Nonequilibrium electron-phonon energy relaxation and valley-resolved Joule heating in monolayer MoS₂ MOSFETs

Temperature rise and hotspot formation induced by self-heating severely affect the performance and reliability of two-dimensional (2D) electronic devices. Here, a first-principles-based multiscale framework combining density functional theory, TCAD simulations, and electron Monte Carlo methods is developed to investigate nonequilibrium electron–phonon energy dissipation in monolayer MoS₂ field-effect transistors. A unified scattering model incorporating deformation-potential, polar optical phonon, piezoelectric, and ionized impurity scattering is established entirely from first-principles parameters. Simulations reveal pronounced nonequilibrium transport, where Joule-heating hotspots are spatially displaced from electric-field maxima, indicating nonlocal energy relaxation of hot electrons. Increasing gate voltage drives a transition from single- to dual-hotspot regimes through carrier accumulation and electric-field redistribution. Polar optical phonon emission and intervalley scattering dominate energy dissipation, while multi-degenerate Q valleys provide efficient energy-relaxation pathways at high carrier density. Phonon-mode-resolved analysis further reveals strong nonequilibrium energy partition among phonon branches, with the high-frequency LO2 mode contributing up to 45% of the dissipated energy. These findings establish a unified physical picture linking hotspot evolution, valley scattering, and phonon-mode nonequilibrium, and provide a transferable framework for investigating electron–phonon energy conversion in low-dimensional semiconductor devices.