Effects of Negative-Bias Operation and Optical Stress on Dark Current in CMOS Image Sensors

A negative-bias operation of the transfer gate has revealed a major origin of dark current defects of CMOS image sensors. Charge injection from the photodiode to the substrate at the negative-bias operation has been avoided by an improved well structure. A strong visible light has been observed to cause damage with an increase in the dark current under the normal operating condition, and the damage has been annealed in the power-off mode. This indicates that the strong light possibly causes a threshold voltage shift, which is explained by the photon-assisted tunneling or emission mechanisms. Multiple stress-and-anneal cycles have been found to cause an optical hardening effect, which can be explained by immobile trapped holes.