A method of forming an image sensor is disclosed. A partially processed semiconductor wafer is provided, containing a p-type region. An n-type photodiode region is formed within the p-type region. A field oxide isolation region is then formed which extends beyond the p-type region and also covers the p-type region except for an active region and an overlap part of the n-type photodiode region. An n-channel MOSFET is fabricated in the active region with one of the source/drain regions of the MOSFET extending over the overlap part of the n-type photodiode region. A blanket transparent insulating layer is then deposited.