This work investigates the conduction band structure of silicon nanowires, its dependence with the wire width and its influences on the electrical performances of Si nanowire-based MOSFET's working in the ballistic regime. The energy dispersions relations for Si nanowires have been calculated using a sp(3) tight-binding model and the ballistic response of n-channel devices with a 3D Poisson-Schrodinger solver considering a mode-space approach and open boundary conditions (NEGF formalism). Results are compared with data obtained considering the parabolic bulk effective mass approximation, highlighting in this last case the overestimation of the I-on current, up to 60% for the smallest (1.36 nm x 1.36 nm Si wire) devices. (c) 2006 Elsevier Ltd. All rights reserved.