Fundamental Mechanisms of Total-Ionizing-Dose Degradation in Waveguide-Coupled Germanium-on-Silicon p-i-n Photodiodes

In this work, we investigate 10-keV X-ray-induced TID effects in VPIN photodiodes using calibrated TCAD simulations. We demonstrate that positive trapped charges near the space-charge region locally increase the electric field magnitude, enhancing Shockley–Read–Hall (SRH) generation-recombination and trap-assisted tunneling (TAT) rates, thereby impacting dark current, forward current, and the ideality factor. Simulations indicate that charge trapping at the top-corner Ge/SiO2 interfaces accurately reproduces experimental trends. In contrast, photocurrent and junction capacitance remain largely unaffected, attributed to the large area-to-perimeter ratio of the device, which reduces the influence of localized interface degradation.