A low voltage photodetector structure including a semiconductor device layer, which may be Ge, is disposed over a substrate semiconductor, which may be Si, for example within a portion of a waveguide extending laterally within a photonic integrated circuit (PIC) chip. In exemplary embodiments where the device layer is formed over an insulator layer, the insulator layer is removed to expose a surface of the semiconductor device layer and a passivation material formed as a replacement for the insulator layer within high field regions. In further embodiments, controlled avalanche gain is achieved by spacing electrodes in a metal-semiconductor-metal (MSM) architecture, or complementary doped regions in a p-i-n architecture, to provide a field strength sufficient for impact ionization over a distance not significantly more than an order of magnitude greater than the distance that a carrier must travel so as to acquire sufficient energy for impact ionization.