A photo-detecting device includes a first semiconductor layer with a first dopant, a light-absorbing layer, a second semiconductor layer, and a semiconductor contact layer. The second semiconductor layer is located on the first semiconductor layer and has a first region and a second region, the light absorbing layer is located between the first semiconductor layer and the second semiconductor layer and has a third region and a fourth region, the semiconductor contact layer contacts the first region. The first region includes a second dopant and a third dopant, the second region includes second dopant, and the third region includes third dopant. The semiconductor contact layer has a first thickness greater than 50 Å and smaller than 1000 Å.