An implanting method for forming a photodiode comprises providing a substrate with a first conductivity, growing an epitaxial layer on the substrate, implanting ions with a second conductivity in the epitaxial layer from a front side of the substrate and implanting ions with the first conductivity in the epitaxial layer from the front side of the substrate to form a photo active region adjacent to the front side and a photo inactive region underneath the photo active region. By employing the implanting method, an average doping density of the photo active region is approximately ten times more than an average doping density of the photo inactive region.