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In this paper, we investigate band-structure effects on the transport characteristics of ultrascaled silicon nanowire (SNW) FETs by means of quantum transport simulations. To this purpose, a new approach is used for the solution of the open-boundary Schrodinger equation in the SNW, accounting for the appropriate dispersion relationships of the subbands induced by the confinement of the 1-D electron gas. The model is validated by comparison with 3-D atomistic simulations based on the tight-binding approach, and simulation results are compared with a simpler effective-mass model with either constant and fitted (not bulk-like) transport effective masses. The proposed model predicts: (1) the possibility of negative differential output conductance in thin SNW-FETs related with the finite energy extension of the subbands; (2) an increase of the intrinsic transit time, corresponding to a reduced electron average velocity; and (3) a degradation of the subthreshold slope at short channel lengths, due to enhanced tunneling currents.