  The quantum anomalous Hall (QAH) effect has recently been realized in thin
films of intrinsic magnetic topological insulators (IMTIs) like MnBi$_2$Te$_4$.
Here we point out that that the QAH gaps of these IMTIs can be optimized, and
that both axion insulator/semimetal and Chern insulator/semimetal transitions
can be driven by electrical gate fields on the $\sim 10$ meV/nm scale. This
effect is described by combining a simplified coupled-Dirac-cone model of
multilayer thin films with Schr{\"o}dinger-Poisson self-consistent-field
equations.
