Photo sensor in a photo diode

A photo sensor in a photo diode positioned on a semiconductor wafer, the surface of the semiconductor wafer comprising a silicon substrate containing first-type dopants, and a well of first-type dopants positioned in a predetermined region on the substrate, the photo diode comprising a MOS transistor of second-type dopants positioned on the surface of the well, a photo sensor positioned beside the well and electrically connected to the MOS transistor, and an insulation layer positioned on the surface of the substrate surrounding the photo sensor, the photo sensor comprising:

• a first doped region of second-type dopants positioned on the surface of the photo sensor;
• and
• a second doped region of second-type dopants positioned under the first doped region, the periphery of the second doped region in contact with the insulation layer, and a portion of the second doped region at least partially under the insulation layer;
• wherein the dopant density of the second doped region is less than that of the first doped region, and the second doped region functions to reduce the electrical field around the first doped region so as to reduce leakage current.