Valleytronics, based on the valley degree of freedom rather than charge, is a promising candidate for next-generation information devices beyond complementary metal-oxide-semiconductor (CMOS) technology1-4. Although many intriguing valleytronic properties have been explored based on excitonic injection or the non-local response of transverse current schemes at low temperature4-7, demonstrations of valleytronic building blocks similar to transistors in electronics, especially at room temperature, remain elusive. Here, we report a solid-state device that enables a full sequence of generating, propagating, detecting and manipulating valley information at room temperature. Chiral nanocrescent plasmonic antennae8 are used to selectively generate valley-polarized carriers in MoS2 through hot-electron injection under linearly polarized infrared excitation. These long-lived valley-polarized free carriers can be detected in a valley Hall configuration9-11 even without charge current, and can propagate over 18 μm by means of drift. In addition, electrostatic gating allows us to modulate the magnitude of the valley Hall voltage. The electrical valley Hall output could drive the valley manipulation of a cascaded stage, rendering the device able to serve as a transistor free of charge current with pure valleytronic input/output. Our results demonstrate the possibility of encoding and processing information by valley degree of freedom, and provide a universal strategy to study the Berry curvature dipole in quantum materials.