  Atomistic quantum transport simulation of realistically large devices is
computationally very demanding. The widely used mode space (MS) approach can
significantly reduce the numerical cost but good MS basis is usually very hard
to obtain for atomistic full-band models. In this work, a robust and parallel
algorithm is developed to optimize the MS basis for atomistic nanowires. This
enables tight binding non-equilibrium Green's function (NEGF) simulation of
nanowire MOSFET with realistic cross section of $\rm 10nm\times10nm$ using a
small computer cluster. This approach is applied to compare the performance of
InGaAs and Si nanowire nMOSFETs with various channel lengths and cross
sections. Simulation results with full-band accuracy indicate that InGaAs
nanowire nMOSFETs have no drive current advantage over their Si counterparts
for cross sections up to about $\rm 10nm\times10nm$.
