Most Foundries provide model that are not accurate for weak (sub-threshold) and moderate inversion, or other ultra-power considerations. How to solve this issue?
Foundries generally gives you BSIM models. In most cases they provide good fitting in weak and strong inversion. Besides, BSIM model is of not much help to the custom analog designer when sizing transistors due to the lack of physical meaning in many of its parameters. You can solve the issue if you know how to extract EKV and/or ACM parameters from such BSIM models to provide an accurate and continuous model from weak to strong inversion. The ACM model, for example, is composed of very simple expressions, valid for any inversion level, conserves charge and preserves the source-drain symmetry of the transistor.
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