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Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices1,3. However, owing to metal-induced gap states (MIGS)4-7, energy barriers at the metal-semiconductor interface-which fundamentally lead to high contact resistance and poor current-delivery capability-have constrained the improvement of two-dimensional semiconductor transistors so far2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS2; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS2, WS2 and WSe2. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law.
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The electrical contact resistance at a metal-semiconductor interface has been an increasingly critical, yet unsolved, issue for the semiconductor industry, hindering the ultimate scaling and the performance of electronic devices9. The main cause of this resistance is the energy barrier-the Schottky barrier-formed between the metal electrode and semiconductor8, owing to (I) the energy difference between the metal work function and the semiconductor electron affinity, and (II) MIGS, resulting in Fermi-level pinning4-7. When a semiconductor is in close proximity to a metal surface, the extended wavefunction from the metal perturbs the environment of the semiconductor, leading to rehybridizations of the semiconductor's original wavefunctions. MIGS are a result of such perturbation, where new states in resonance with the metal states emerge in the bandgap (Fig. 1a), as compared to the original density of states (DOS) of the semiconductor (such as MoS2) before contact (Fig. 1c). Resembling the contours of metal DOS after the band alignment, the density of the MIGS is contributed by the valence...