  The sp3d5s*-spin-orbit-coupled atomistic tight-binding (TB) model is used for
the electronic structure calculation of Si nanowires (NWs), self consistently
coupled to a 2D Poisson equation, solved in the cross section of the NW. Upon
convergence, the linearized Boltzmann transport theory is employed for the
mobility calculation, including carrier scattering by phonons and surface
roughness. As the channel is driven into inversion, for [111] and [110] NW
devices of diameters D>10nm the curvature of the bandstructure increases and
the hole effective mass becomes lighter, resulting in a approx. 50% mobility
increase. Such improvement is large enough to compensate for the detrimental
effect of surface roughness scattering. The effect is very similar to the
bandstructure variations and mobility improvement observed under geometric
confinement, however, in this case confinement is caused by electrostatic
gating. We provide explanations for this behavior based on features of the
heavy-hole band. This effect could be exploited in the design of p-type NW
devices. We note, finally, that the 'apparent' mobility of low dimensional
short channel transistors is always lower than the intrinsic channel diffusive
mobility due to the detrimental influence of the so called 'ballistic'
mobility.
