Vertical p-i-n junctions are key components for optoelectronics to achieve fast response speed. However, a critical bottleneck lies in the complex fabrication techniques and the performance tradeoff between high responsivity and fast speed, especially under self-powered mode. Here, we illustrate the superiority of 2D materials-based vertical p-i-n photodiodes with maximized optical absorption in intrinsic layer (high responsivity), the efficient photocarrier separation (self-power ability), and the high-field drift velocity (fast speed). By optimizing the photocarrier generation/transfer dynamics via doping and thickness engineering, our device with zero voltage bias achieves high built-in electric field, leading to a high responsivity of 0.388 A W⁻¹ and an EQE of 90.5% at 532 nm, a short intrinsic response time of sub-10 ps, a fast switching response time of 23 ns, and a high power conversion efficiency of 6.5%. Our work lays the foundation to resolve the responsivity-speed dilemma without the constraint of lattice mismatch.

Two-dimensional (2D) semiconductors have been considered as promising candidates to fabricate ultimately scaled field-effect transistors (FETs), due to the atomically thin thickness and high carrier mobility. However, the performance of FETs based on 2D semiconductors has been limited by extrinsic factors, including high contact resistance, strong interfacial scattering, and unintentional doping. Among these challenges, contact resistance is a dominant issue, and important progress has been made in recent years. In this review, the Schottky–Mott model is introduced to show the ideal Schottky barrier, and we further discuss the contribution of the Fermi-level pinning effect to the high contact resistance in 2D semiconductor devices. In 2D FETs, Fermi-level pinning is attributed to the high-energy metal deposition process, which would damage the lattice of atomically thin 2D semiconductors and induce the pinning of the metal Fermi level. Then, two contact structures and the strategies to fabricate low-contact-resistance short-channel 2D FETs are introduced. Finally, our review provides practical guidelines for the realization of high-performance 2D-semiconductors-based FETs with low contact resistance and discusses the outlook of this field.