Investigating Transfer Gate Potential Barrier by Feed-Forward Effect Measurement

In a 4T pixel, the transfer gate (TG) “OFF” surface potential is one of the important parameters, which determines the pinned photodiode (PPD) full well capacity. The feed-forward effect measurement is a powerful tool to characterize the relationship of the PPD injection potential and the feed-forward electrons. In this paper, a parameter Vb is introduced to characterize the TG “OFF” surface potential and Vb can be extracted from the feed-forward measurement result. Using this characterization method, the pixel design and application parameters will be investigated. To better understand and control the full well capacity of the PPD, these effects and parameters will be investigated in this paper. Two test chips, using different technologies, were implemented. Further, the different mechanisms of the feedforward effect will be discussed.