A photodetector is provided. The photodetector includes a bottom electrode region in a semiconductor layer, a light absorption material in the semiconductor layer, and a first buffer layer sandwiched between a bottom surface of the light absorption material and the semiconductor layer. The first buffer layer includes, from bottom to top, a first Si layer, a first SiGe layer, a second Si layer, and a second SiGe layer. A first atomic percentage of Ge in the first SiGe layer is less than a second atomic percentage of Ge in the second SiGe layer. The photodetector further includes a top electrode region over the light absorption material.