Extremely narrow and bulk-like p-type InAs Si nanowire TFETs are studied using (i) a full-band and atomistic quantum transport simulator based on the sp(3)d(5)s* tight-binding model and (ii) a drift diffusion TCAD tool. As (iii) option, a two-band model and the WKB approximation have been adapted to work in heterostructures through a careful choice of the imaginary dispersion. It is found that for ultra-scaled InAs Si nanowire TFETs, the WKB approximation and the quantum transport results agree very well, suggesting that the former could be applied to larger hetero-TFET structures and considerably reduce the simulation time while keeping a high accuracy. (C) 2015 Elsevier Ltd. All rights reserved.