Inherent transmission probability limit between valence-band and conduction-band states and calibration of tunnel-FET parasitics

Tunnel-FETs (TFETs) can achieve a subthreshold swing smaller than 60mV/dec and are considered as promising candidates for future low-power technology nodes [1-3]. The carrier injection into the channel of a TFET occurs via band-to-band tunneling (BTBT) between valence- and conduction-band states. Transmission probabilities between these states can be properly modeled using the Wentzel-Kramers-Brillouin (WKB) approximation, provided the changes in electrostatic potential are small compared to the length of a unit cell. However, the tunnel path length in highly optimized TFETs is typically less than 2 nm in the on-state and the WKB approximation is at its limits. In particular, predictions of 100% transmission probability for tunnel path lengths close to /tun = 0 nm are overestimated, since the mismatch between valence- and conduction-band states is not taken into account.