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Two-dimensional materials are an emerging class of new materials with a wide range of electrical properties and potential practical applications. Although graphene is the most well-studied two-dimensional material, single layers of other materials, such as insulating BN (ref. ) and semiconducting MoS₂ (refs , ) or WSe₂ (refs , ), are gaining increasing attention as promising gate insulators and channel materials for field-effect transistors. Because monolayer MoS₂ is a direct-bandgap semiconductor^, due to quantum-mechanical confinement^,,, it could be suitable for applications in optoelectronic devices where the direct bandgap would allow a high absorption coefficient and efficient electron–hole pair generation under photoexcitation. Here, we demonstrate ultrasensitive monolayer MoS₂ phototransistors with improved device mobility and ON current. Our devices show a maximum external photoresponsivity of 880 A …