Abstract: A photodetector circuit incorporates an avalanche photodiode structure having a contact layer (14) forming an ohmic contact over an annular region (18) with the annular guard ring (8). In the fabrication process, the starting substrate can either be the handle wafer of a p? silicon-on-insulator wafer, or a p-Si substrate with an insulating SiO2 layer (4). A window (6) is produced in the insulating layer (4) by conventional photolithographic and etching. A n+ guard ring (8) is created by diffusing donor impurities into the substrate, and a thinner insulating SiO2 layer (22) is thermally grown so as to cover the exposed surface of the substrate within the window (6). P-type dopant is then implanted through the thin oxide layer to increase the doping level near the surface of the substrate. Subsequently a second window (24) is made in the insulating layer (22), and the layer (12) is then epitaxially grown selectively on the area of the substrate exposed by the window (24) in the insulating layer (22).