  Nanoelectronics requires the development of a priori technology evaluation
for materials and device design that takes into account quantum physical
effects and the explicit chemical nature at the atomic scale. Here, we present
a cross-platform quantum transport computation tool. Using first-principles
electronic structure, it allows for flexible and efficient calculations of
materials transport properties and realistic device simulations to extract
current-voltage and transfer characteristics. We apply this computational
method to the calculation of the mean free path in silicon nanowires with
dopant and surface oxygen impurities. The dependence of transport on basis set
is established, with the optimized double zeta polarized basis giving a
reasonable compromise between converged results and efficiency. The
current-voltage characteristics of ultrascaled (3 nm length) nanowire-based
transistors with p-i-p and p-n-p doping profiles are also investigated. It is
found that charge self-consistency affects the device characteristics more
significantly than the choice of the basis set. These devices yield
source-drain tunneling currents in the range of 0.5 nA (p-n-p junction) to 2 nA
(p-i-p junction), implying that junctioned transistor designs at these length
scales would likely fail to keep carriers out of the channel in the off-state.
