Analysis and Modeling of the Light-Dependent Full Well Capacity of the 4-T Pixel in CMOS Image Sensors

The full well capacity (FWC) of the four-transistor pixel in CMOS image sensors (CISs) should be a constant which is decided by photodiode structure, doping, and manufacturer, whereas the measured FWC in experiments varies with the incident light intensity. In this paper, a model for the light-dependent FWC is developed. The model includes two sources that contribute to the extra light-dependent charge. They are collected along with the intrinsic full well charge in the photodiode or the floating diffusion. The first source comes from the equilibrium between the photocurrent and the photodiode forward current. The other source is the extra collected charges during charge transfer. An analysis model is developed to describe these two light-dependent sources, and the saturated output shows logarithmic and linear relationship with light intensity at low- and high-light condition, respectively. A device level simulation using technology computer aided design software was performed to verify the proposed model. Furthermore, a prototype chip with 15-μm × 15-μm pixels fabricated in a 0.18-μm CIS process was tested. The simulation and measurement results exhibit fairly good consistency with the proposed model. The estimation of the intrinsic FWC is also discussed.