  Quantum spin Hall (QSH) insulator with large gap is highly desirable for
potential spintronics application. Here we realize electrically tunable QSH
insulator with large gap in van der Waals heterobilayer of monolayer transition
metal dichalcogenide (TMD) and hexagonal BAs. When the type II band alignment
gets inverted in an electric field, the hybridization by interlayer hopping
between the spin-valley locked valence band edges in TMD and the BAs conduction
band edges leads to a stacking-configuration dependent topological band
inversion. In the non-interacting limit, the double spin degeneracy of BAs
leaves an un-hybridized conduction band inside the gap, so the heterobilayer is
a spin-valley locked metal instead of a QSH insulator. With the Coulomb
interaction accounted in the double-layer geometry, the interaction with the
hybridization induced electric dipole shifts this un-hybridized conduction band
upwards in energy, giving rise to a sizable global QSH gap. Consequently, this
heterobilayer provides a platform for engineering electrically tunable QSH
insulator with sizable band gap. In the long period moir\'e pattern with the
spatial variation of local stacking-configurations, the competition between
Coulomb interaction and interlayer hopping leads to superstructures of QSH
insulators and excitonic insulators.
