An Investigation on Effective Mobility in Nano-wire FETs under Quasi Ballistic Conditions

In this work we investigate the electron mobility in nanowire FETs operating under quasi-ballistic conditions. Starting from a general expression of the current-voltage characteristics worked out in a previous paper, we extract the drain current at vanishingly-low drain voltages and find the functional dependence of the effective mobility on the device length, the mean-free path and the barrier height. The resulting expression is nonlocal for short gate lengths, but may be useful for the interpretation of experimental measurements. The main result of this work is that the combined effect of acoustic-phonon and surface-roughness scattering leads to a nearly uniform mean-free path, at least for nanowire FETs with a diameter around 5 nm. Thus, mobility degradation at large gate voltages is predominantly due to carrier degeneracy, rather than an enhanced scattering rate.