Performances of Strained Nanowire Devices: Ballistic Versus Scattering-Limited Currents

We discuss the performances of (001) and (110) oriented gate-all-around silicon nanowire (Si NW) transistors within a nonequilibrium Green´s functions framework, taking surface roughness and phonon scatterings into account. We show, in agreement with previous studies, that uniaxial tensile (respectively, compressive) strains can significantly improve the mobility of electrons (respectively, holes) in the channel. This does not, however, necessarily result in a comparable enhancement of the device performances. Indeed, the current in short channels is limited by both the scattering and the number of sub-bands available for carrier transport in quantum confined systems (intrinsic “ballistic” resistance). The dependence of the mobility and ballistic resistance on strains can be different, which calls for a careful design of the devices. We show, in this respect, that (110) Si NWs provide the best opportunities for strain engineering in ultimate short channel transistors.