P‑Type\nVertical FETs Realized by Using Fermi-Level\nPinning-Free 2D Metallic Electrodes

In two-dimensional (2D) nanomaterial electronics, vertical\nfield-effect\ntransistors (VFETs), where charges flow perpendicular to the channel\nmaterials, hold promise due to the ease of forming ultrashort channel\nlengths by utilizing the thinness of 2D materials. However, the poor\nperformance of p-type VFET arises from the lack of a gate-field-penetrating\nelectrode with suitable work functions, which is essential for VFET\noperation. This motivated us to replace graphene (work function of\n∼4.5 eV) with a high-work-function electrode to achieve the\ndesired VFET characteristics. In this study, we demonstrate that WSe<sub>2</sub>-based p-type VFETs with a high on/off ratio of ∼10<sup>5</sup> can be realized using van-der-Waals contacts formed with\nhigh-work-function 2D metals (i.e., 2H-TaS<sub>2</sub>, NbSe<sub>2</sub>, and NbS<sub>2</sub>), which form a p-type ohmic contact to the\nWSe<sub>2</sub> channel by suppressing Fermi-level pinning. Furthermore,\nwe successfully fabricate a 2D metal-incorporating pseudocomplementary\nFET structure, demonstrating a great potential to significantly reduce\nthe scaling factor by dense structure and vertical operation.