NMOS Drain Current Calculator

Example Data Table

Formula Used

Body adjusted threshold:

VTH = VTH0 + α(T − TREF) + γ(√(2φF + VSB) − √(2φF))

Overdrive voltage:

VOV = VGS − VTH

Cutoff:

If VOV ≤ 0, then ID = 0. Weak inversion may be estimated when enabled.

Linear / triode region:

If 0 < VDS < VOV, then ID = β[(VOV × VDS) − VDS²/2](1 + λVDS)

Saturation region:

If VDS ≥ VOV, then ID = 0.5βVOV²(1 + λVDS)

Computed beta:

β = μnCox × W/L

How to Use This Calculator

1. Enter VGS, VDS, and VSB for the NMOS bias point.
2. Enter threshold voltage, body effect coefficient, and surface potential.
3. Choose direct beta or compute beta from μnCox and W/L.
4. Add channel length modulation if your model includes λ.
5. Set temperature values when threshold drift matters.
6. Enable weak inversion only for cutoff leakage estimates.
7. Press Calculate to view current above the form.
8. Use CSV or PDF export to save the result.

Understanding NMOS Drain Current

An NMOS transistor controls drain current with gate voltage, drain voltage, threshold voltage, and device strength. The calculator separates the operating point into cutoff, linear, or saturation regions. This matters because each region uses a different equation. A small error in region choice can change the current result by a large amount.

Why This Calculator Helps

Manual NMOS work often involves repeated checks. First, the body adjusted threshold is found. Next, the overdrive voltage is compared with the drain source voltage. Then, the correct current expression is applied. This page handles those steps and also reports power, transconductance, and output resistance when possible.

Important Input Choices

Use direct beta when a model card already gives device gain. Use computed beta when mobility capacitance and width length ratio are known. The body effect inputs are useful when the source is not tied to the body. Channel length modulation improves saturation estimates by adding finite output resistance.

Practical Design Notes

The result is a square law estimate. It is helpful for learning, bias planning, and quick hand checks. Real devices can differ because of velocity saturation, short channel effects, leakage, temperature drift, and process variation. For final silicon or board work, compare this result with a simulator and a device data sheet.

Treat the answer as a bias estimate, not a foundry model. It is best for long channel learning cases, homework checks, and first pass analog design reviews before detailed simulations are available locally.

Using The Results

A saturation result usually suggests current source behavior. A linear result suggests resistor like behavior. A cutoff result means the gate overdrive is not positive. If subthreshold mode is enabled, the tool estimates weak inversion current instead of forcing zero current.

Good Measurement Practice

Keep units consistent before entering values. Enter beta in mA per volt squared if using the direct option. Enter mobility capacitance in microamps per volt squared when using the computed option. Check that VSB is physically valid and that the square root body expression stays positive.

Export And Review

The CSV export stores the main numeric results. The PDF export creates a simple summary for notes or reports. Save both when comparing several transistor bias points.

FAQs

What is NMOS drain current?

It is the current flowing from drain to source in an NMOS device. It depends on gate bias, drain bias, threshold voltage, device geometry, and operating region.

What is the overdrive voltage?

Overdrive voltage is VGS minus the effective threshold voltage. Positive overdrive usually turns the device on and controls the square law current estimate.

When is an NMOS in saturation?

In this model, saturation occurs when VDS is greater than or equal to VOV. The drain current mainly depends on VOV, with modulation added by λ.

When is an NMOS in the linear region?

The linear region occurs when VOV is positive and VDS is less than VOV. The device behaves more like a voltage controlled resistor.

What does body effect change?

Body effect changes the threshold voltage when source and body are not at the same potential. A higher VSB usually increases threshold and reduces drain current.

What is beta in this calculator?

Beta is the device gain factor. You can enter it directly, or calculate it from process transconductance parameter μnCox and the W/L ratio.

What does channel length modulation do?

Channel length modulation increases current with VDS in saturation. It also creates a finite output resistance instead of an ideal current source.

Is this accurate for all MOSFETs?

No. It is a square law estimate. Short channel devices, power MOSFETs, and real processes may need manufacturer models or circuit simulation.