Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to one-dimensional materials, it is relatively easy to fabricate complex structures from them. The most widely studied two-dimensional material is graphene 1, 2, both because of its rich physics 3, 4, 5 and its high mobility 6. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors 7. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained silicon films 8, 9, 10, 11, 12, 13 or requires high voltages 14, 15. Although single layers of MoS 2 have a large intrinsic bandgap of 1.8 eV (ref. 16), previously reported mobilities in the 0.5–3 cm 2 V− 1 s− 1 range 17 are too low for practical devices. Here, we use a halfnium oxide gate dielectric to demonstrate a room-temperature single-layer MoS 2 mobility of at least 200 cm 2 V− 1 s− 1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temperature current on/off ratios of 1× 10 8 and ultralow standby power dissipation. Because monolayer MoS 2 has a direct bandgap 16, 18, it can be used to construct interband tunnel FETs 19, which offer lower power consumption than classical transistors. Monolayer MoS 2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.