A tight-binding study of the ballistic injection velocity for ultrathin-body SOI MOSFETs

This paper examines the validity of the widely used parabolic effective mass approximation by computing the ballistic injection velocity of a double-gate, ultrathin-body (UTB) n-MOSFET. The energy dispersion relations for a Si UTB are first computed by using a 20-band sp(3)d(5) s*-SO semiempirical atomistic tight-binding (TB) model coupled with a self-consistent Poisson solver. A semiclassical ballistic FET model is then used to evaluate the ballistic injection velocity of the n-type UTB MOSFET based on both an TB dispersion relation and parabolic energy bands. In comparison with the TB approach, the parabolic band model with bulk effective masses is found to be reasonably accurate as a first-order approximation until down to about 3 nm, where the ballistic injection velocity is significantly overestimated. Such significant nonparabolicity effects on ballistic injection velocity are observed for various surface/transport orientations. Meanwhile, the injection velocity shows strong dependence on the device structure as the thickness of the UTB changes. Finally, the injection velocity is found to have the same trend as mobility for different surface/transport orientations, indicating a correlation between them.