Dirac Surface States and Nonlocal Quantum Tunneling in Topological Semiconductor Mo$_2$SeTe$_3$ for High-Performance Tunnel FETs

A first-principles and device-level study of the quasi-two-dimensional transition-metal chalcogenide Mo$_2$SeTe$_3$ is performed. The material is found to be a weak topological semiconductor with a finite bulk band gap and symmetry-protected Dirac surface states, indicating strong potential for next-generation low-power quantum electronic devices. An SOC-driven band inversion accompanied by an indirect semiconducting gap of approximately 0.75 eV is observed. Topological nontriviality is rigorously confirmed through Wannier charge-center evolution and $\mathbb{Z}_2$ invariant analysis, yielding weak topological indices of $(0;001)$, while iterative Green's-function surface-state calculations corroborate Dirac-cone conducting states traversing the bulk gap on symmetry-preserving surfaces. Mo$_2$SeTe$_3$ additionally exhibits exceptional dynamical and mechanical stability, pronounced optical anisotropy, high dielectric polarizability, broad infrared-to-visible optical absorption, a large static dielectric constant, and substantial birefringence, making it favorable for photonic and optoelectronic applications. Thermoelectric transport analyses further reveal enhanced carrier mobility and a competitive figure of merit under $n$-type doping near room temperature. A dual-source tunnel field-effect transistor (TFET) is implemented via TCAD simulations with nonlocal band-to-band tunneling, yielding subthreshold switching below the thermionic limit, a high ON/OFF current ratio, and enhanced tunneling efficiency driven by SOC-induced orbital hybridization and topologically enhanced interband coupling. The concurrent realization of nontrivial bulk-boundary correspondence, robust transport properties, and steep-slope switching characteristics establishes Mo$_2$SeTe$_3$ as a multifunctional quantum material platform for topological and next-generation energy-efficient nanoelectronic devices.