Trap Assisted Tunneling and Its Effect on Subthreshold Swing of Tunnel FETs

We provide a detailed study of the oxide- semiconductor interface trap assisted tunneling (TAT) mechanism in tunnel FETs 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 regions. 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.