In this paper, we analyze, based on a two-dimensional drift-diffusion simulation, how variations in the structural components of an InGaAs/InP separate absorption, grading, charge, and multiplication photodiode (SAGCM) alter its performance. The model is employed in conjunction with experimental measurements to enhance the understanding of the device performance. Calibration of the model to the material system and growth technique is performed via the analysis of a simpler, alternate structure. Excellent agreement between the calculated results and experimental measurements of the breakdown voltage, dark current, mesa punchthrough voltage, photoresponse, and gain are obtained.