Unipolar p-type monolayer WSe2 field-effect transistors with high current density and low contact resistance enabled by van der Waals contacts

High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 µA·µm⁻¹) and high contact resistance (larger than 100 kΩ·µm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe₂ grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe₂ FETs with superior device performance: room temperature on-state current density exceeding 100 µA·µm⁻¹, contact resistance of 12 kΩ·µm, on/off ratio over 10⁷, and field-effect hole mobility of ~ 103 cm²·V⁻¹·s⁻¹. Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe₂ with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.