Unified Analytic Framework for Thickness‐Dependent Transport and Trap‐State Modulation in Ultrathin W:In 2 O 3 Field‐Effect Transistors

3D integration demands ultrathin oxide transistors that combine strong gate control, high mobility, steep subthreshold swing, and normally off operation within back‐end‐of‐line (BEOL) thermal budgets below 400°C. Yet, conventional amorphous oxides lose stability and suffer from disorder‐limited transport below ~10 nm, while crystalline or doped In 2 O 3 , though more robust, remains constrained by trap states, interface dipoles, and surface roughness at 2–3 nm. This work presents a unified analytical framework that quantitatively links band transport, trap‐tail conduction, interface‐trap diffusion, and roughness‐limited scattering in In 2 O 3 ‐based field‐effect transistors (FETs). The model reproduces transfer characteristics from subthreshold to above‐threshold and captures key differences between pristine and W‐doped In 2 O 3 (IWO), including suppressed off‐current, improved subthreshold swing, and mitigation of thickness‐dependent mobility degradation. Using sputtered IWO and pristine channels from ~2 to ~13 nm, the framework introduces two physical descriptors—the volumetric trap density N t and energetic width T t —together with a gate‐ and thickness‐aware mobility law . Extracted flat‐band carrier densities and Fermi‐level shifts reveal that thermal carrier suppression in IWO yields a linear reduction in off‐current with decreasing thickness. DFT and DOS analyses confirm smoother IWO/high‐ interfaces and lower trap densities, enabling predictive design of bias‐stable, normally off oxide transistors for 3D‐integrated BEOL logic.