FTG is a pixel concept that enables non-destructive readout in plain CMOS (CIS) pixels. By repeating the measurement multiple times, the accuracy of the photocharge readout improves with the square root of the number of repetitions. Figure 1 illustrates schematic cross section of a FTG pixel next to NFTG.
Figure 1: schematic cross section of a FTG pixel.
During each cycle, the photocharge is initially held beneath a positively biased non-floating transfer gate (NFTG) in Figure 1. The FTG made high and left floating, then the photocharge is pushed away from the NFTG and collected by the FTG. The charge difference is sensed across the CTIA. Dark current shot noise (DCSN) is excluded by flushing the PPD between each CDS sample.
The noise for a single CDS sample is approximately 8eRMS, while the noise averaged over 1024 CDS samples is around 0.25eRMS.
Figure 2: Noise measurements on FTG testchip pixel “Variant C”.