  We present a quasi-analytical model for Tunnel Field Effect Transistors
(TFETs) that includes the microscopic physics and chemistry of interfaces and
non-idealities. The ballistic band-to-band tunneling current is calculated by
modifying the well known Simmons equation for oxide tunneling, where we
integrate the Wentzel-Kramers-Brillouin (WKB) tunneling current over the
transverse modes. We extend the Simmons equation to finite temperature and
non-rectangular barriers using a two-band model for the channel material and an
analytical channel potential profile obtained from Poisson's equation. The
two-band model is parametrized first principles by calibrating with hybrid
Density Functional Theory calculations, and extended to random alloys with a
band unfolding technique. Our quasi-analytical model shows quantitative
agreement with ballistic quantum transport calculations. On top of the
ballistic tunnel current we incorporate higher order processes arising at
junctions coupling the bands, specifically interface trap-assisted tunneling
and Auger generation processes. Our results suggest that both processes
significantly impact the off-state characteristics of the TFETs - Auger in
particular being present even for perfect interfaces. We show that our
microscopic model can be used to quantify the TFET performance on the atomistic
interface quality. Finally, we use our simulations to quantify circuit level
metrics such as energy consumption.
