Abstract: A method in which thin-film p-i-n heterojunction photodiodes are formed by selective epitaxial growth/deposition on pre-designated active-area regions of standard CMOS devices. The thin-film p-i-n photodiodes are formed on active areas (for example n+-doped), and these are contacted at the bottom (substrate) side by the “well contact” corresponding to that particular active area. There is no actual potential well since that particular active area has only one type of doping. The top of each photodiode has a separate contact formed thereon. The selective epitaxial growth of the p-i-n photodiodes is modular, in the sense that there is no need to change any of the steps developed for the “pure” CMOS process flow. Since the active region is epitaxially deposited, there is the possibility of forming sharp doping profiles and band-gap engineering during the epitaxial process, thereby optimizing several device parameters for higher performance.