Threshold voltage and bulk inversion effects in nonclassical CMOS devices with undoped ultra-thin bodies

Novel, or nonclassical effects due to carrier distribution in double-gate (DG) CMOS devices (e.g., FinFETs) with undoped ultra-thin silicon bodies (UTBs) are analyzed and modeled. The classical analysis of gate-gate charge coupling and threshold voltage (Vt) of fully depleted SOI MOSFETs by Lim and Fossum [Lim H-K, Fossum JG. Threshold voltage of thin-film silicon-on-insulator (SOI) MOSFETs. IEEE Trans Electron Dev 1983;ED-30(October): 1244-51.] is generalized to account for bulk inversion in UTBs, carrier-energy quantization, and short-channel effects (SCEs). The generalized charge coupling model physically and generically characterizes Vt for arbitrary gate biases, and explains an enhanced coupling in independent-gate (IG) FinFETs associated with bulk inversion. The impact of the carrier distribution on SCEs is also discussed. The effect on Vt of sparse, random dopants in the UTB is shown to be insignificant. Finally, bulk inversion is noted to prevail in strong inversion, where its net effect on device performance is beneficial.