This paper investigates the critical Sentaurus role of physics-based model selection and calibration in the simulation of sub-5nm silicon Gate-AllAround Field-Effect Transistors (GAAFETs) using the TCAD environment. As transistor dimensions shrink below 5nm, classical modeling approaches become insufficient due to the increasing influence of quantum confinement, carrier mobility degradation, and short-channel effects. To address these challenges, we explore an optimized combination of physical models including drift-diffusion transport, density-gradient quantum corrections, and advanced mobility degradation models that account for surface roughness, phonon scattering, and high-field saturation. Additionally, essential effects such as band-to-band tunneling, bandgap narrowing, and Fermi–Dirac carrier statistics are integrated into the simulation flow. A robust calibration strategy is also proposed, aligning simulation outputs with experimental benchmarks through iterative parameter tuning. The findings confirm that accurate model selection and parameter calibration are indispensable for realistic and predictive simulation of nextgeneration GAAFET devices, especially at atomic-scale technology nodes.