How this calculator works
The ideal long-channel enhancement nMOS model has cut-off below threshold, a triode region when drain voltage is below gate overdrive, and saturation above that boundary. Geometry and oxide capacitance determine the square-law coefficient. Channel-length modulation, body effect, mobility degradation, subthreshold leakage and short-channel behaviour are excluded. This is a teaching model, not a power-MOSFET thermal or switching model.
Formula
K = mu Coxide W/(2L); Vov = Vgs-Vt. Cut-off: Id = 0. Triode: Id = K(2 Vov Vds - Vds²). Saturation: Id = K Vov².
Worked example
With K = 0.001 A/V², threshold 1 V and gate voltage 3 V, a 1 V drain voltage gives 3 mA in triode. Raising drain voltage to 3 V gives 4 mA in saturation.
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Frequently asked questions
Why does another reference use K/2?
Some define K without the one-half factor. This calculator includes it in K, so compare definitions before entering a value.
Does saturation mean the device is fully on?
In MOSFET analogue terminology saturation is the approximately current-source region. Low-loss switching usually uses low drain voltage in the triode region.
Is the resistance the datasheet RDS(on)?
No. It is Vds/Id at this modelled operating point. Datasheet on-resistance is specified at particular gate drive, temperature and current.