Analytical Modeling of Exposure Process in Pinned Photodiode CMOS Image Sensors

The output features of pixels in CMOS image sensors (CISs) are influenced by different exposure conditions. This article presents an analytical model to describe the output characteristics of the exposure process in pinned photodiode (PPD) CMOS image sensors with the accumulation of three charge sources: photogenerated charge, p-n junction-generated charge, and emission charge. In the proposed model, the difference between the time-based and light intensity-based photo response process is illustrated. The model also reveals the relationship between photodiode potential and light intensity at different exposure values. At the low exposure values, the PD potential increases with light intensity, and a contrary trend is observed at the high exposure values. This concludes that a larger linear output range can be obtained in high light intensity conditions. Furthermore, the improved model provides development analysis in terms of light intensity influence for long exposure time noise. The models were verified with technology computer-aided design simulation and the test devices were fabricated using a 0.18-μm CIS process. The model demonstrates good consistency with simulation and measured results.