Source-to-drain tunneling is investigated for Si triple-gate nanowire transistors. The full-band quantum transport problem is solved in an atomistic basis using the nearestneighbor
tight-binding method. It is self-consistently coupled to the threedimensional calculation of the electrostatic potential in the device using the finite element method. This procedure is applied to the computation of
transfer characteristics of transistors with different channel orientations such as , , , and  for gate lengths ranging from 4 nm to 13 nm. The subthreshold swing
is then extracted from the results to determine the scaling limit of nanowire transistors.