Influence of Pixel Design on Charge Transfer Performances in CMOS Image Sensors

The influence of pixel design on image lag is investigated by focusing on two different aspects which impact the charge transfer. First, it is confirmed that the transfer gate (TG) channel doping profile strongly affects image lag. Introducing a step under the TG in the potential diagram, due to the doping implant differences in the channel, enables very good transfer performances by limiting spillback of the charge to the photodiode. On the other hand, it is demonstrated that the overlap between the two implants used to create the step can produce a potential barrier under the TG which extension increases the image lag. Then, the influence of pixel layout geometrical parameters (e.g., the photodiode size, the TG length, and the floating diffusion area) on the charge transfer efficiency is clarified. The whole study conclusions allow identifying the design parameter limiting the transfer efficiency in a given design and the possible design-based solutions to improve it.