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Full-quantum device simulations on p-type Si nanowire field-effect transistors based on the k · p method, using the k ·p parameters tuned against the sp3s* tight-binding method, are carried out. Full transport calculations from both methods agree reasonably well, and the spin-orbit coupling effect is found to be negligible in the final current-voltage characteristics. Use of the highly efficient simulator based on the 3 × 3 k ·p Hamiltonian is therefore justified, and simulations of nanowire devices with cross sections from 3 × 3 nm2 up to 10 × 10 nm2 are performed. The subthreshold characteristics, threshold voltages, and ON-state currents for the three respective transport directions of the , , and  directions are examined. The device characteristics for the  and  directions are quite similar in every respect, and the  direction has the advantage with regard to the subthreshold behavior when the channel length is aggressively scaled down. The on-current magnitudes for the three respective orientations do not differ much, although the on-current in the  direction is a little smaller, compared with that in the other two directions when the channel width becomes smaller. An uncoupled mode space approach has been used to determine the contributions from individual heavy and light hole subbands, enabling an insightful analysis of the device characteristics.