Field-Effect Transistors with Accumulation-Type Ohmic Contacts*

We report the creation of accumulation-type ohmic contacts between an intrinsic 2D semiconductor and a degenerately doped 2D semiconductor acting as a 2D metal. Our back-gated WSe₂ field-effect transistors (FETs) with bottom-contacts exhibit excellent device performance, including linear output characteristics, a high on/off ratio of 10⁸, and two-terminal extrinsic field-effect mobility approaching the phonon-limited intrinsic mobility. As the temperature decreases, the drain-source current increases while the output characteristics remain linear. Because the back-gate does not electrostatically modulate the intrinsic WSe₂ in the drain/source contact regions due to screening by the bottom-contacts, the observed excellent device performance and ohmic behavior can be attributed to the formation of accumulation-type ohmic contacts that are free of a Schottky barrier. The accumulation-type ohmic contacts are further confirmed by dual-gated measurements, where a positive top-gate voltage is applied to deliberately deplete the holes in WSe₂ at the drain/source contacts.