  Ultra-thin-body (UTB) channel materials of a few nanometers in thickness are
currently considered as candidates for future electronic, thermoelectric, and
optoelectronic applications. Among the features that they possess, which make
them attractive for such applications, their confinement length scale,
transport direction, and confining surface orientation serve as degrees of
freedom for engineering their electronic properties. This work presents a
comprehensive study of hole velocities in p-type UTB films of widths from 15nm
down to 3nm. Various transport and surface orientations are considered. The
atomistic sp3d5s*-spin-orbit-coupled tight-binding model is used for the
electronic structure, and a semiclassical ballistic model for the carrier
velocity calculation. We find that the carrier velocity is a strong function of
orientation and layer thickness. The (110) and (112) surfaces provide the
highest hole velocities, whereas the (100) surfaces the lowest velocities,
almost 30% lower than the best performers. Additionally, up to 35% velocity
enhancements can be achieved as the thickness of the (110) or (112) surfaces is
scaled down to 3nm. This originates from strong increase in the curvature of
the p-type UTB film subbands with confinement, unlike the case of n-type UTB
channels. The velocity behavior directly translates to ballistic on-current
trends, and correlates with trends in experimental mobility measurements.
