P-Type Vertical FETs Realized by Using Fermi-Level Pinning-Free 2D Metallic Electrodes

In two-dimensional (2D) nanomaterial electronics, vertical field-effect transistors (VFETs), where charges flow perpendicular to the channel materials, hold promise due to the ease of forming ultrashort channel lengths by utilizing the thinness of 2D materials. However, the poor performance of p-type VFET arises from the lack of a gate-field-penetrating electrode with suitable work functions, which is essential for VFET operation. This motivated us to replace graphene (work function of ∼4.5 eV) with a high-work-function electrode to achieve the desired VFET characteristics. In this study, we demonstrate that WSe₂-based p-type VFETs with a high on/off ratio of ∼10⁵ can be realized using van-der-Waals contacts formed with high-work-function 2D metals (i.e., 2H-TaS₂, NbSe₂, and NbS₂), which form a p-type ohmic contact to the WSe₂ channel by suppressing Fermi-level pinning. Furthermore, we successfully fabricate a 2D metal-incorporating pseudocomplementary FET structure, demonstrating a great potential to significantly reduce the scaling factor by dense structure and vertical operation.