Dark Current Blooming in Pinned Photodiode CMOS Image Sensors
Résumé
This paper demonstrates the existence of dark current blooming in pinned photodiode CMOS image sensors with the support of both experimental measurements and TCAD simulations. It is usually assumed that blooming can appear only under illumination, when the charge collected by a pixel exceeds the full well capacity (i.e. when the photodiode becomes forward biased). In this work, it is shown that blooming can also appear in the dark by dark current leakage from hot pixels in reverse bias (i.e. below the full well capacity). The dark current blooming is observed to propagate up to nine pixels away in the experimental images and can impact hundreds of pixels around each hot pixel. Hence, it can be a major image quality issue for state-of-the-art pinned photodiode CMOS Image Sensors used in dark current limited applications such as low-light optical imaging and should be taken into account in the dark current subtraction process. This work also demonstrates that one of the key parameter for dark current optimization, the transfer gate bias during integration, has to be carefully chosen depending on the application because the optimum bias for dark current reduction leads to the largest dark current blooming effects.
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