Patent ID: 7564083

Claim:
An active pixel sensor cell (APS cell), the APS cell comprising: an isolation region positioned in a semiconductor substrate and surrounding the APS cell to isolate the APS cell, the isolation region having an isolation interface with other elements of the APS cell; a photodiode positioned in the semiconductor substrate, the photodiode comprising a first doped region; a guard ring comprising a first guard ring end and a second guard ring end surrounding the first doped region and having a gap, the guard ring being positioned at the isolation interface surrounding the first doped region; a first transistor comprising a first gate electrically connected to the first doped region with a conductive structure; and a second transistor comprising a common drain with the first transistor, a source electrically connected to the first doped region, and a second gate positioned on the semiconductor substrate above the gap, the common drain further comprises a lightly doped drain (LDD), wherein when the second transistor is turned on, a chemical channel is occurred in the semiconductor substrate underneath the second gate; wherein the second gate does not cover the isolation interface that is positioned between the conductive structure and the second gate, the length of the chemical channel is determined by the spacing between the first doped region and the LDD, and the first guard ring end and the second guard ring end extend to the LDD region.