Equivalent Circuit Model of an Avalanche Photodetector Based on Verilog-A

• 1、School of Electronic Engineering,Beijing University of Posts and Telecommunications,Beijing 100876

Abstract：Single-photon avalanche diodes (SPADs), featuring high gain and single-photon sensitivity, are key devices for weak optical signal detection. This paper proposes an improved SPAD equivalent-circuit simulation model based on Verilog-A, aiming to address the limitations of existing three-switch models in capturing non-ideal effects, as well as the lack of intuitiveness and the complexity of purely physics-based models. The proposed model innovatively embeds Verilog-A modules that describe dark count rate (DCR) and afterpulsing (AP) into the classical three-switch topology, achieving both physical-mechanism accuracy and circuit-level simulation intuitiveness. The model comprehensively accounts for the thermal carrier generation and band-to-band tunneling mechanisms that contribute to dark counts. A digital pulse module with adjustable width is incorporated to ensure robust and stable switch triggering. Cadence Spectre simulation results based on 4H-SiC material parameters demonstrate that the proposed model can accurately reproduce the dynamic response and stochastic noise characteristics of SPADs in a passive-quenching circuit, thereby validating the effectiveness of the model.

Keywords： Avalanche photodiode Equivalent circuit model Verilog-A Dark count rate Afterpulsing