Formula Used
Square-law saturation method:
ID = 0.5 × μCox × (W / L) × (VGS − Vth)²
Vth = VGS − √[2ID / {μCox × (W / L)}]
Body-effect method:
Vth = Vth0 + γ × [√(2φF + VSB) − √(2φF)]
Temperature correction:
Vth(T) = Vth(reference) + coefficient × (T − reference T)
Linear extrapolation:
Slope = (ID2 − ID1) / (VGS2 − VGS1)
Vth = VGS1 − ID1 / Slope
How to Use This Calculator
Select the threshold extraction method first. Use square-law when the MOSFET is operating in saturation. Enter drain current, gate-source voltage, process value, width, and length.
Use body-effect mode when Vth0, γ, φF, and VSB are known. Use linear extrapolation when two measured ID and VGS points are available.
Use the constant-current method when your test rule defines threshold voltage at a fixed drain current. Choose PMOS when you want a signed negative result.
Press the calculate button. The result appears below the header and above the form. Then export the table as CSV or PDF.
Example Data Table
| Method |
VGS |
ID |
μCox |
W/L |
VSB |
Estimated Vth |
| Square-law with body correction |
2.50 V |
1.00 mA |
120 µA/V² |
10 |
0.20 V |
About 1.25 V |
| Body-effect equation |
Not used |
Not used |
Not used |
Not used |
0.20 V |
About 0.75 V |
| Linear extrapolation |
1.20 V and 2.20 V |
0.05 mA and 1.20 mA |
Not used |
Not used |
Not used |
About 1.16 V |
Understanding MOSFET Threshold Voltage
MOSFET threshold voltage is a key device parameter. It tells when a conducting channel begins to form. In simple circuit work, it marks the gate voltage where the transistor starts to turn on. In detailed design, it affects bias current, switching speed, leakage, noise margin, and power loss.
Why Threshold Voltage Matters
A low threshold device turns on with less gate voltage. That helps low voltage logic. It can also increase leakage when the device should be off. A high threshold device usually leaks less. It may need more drive voltage and may switch slower in some designs.
Main Extraction Methods
This calculator supports several practical approaches. The square-law method uses a long-channel saturation model. It works best when channel length modulation and mobility reduction are small. The body-effect equation adjusts threshold when the body and source are not at the same voltage. Linear extrapolation uses two measured points and extends the current curve back to the voltage axis. The constant-current method follows a lab rule. It defines threshold at a selected current level.
Body Bias and Temperature
Body bias changes the surface charge needed to form a channel. For an NMOS device, a positive source-body voltage usually raises threshold. Temperature also changes threshold. Many silicon devices show a negative temperature coefficient. That means threshold often falls as temperature rises. The exact value depends on process, geometry, and bias conditions.
Using Results Carefully
The result is an engineering estimate. Real MOSFET behavior can differ from ideal equations. Short-channel devices need more advanced models. Strong drain fields, mobility loss, series resistance, and subthreshold conduction can shift the extracted value. Use measured data when accuracy matters. Compare more than one method when possible. Check units before relying on any answer. Use this tool for study, design review, reports, and quick lab verification.
FAQs
What is MOSFET threshold voltage?
It is the gate-source voltage where a MOSFET begins forming a conductive channel. It is not a perfect on-off point. It depends on current definition, body bias, temperature, and device process.
Which method should I choose?
Use square-law for ideal saturation estimates. Use body-effect when Vth0 and substrate bias terms are known. Use linear extrapolation for measured ID-VGS data. Use constant-current when your lab standard defines threshold at a fixed current.
Can this calculator handle PMOS devices?
Yes. Enter voltage and current magnitudes. Then choose PMOS. The calculator returns a signed negative threshold value, which matches common PMOS notation.
Why does body bias change Vth?
Body bias changes depletion charge near the channel. More depletion charge usually requires more gate voltage to create inversion. This shift is modeled with γ, φF, and VSB.
What is μCox?
μCox is the process transconductance factor. It combines carrier mobility and oxide capacitance per area. The square-law equation uses it with W/L to estimate channel current.
Does temperature always reduce threshold voltage?
Often it does for silicon MOSFETs. A common estimate is a negative millivolt-per-degree coefficient. Actual behavior depends on technology, geometry, and operating region.
Why is my result negative?
A negative result usually appears when PMOS is selected. It can also happen if entered data is inconsistent, such as very high current for a low gate voltage.
Is square-law accurate for modern MOSFETs?
It is useful for learning and rough estimates. Modern short-channel devices may need SPICE models. Effects like velocity saturation and channel length modulation can change the real threshold extraction.