stage IR photodetectors and generate increased interest in the development of ICIPs and related devices for useful applications.