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A transfer transistor structure achieving complete charge transfer in a CMOS image sensor pixel is proposed to eliminate image lag. This structure adopted double N implants for photodiode to ensure a wide channel of charge transfer, a surface P type preventing from dark current and especially a non-uniform doped poly-silicon gate to adjust the potential profile in channel. The simulation results show that if a light pulse of 10000lux illuminates for one integration cycle, the signal voltage of the following frame drops to 1/50000 of the first one. The electron density in N buried layer after charge transfer is around 108/cm3, far below the intrinsic value.