A semiconductor substrate is provided on which a plurality of shallow trench isolations (STI) defining a plurality of active areas are formed. The active areas comprise a photo sensing region, and a plurality of photodiodes are formed in each photo sensing region. Then a local oxidation of silicon isolation (LOCOS) layer is formed by performing a LOCOS process. Thereafter a plurality of gates are respectively formed in each active area, where the gates partially overlap the LOCOS layer. Finally doped regions are formed in the semiconductor substrate where the gate does not cover the LOCOS layer.