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Nanowire band-to-band tunneling field-effect transistors (TFETs) are simulated using the Wentzel–Kramers–Brillouin (WKB) approximation and an atomistic, full-band quantum transport solver including direct and phonon-assisted tunneling (PAT). It is found that the WKB approximation properly works if one single imaginary path connecting the valence band (VB) and the conduction band (CB) dominates the tunneling process as in direct band gap semiconductors. However, PAT is essential in Si and Ge nanowire TFETs where multiple, tightly-coupled, imaginary paths exist between the VB and the CB.