Abstract-This letter presents an electrical method to reduce dark current as well as increase well capacity of four-transistor pixels in a CMOS image sensor, utilizing a small negative offset voltage to the gate of the transfer (TX) transistor particularly only when the TX transistor is off. As a result, using a commercial pixel in a 0.18 µm CMOS process, the voltage drop due to dark current of the pinned photodiode (PPD) is reduced by 6.1 dB and the well capacity is enhanced by 4.4 dB, which is attributed to the accumulated holes and the increased potential barrier near the PPD, respectively.Index Terms-CMOS image sensor (CIS), dark current, fourtransistor pixel, hole accumulation diode, imager, pinned photodiode (PPD), well capacity.
, respectively, the transconductances and driving currents of both devices were enhanced by 7 to 37% and 6 to 72%. These improvements seemed responsible for the formation of a lightly doped retrograde high-electron-mobility Si surface channel in nMOSFETs and a compressively strained high-hole-mobility Si 0.8 Ge 0.2 buried channel in pMOSFETs. In addition, the nMOSFET exhibited greatly reduced subthreshold swing values (that is, reduced standby power consumption), and the pMOSFET revealed greatly suppressed 1/f noise and gate-leakage levels. Unlike the conventional strained-Si CMOS employing a relatively thick (typically > 2 µm) Si x Ge 1-x relaxed buffer layer, the strained-SiGe CMOS with a very thin (20 nm) Si 0.8 Ge 0.2 layer in this study showed a negligible self-heating problem. Consequently, the proposed strained-SiGe CMOS design structure should be a good candidate for low power and high performance digital/analog applications.
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