Abstract: A method of fabricating a photodiode and at least one MOS device within a first active region and a second active region, respectively, of a substrate is disclosed. First, a gate structure is formed on the substrate within the second active region, and lightly-doped regions are formed by introducing first dopants into the substrate through the gate structure as masking. Then, a diffusion region is formed in the substrate within the first active region by ion implantation. Then, an insulating layer is formed to overlie the first and second active region, a portion of which within the second active region is thereafter patterned to sidewall spacers on the sidewalls of the gate structure. Subsequently, heavily-doped regions are formed by introducing second dopants throughout the second active region into the substrate by the gate structure and sidewall spacers as masking. In addition, the insulating layer can be thinned before the step of patterning the insulating layer to form the sidewall spacers is performed.