Germanium is living a renaissance in the field of silicon photonics, where it is used as absorption element in waveguide photodetectors. Since a moderate reverse bias is enough to deplete the Ge absorber, pushing the carriers in the velocity saturation regime, we discuss here the main challenges involved in the three-dimensional electrooptical modeling of Ge-on-Si p-i-n detectors taking into account high-electric-field velocity saturation effects.