  Full-band atomistic quantum transport simulations based on first principles
are employed to assess the potential of band-to-band tunneling FETs (TFETs)
with a 2-D channel material as future electronic circuit components. We
demonstrate that single-layer (SL) transition metal dichalcogenides are not
well suited for TFET applications. There might, however, exist a great variety
of 2-D semiconductors that have not even been exfoliated yet; this paper
pinpoints some of the most promising candidates among them to realize highly
efficient TFETs. SL SnTe, As, TiNBr, and Bi are all found to ideally deliver
ON-currents larger than 100{\mu}A/{\mu}m at 0.5-V supply voltage and 0.1
nA/{\mu}m OFF-current value. We show that going from single to multiple layers
can boost the TFET performance as long as the gain from a narrowing bandgap
exceeds the loss from the deteriorating gate control. Finally, a 2-D van der
Waals heterojunction TFET is revealed to perform almost as well as the best SL
homojunction, paving the way for research in optimal 2-D material combinations.
