There are lots of published work on sub-threshold operation and models of MOS devices (example: on Journal of Solid-State Circuits, IEEE). For a good and highly-credited one, could you recommend me good sources to study this?
Nguyen: Best regards. Try to gather the following two excellent textbooks that cover the MOSFET subthreshold current modeling in detail:
(1) Operation and Modeling of the MOS Transistor, The Oxford Series in Electrical and Computer Engineering, 3rd edition, Yannis P. Tsividis and Colin C McAndrew
(This is one of the foremost and best resource for your objective)
(2) Fundamentals of Modern VLSI Devices, 2nd edition, Cambridge University Press, Yuan Taur, Tak H. Ning (This is second best resource for your objective).
Additionally I am attaching the following reference articles for your overview. Gathering the ebook pdf format of the above two books should give you the best possible resources along with the first article attached here by C.C. Enz at EPFL. MIT Open Courseware reserve lecture notes are also beneficial, one that I attached here.
If my suggestions are helpful to you, I would request to recommend my answer.
Nguyen: Best regards. I do not know whether you have individual IEEE Xplore access or through your institution, but I am attaching the link to a IEEE Transactions on Electron Devices journal paper by eminent researcher Professor Dr. Michael Shur and Professor Dr. Tor A Fjeldly who extracted the modeling equations in this paper from device structural related electrostatics analysis. Through your institutional subscription to IEEE journals and conferences listed in IEEE Xplore, you should be able to download this extremely important paper relevant to your objective.
The link is: https://ieeexplore.ieee.org/abstract/document/249436
If this information is useful, please recommend my answer.