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The threshold voltage Vt variability in the scaled crystal channel and poly-Si channel double-gate fin-type metal-oxide-semiconductor field-effect transistors (FinFETs) with different gate oxide thicknesses Tox has been systematically analyzed. By investigating the Tox dependence of Vt variations in crystal channel FinFETs, the gate-stack origin sources, i.e., work function (φm) variation and gate oxide charge (Qox) variation sources, were successfully separated. It was found that the atomically flat Si-fin sidewall channels fabricated by the orientation-dependent wet etching contribute to the reduction of gate-stack origin Vt variations. Moreover, it was experimentally found that the standard Vt deviation (σVt) of poly-Si channel FinFETs is three times higher than that of crystal channel ones, and a good subthreshold slope in the poly-Si channel FinFETs was obtained with gate length Lg down to poly-Si grain size.