  We investigate the effect of a vertical electric field on a Dirac semimetal
thin film. We show that through the interplay between the quantum confinement
effect and the field-induced coupling between sub-bands, the sub-band gap can
be tuned and inverted, during which the system undergoes a topological phase
transition between a trivial band insulator and a quantum spin Hall insulator.
Consequently, one can electrically switch the topological edge channels on and
off, making the system a promising platform for constructing a topological
field effect transistor.
