Ge-on-Si waveguide photodetectors achieving remarkable bandwidths exceeding 250 GHz and compatible with mature CMOS technology have been demonstrated recently (S. Lischke et al., Nat. Photon. 2021). The present thesis leverages commercial 3D multiphysics simulation tools, specifically Synopsys TCAD Sentaurus and RSoft Photonic Device Tools, to develop a model for the computer-aided design of this novel concept of device. In the perspective of providing guidance to manufacturers, this model has been tailored to predict key performance metrics of the detector, namely modulation bandwidth and responsivity. After validating and calibrating the model versus experimental results in the literature, the performance of the detector has been related to its technological parameters, such as geometry and doping, and to various operating conditions, including power and wavelength of the light source and bias.