  In a CMOS image sensor pixel, fast and complete charge transfer from pinned
photodiode (PPD) is desired and necessary in some applications. In special
cases such as time-of-flight imaging or large pinned photodiodes, the PPD
potential well shape highly affects the charge transfer performance and should
be engineered carefully. In the present work, a PPD structure named multi-level
PPD is introduced and examined through simulation study. Moreover, a fast and
effective way to analyze the pinning process for a lag-free design is
introduced. It is concluded that the proposed PPD achieves fast and complete
charge transfer without additional implementation masks or process steps. The
proposed PPD is compared with a similar conventional rectangular pixel and 31%
reduction in the charge transfer time is observed.
