  The topological quantum field-effect transition in buckled 2D-Xenes can
potentially be engineered to enable sub-thermionic transistor operation coupled
with dissipationless ON-state conduction. Substantive device design strategies
to harness this will necessitate delving into the physics of the quantum field
effect transition between the dissipationless topological phase and the band
insulator phase. Investigating workable device structures, we uncover
fundamental sub-threshold limits posed by the gating mechanism that effectuates
such a transition, thereby emphasizing the need for innovations on materials
and device structures. Detailing the complex band translation physics related
to the quantum spin Hall effect phase transition, it is shown that a gating
strategy to beat the thermionic limit can be engineered at the cost of
sacrificing the dissipationless ON-state conduction. It is then demonstrated
that an out-of-plane antiferromagnetic exchange introduced in the material via
proximity coupling can incite transitions between the quantum spin-valley Hall
and the spin quantum anomalous Hall phase, which can ultimately ensure the
topological robustness of the ON state while surpassing the thermionic limit.
Our work thus underlines the operational criteria for building topological
transistors using quantum materials that can overcome the Boltzmann's tyranny
while preserving the topological robustness.
