This work presents a 3D multiphysics model to investigate a lateral Ge-on-Si waveguide photodetector. The simulation is performed using commercial tools: Synopsys TCAD Sentaurus for the electrical part, implementing the Drift-Diffusion method, and Synopsys RSoft for the light propagation problem using FDTD. The first section introduces the context and reasons that prompted the start of this work. In the second section, the actual simulation is presented, taking inspiration from the work of S. Lichke et al. (Nat. Photon 2021). The simulation results for the electro-optic frequency response showed a good agreement with the measurements, with a relative mean error of 8% for frequencies over 200 GHz. The appendix collects all the complications and respective solutions encountered during the device simulation.