We investigate influences of series resistances on the performance of 1.55 μm waveguide-type germanium photodetectors (Ge-PDs) on a silicon-on-insulator substrate. The current–voltage characteristics, responsivities, saturation photo-current characteristics, electrical reflection coefficients, and photodetection frequency responses of Ge-PDs, having different series resistances, are measured, and their equivalent circuit models are established. By analyzing the resulting circuit model parameters, we determine how much Ge-PD series resistances influence Ge-PD saturation photo-currents and photodetection bandwidth. These results should be of great use for optimization of Ge-PD fabrication processes and device parameters for target applications.