Atomistic Approach for Nanoscale Devices at the Scaling Limit and Beyond - Valley Splitting in Si

Band-structure effects on channel carrier density in the ultrathin-body end of the ITRS roadmap silicon (100) n-type metal oxide semiconductor field effect transistors (MOSFETs) are assessed here using a semi-empirical nearest-neighbor sp 3 d 5 s à tight-binding model with spin-orbit interaction. The calculations focus on the body thickness range between 10 and 18 atomic layers ($1:5{2:5 nm). At this range, the standard effective mass approach is limited by its inability to capture the conduction band nonparabolicity effects and the subband splitting. The tight-binding simulations show interesting effects of ground-state subband splitting in this thickness range, and as a result of this, the channel charge density was found to fluctuate by as much as 30%. Additionally, it was observed that strict process tolerance is necessary in this thickness range in order to maintain an acceptable threshold voltage variation.