Via-geometry-engineered series resistance for simultaneous enhancement of bandwidth and saturation in Ge-Si...

High speed and high saturation power germanium-silicon (Ge-Si) photodetectors (PDs) are highly desirable for microwave photonic links. However, the impact of series resistance on their performance remains poorly understood. In this work, the series resistance is engineered by optimizing the via-opening geometry while keeping the vertical PIN junction unchanged. A set of waveguide-integrated Ge-Si PDs with identical active regions but different via geometries is systematically characterized. The series resistance is extracted through equivalent-circuit fitting of the measured small-signal frequency responses, and the saturation behavior is quantified using photocurrent-optical power characteristics. Devices with reduced series resistance consistently exhibit both enhanced 3 dB bandwidth and improved saturation output capability. These improvements are attributed to the dual role of lower series resistance, which alleviates RC limited bandwidth and suppresses current-induced voltage drop, thereby maintaining the effective junction reverse bias and delaying output compression. The results provide a process compatible strategy for realizing high speed and high power Ge-Si PDs.