Example Data Table
| Method | Key Inputs | Estimated Threshold | Use Case |
|---|---|---|---|
| Saturation | VGS 3.3 V, ID 0.002 A, k 0.004 A/V² | 2.3000 V | Bench current estimate |
| Linear | VGS 2.5 V, ID 0.001 A, k 0.01 A/V², VDS 0.1 V | 1.4500 V | Low drain voltage check |
| Body effect | VTH0 0.7 V, gamma 0.4, phi 0.35 V, VSB 0.5 V | 0.8035 V | Body biased device check |
Formula Used
Saturation model: ID = k / 2 × (VGS - VTH)²
Rearranged: VTH = VGS - √(2ID / k)
Linear model: ID = k × ((VGS - VTH) × VDS - VDS² / 2)
Rearranged: VTH = VGS - (ID / (k × VDS) + VDS / 2)
Body effect: VTH = VTH0 + gamma × (√(2phi + VSB) - √(2phi))
Temperature correction: VTH adjusted = VTH + coefficient × (T - Tref) / 1000
Gate margin: margin = available gate drive - absolute adjusted threshold voltage
How to Use This Calculator
Choose the method that matches your known values.
Use the saturation method when drain current follows square-law behavior.
Use the linear method when VDS is small and known.
Use the body effect method when source-body voltage changes threshold.
Enter all values in volts, amperes, and amperes per square volt.
Set the temperature fields when thermal correction is needed.
Press calculate to show the result above the form.
Use CSV or PDF download for project records.
Understanding Threshold Voltage
Threshold Voltage Basics
Threshold voltage is the gate voltage that starts controlled channel conduction. It is important in switching, biasing, and logic design. A low value can turn a device on early. A high value may require more drive. This calculator gives several useful estimates. It can use measured drain current, a square law model, or a body effect model.
Why This Value Matters
Designers use threshold voltage when choosing transistors for relays, converters, sensors, and microcontroller outputs. A logic pin must provide enough gate voltage. Power loss also depends on how strongly the channel is enhanced. The result is not a full data sheet replacement. It is a planning aid for early design checks.
Measurement Based Estimate
In saturation mode, the drain current model is simple. Current rises with the square of overdrive voltage. Overdrive means gate source voltage minus threshold voltage. If drain current and transconductance parameter are known, threshold voltage can be rearranged. In linear mode, drain voltage also affects the expression. Both modes are included, so bench readings can be reviewed quickly.
Body Effect Estimate
A MOS device may have a source body voltage. This voltage changes the threshold. The body effect equation uses zero bias threshold voltage, body factor, Fermi potential, and source body voltage. It helps with analog circuits, source followers, and substrate biased designs. Use positive source body voltage for standard n channel body effect checks.
Using The Result
Always compare the answer with a real device curve. Data sheets often give threshold voltage at a small drain current. That number does not mean the device is fully on. For power switching, check gate charge and on resistance too. For precision analog work, confirm temperature effects and process spread.
Good Input Practice
Use volts, amperes, and amperes per square volt. Keep units consistent. Enter measured values from the same operating point. If results look unrealistic, check current scaling first. Milliamps and microamps are common mistake sources. The PDF and CSV downloads help keep calculations with project notes.
Advanced Review Notes
Small signal devices, power MOSFETs, and process models use different test conditions. Record temperature, drain voltage, and chosen mode. This makes later reviews clearer and reduces wrong part choices during final hardware checks.
FAQs
What is threshold voltage?
Threshold voltage is the gate-source voltage where a MOS channel begins conducting by a defined test condition. It is not the same as full turn-on voltage.
Can this calculator be used for PMOS devices?
Yes. Use magnitudes for the inputs. Select P channel to show the signed threshold convention as a negative value.
What does k mean in this calculator?
k is the device transconductance parameter. It often represents mobility, oxide capacitance, and geometry terms combined into amperes per square volt.
When should I use the saturation method?
Use it when the transistor is operating in saturation and a square-law model is suitable for your first estimate.
When should I use the linear method?
Use it when drain-source voltage is small, known, and the device is operating in the linear or triode region.
Why does body effect increase threshold voltage?
Source-body bias changes the surface potential needed to form a channel. The equation models that shift with gamma and phi.
Why is my result negative?
A negative result can occur from inconsistent inputs, strong temperature correction, or selected PMOS signed convention. Check current scale and device constants.
Is threshold voltage enough for switching design?
No. Also check on resistance, gate charge, drive voltage, thermal limits, and current rating before selecting a switching transistor.