  The mobility of p-type nanowires (NWs) of diameters of D=12nm down to D=3nm,
in [100], [110], and [111] transport orientations is calculated. An atomistic
tight-binding model is used to calculate the NW electronic structure.
Linearized Boltzmann transport theory is applied, including phonon and surface
roughness scattering (SRS) mechanisms, for the mobility calculation. We find
that large mobility enhancements (of the order of 4X) can be achieved as the
diameter of the [110] and even more that of the [111] NWs scales down to D=3nm.
This enhancement originates from the increase in the dispersion curvatures and
consequently the hole velocities as the diameter is scaled. This benefit over
compensates the mobility reduction caused by SRS as the diameter reduces. The
mobility of the [100] NWs, on the other hand, is the lowest compared the other
two NW orientations, and additionally suffers as the diameter scales. The
bandstructure engineering techniques we describe are a generic feature of
anisotropic bulk bands, and can be also applied to 2D thin-body-layers as well
as other channel materials.
