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Equivalent Circuit Model of the RF Characteristics of Multi-Stage Infrared Photodetectors

Multi-stage infrared photodetectors such as quantum cascade detectors (QCDs) and interband cascade infrared photodetectors (ICIP) have been demonstrated with high detectivity and high-frequency operation. In the previous work published by Rui. Q. Yang [Applied Physics Letters 2021, 119 (14), 141107], an equivalent circuit model is used to derive the relation among quantum efficiency, collection efficiency, and particle conversion efficiency. The ultimate detectivities in multi-stage infrared photodetectors were discussed based on that model. However, the RF performance of multi-stage infrared photodetectors was rarely studied theoretically so far. In this work, a generalized equivalent circuit model is proposed with the AC term included. The detailed calculations and relevant discussion are provided to show how the normalized frequency response function of each stage impacts the overall 3 dB performance of the device. In the limit of weak absorption, such as QCDs or ICIP with very weak and identical absorption in each stage, the multi-stage infrared photodetectors show significant improvement in RF performance. Moreover, how the current mismatching issue between stages impacts 3 dB bandwidth performance quantitatively is derived and discussed for identical absorption stages architecture. The result and insights gained from this work can be beneficial for the understanding and optimization of high-speed multi-stage detectors.