A novel equivalent circuit for separate absorption grading charge multiplication (SAGCM) APDs composed of basic circuit components is developed. The model is applied to simulate the frequency performance of APDs, and the simulating results show a well-reasonable agreement with the physical model calculation and experimental data. The influence of three important factors on frequency performance, including carrier transit time, avalanche buildup time, and parasitic elements (consisted of resistance, inductance, and capacitance of APDs) are investigated. Using this model, the cosimulation of the packaged APD$\backslash$transimpedance amplifier (TIA)-module is also carried out. We found that the resonance of parasitic inductance and capacitance can be utilized to compensate the attenuation of high-speed APD$\backslash$TIA circuit board.
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