Abstract: A photodiode structure including a silicon substrate, an oxide layer on the silicon substrate, a silicon on insulator region on the oxide layer, a germanium absorption region, a silicon nitride waveguide, a cathode region, and an anode region is provided. The germanium absorption region is at least partially disposed in a recess of the silicon on insulator region. The germanium absorption region includes a top surface having a first width and a bottom surface having a second width, the first width being greater than the second width. The cathode region is formed at a first side of the germanium absorption region, and the anode region is formed at a second side of the germanium absorption region that is opposite the first side.