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We present a comparative study of the surfaceroughness (SR)-induced variability at low supply voltage VDD = 0.3 V in nanowire InAs tunnel FETs and strained-silicon (sSi) MOSFETs. By exploiting a 3-D full-quantum approach based on the Non-Equilibrium Green's Function formalism, we show that the Ion variability in InAs tunnel FETs is much smaller than the Ioff variability, whereas for VDD = 0.3 V, the sSi MOSFETs working in the subthreshold regime present similar Ion and Ioff variability. We explain the smaller Ion compared with Ioff variability of InAs tunnel FETs by noting that in the source depletion region, where tunneling mainly occurs for VGS = VDD, microscopic subband fluctuations induced by SR are small compared to macroscopic band bending due to the built-in potential of the source junction and to the gate bias. This results in SR-induced variability that is larger in InAs tunnel FETs than in sSi MOSFETs.