An analytical physically based analysis for undoped FinFET devices in the subthreshold and near-threshold regimes has been developed by solving the 3-D Poisson equation, in which the mobile-charge term was included. From this analysis, a subthreshold-swing model has been developed; this model is also based on a new physically based analysis of the conduction path. The subthreshold-swing model has been verified by comparison with 3-D numerical simulations and measured values; a very good agreement with both 3-D numerical simulation and the experimental results was observed.
El Hamid, H. A., Guitart, J. R., Kilchytska, V., Flandre, D., & Iniguez, B. (2007). A 3-D analytical physically based model for the subthreshold swing in undoped trigate FinFETs. IEEE Transactions on Electron Devices, 54(9), 2487-2496. https://doi.org/10.1109/TED.2007.902415 (Original work published 2007)