Electrical characterization of transfer transistors with pure and nitirided gate oxides in four-transistor CMOS image pixels

Abstract

Transfer transistors having pure (TX-TR-PO) and nitrided (TX-TR-NO) gate oxides in four-transistor CMOS image pixels were electrically characterized by both 1/f noise and static measurements. Reduced 1/f noise levels in TX-TR-NO denoted that nitridation of oxides contributed to maintaining the p+ surface passivation layer highly doped near its gate-edge, by suppressing boron out-diffusion into gate-poly. Although TX-TR-NO in this study had a relatively high potential barrier at the photodiode/TX-TR interface, the static conductance analysis showed that the barrier could be sufficiently lowered in real pixel operations through either ‘deep depletion’ or ‘floating-diffusion voltage-induced barrier lowering’. Moreover, the potential barrier existing in TX-TR-NO was suggested to play a key role in reducing the sensor module's dark current level, kTC noise and image lag failure rates by preventing charge's flowing back to photodiode during TX-TR's transition from ‘off’ to ‘on’. The proposed TX-TR design is expected to effectively improve the charge transfer characteristics of CMOS image pixels, especially under low operation voltage conditions (⩽2.5 V).