  We provide a detailed study of the interface Trap Assisted Tunneling (TAT)
mechanism in tunnel field effect transistors to show how it contributes a major
leakage current path before the Band To Band Tunneling (BTBT) is initiated.
With a modified Shockley-Read-Hall formalism, we show that at room temperature,
the phonon assisted TAT current always dominates and obscures the steep turn ON
of the BTBT current for common densities of traps. Our results are applicable
to top gate, double gate and gate all around structures where the traps are
positioned between the source-channel tunneling region. Since the TAT has
strong dependence on electric field, any effort to increase the BTBT current by
enhancing local electric field also increases the leakage current. Unless the
BTBT current can be increased separately, calculations show that the trap
density Dit has to be decreased by 40-100 times compared with the state of the
art in order for the steep turn ON (for III-V materials) to be clearly
observable at room temperature. We find that the combination of the intrinsic
sharpness of the band edges (Urbach tail) and the surface trap density
determines the subthreshold swing.
