  Spin-polarized conducting edge currents counterpropagate in quantum spin Hall
(QSH) insulators and are protected against disorder-driven localizations by the
time-reversal symmetry. Using these spin-currents for device applications
require materials having large band gap and fast switchable QSH states. By
means of in-depth first-principles calculations, we demonstrate the large band
gap and fast switchable QSH state in a newly introduced two-dimensional (2D)
material family with 1T$^\prime$-MGe$_2$Z$_4$ (M = Mo or W and Z = P or As).
The thermodynamically stable 1T$^\prime$-MoGe$_2$Z$_4$ monolayers have a large
energy gap around $\sim$237 meV. These materials undergo a phase transition
from a QSH insulator to a trivial insulator with a Rashba-like spin splitting
under the influence of an out-of-plane electric field, demonstrating the
tunability of the band gap and its band topology. Fast topological phase
switching in a large gap 1T$^\prime$-MoGe$_2$Z$_4$ QSH insulators has potential
applications in low-power devices, quantum computation, and quantum
communication.
