High fill factor CMOS image sensor

A CMOS image sensor structure for improving the fill factor due to design rule limitations of a conventional image sensor that incorporates a photo diode and three N-type transistors. In a first embodiment, two N-type transistors are changed to P-type transistors and the P-type transistors are formed directly within the N-well of the photo diode. In a second embodiment, the other reset N-type transistor is changed to a reset diode and the reset diode is also formed directly within the N-well of the photo diode. In a third embodiment, the reset diode and the source follower transistor are implemented using a single transistor. In addition, the output selection transistors inside all three types of CMOS image sensor structures may be deleted to increase the fill factor even further.