  We report a new class of large-gap quantum spin Hall insulators in
two-dimensional transition metal dichalcogenides, namely, MX$_2$ with M=(Mo, W)
and X=(S, Se, and Te), whose topological electronic properties are highly
tunable by external electric field. We propose a novel topological field effect
transistor made of these atomic layer materials and their van der Waals
heterostructures. Our device exhibits parametrically enhanced charge-spin
conductance through topologically protected transport channels, and can be
rapidly switched off by electric field through topological phase transition
instead of carrier depletion. Our work provides a practical material platform
and device architecture for topological quantum electronics.
