A comprehensive atomistic analysis of bandstructure velocities in si nanowires

A 20 band sp³ d⁵ s* spin-orbit-coupled, semi-empirical, atomistic tight-binding (TB) model is used with a semi-classical, ballistic transport model, to theoretically examine the bandstructure carrier velocity under non-degenerate conditions in silicon nanowire (NW) transistors. Infinitely long, uniform, cylindrical and rectangular NWs, of cross sectional diameters/sides ranging from 3nm to 12nm are considered. For a comprehensive analysis, n-type and p-type NWs in [100], [110] and [111] transport orientations are examined. The carrier velocities of p-type [110] and [111] NWs increase by a factor of ~2X as the NWs´ diameter scales from D=12nm down to D=3nm. The velocity of n-type [110] NWs also increases with diameter scaling by ~50% The velocities of n-type [100], and [111], as well as those of p-type [100] NWs show only minor diameter dependence. This behavior is explained through features in the electronic structure of the silicon host material.