This study presents an ultra-thin backside illuminated (BSI) and top-illuminated germanium (Ge) on silicon (Si) photodetector (PD) designed for a wavelength range of 1 to 1.4 microns. Monte Carlo molecular dynamics simulations, incorporating charge transport, were employed to model the electron-hole current within the device, which features integrated nano-micro hole structures aimed at optimizing high-speed performance. This design enables wide-spectral response, high efficiency, and ultra-fast operation in photodetectors. Our simulations explicitly account for the reverse bias of the PDs, incorporating device doping information to analyze the enhancement of photon absorption and the suppression of unwanted electron-hole recombination driven by the physics of electron and hole transport. The Ge thickness of 350 nm facilitates high-speed performance exceeding 60 GHz, while the nanostructures at the bottom of the Ge layer significantly improve optical absorption efficiency, achieving improvements of over fourfold, reaching up to 80%. We utilized BSI PD technology with multi-stack capabilities, allowing for stacking the PD or PD array wafer with an electronic wafer, thereby enabling efficient signal processing and transmission for optical interconnect applications, such as short-reach links in data centers.