Trap assisted tunneling and its effect on subthreshold swing of tunnel field effect transistors

TL;DR

This paper investigates how interface trap assisted tunneling (TAT) significantly impacts leakage currents and subthreshold swing in tunnel FETs, revealing the need to reduce trap densities to observe steep turn-on behavior at room temperature.

Contribution

It provides a detailed analysis of TAT's role in tunnel FETs using a modified Shockley-Read-Hall model, highlighting the impact of trap density and electric field on device performance.

Findings

TAT dominates over BTBT at room temperature in typical trap densities.

Reducing trap density by 40-100 times is necessary to observe steep turn-on.

Surface trap density and Urbach tail influence subthreshold swing.

Abstract

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.