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 sourced-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. (C) 2013 AIP Publishing LLC.