MOSFET drain current (I–V characteristics) calculator
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KEY POINTS
Long-channel MOSFET drain current is β[(VGS − Vth)VDS − VDS²/2] in the linear region and β/2·(VGS − Vth)² in saturation, with β = μCoxW/L. Once VDS exceeds VGS − Vth, pinch-off holds current nearly constant, rising slightly with channel length modulation λ. Off-state current is set by the subthreshold swing, whose theoretical limit at room temperature is 60 mV/dec.
Inputs
The long-channel square law. For channel lengths below about 0.1 µm, velocity saturation makes current depend on (VGS − Vth) closer to the first power than the square, and this formula overestimates current. Off-state current is estimated with Vth defined as the gate voltage at which ID = 100 nA × W/L.
ID–VDS characteristics
Subthreshold swing and off-state current
| S | Ioff | Per width |
|---|---|---|
| 60 mV/dec | 4.64×10⁻¹⁵ A | 4.64×10⁻¹⁰ A/µm |
| 70 mV/dec | 7.20×10⁻¹⁴ A | 7.20×10⁻⁹ A/µm |
| 80 mV/dec | 5.62×10⁻¹³ A | 5.62×10⁻⁸ A/µm |
| 90 mV/dec | 2.78×10⁻¹² A | 2.78×10⁻⁷ A/µm |
| 100 mV/dec | 1.00×10⁻¹¹ A | 1.00×10⁻⁶ A/µm |
Linear and saturation regions
When the gate voltage exceeds the threshold, an inversion layer forms in the channel, and at small drain voltages current is roughly proportional to voltage (the linear region). When the drain voltage reaches VGS − Vth, the inversion layer disappears at the drain end (pinch-off), and current stays almost constant as voltage rises further (the saturation region). Analog amplifiers use the saturation region; the on state of digital switches uses the linear region.
Current keeps creeping up in saturation because the pinch-off point moves toward the source and shortens the effective channel; this is channel length modulation (λ). λ grows as channels get shorter, lowering the output resistance and the intrinsic gain gm·ro, one reason analog design gets harder with scaling.
FAQ
Why is the subthreshold swing limited to 60 mV/dec?
Below threshold, current is set by carriers thermally crossing a barrier, which changes by a factor of ten for every kT/q × ln10, about 60 mV at room temperature.
When does the square law stop working?
For channels shorter than about 0.1 µm, velocity saturation makes current scale closer to (VGS − Vth) than to its square, so the square law overestimates current.
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Last updated: October 23, 2026