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MOSFET Threshold Voltage Calculator

Learn how to calculate the most critical parameter of a MOSFET with our MOSFET threshold voltage calculator.

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Overdrive voltage3 V
Drain current225 mA½k(V_GS − V_th)² — note the SQUARE, so a MOSFET is not a linear amplifier
Transconductance g_m150 mSk × overdrive — it rises with current, unlike a bipolar's which rises with current too but far more steeply
Minimum V_DS for saturation3 Vbelow the overdrive voltage the device leaves saturation and behaves as a resistor instead
At twice the overdrive900 mAfour times the current — the square law again
Threshold voltage2 Vfalls by roughly 2 mV per °C, so a MOSFET turns on harder as it warms — the opposite of a bipolar, and part of why MOSFETs parallel well

The formula

I = I_s(e^(V/nV_T) − 1); I_D = ½k(V_GS − V_th)²

Exponential, square law, and why it matters

A diode's current rises exponentially with voltage — about a decade for every 60 mV at room temperature. That steepness is why a diode behaves like a fixed 0.7 V drop over a wide current range, and why driving one from a voltage source is destructive: 60 mV of error is a factor of ten in current.

A MOSFET in saturation follows a square law in the overdrive voltage, which is far gentler. That makes MOSFETs easier to bias and much better at paralleling, helped by their threshold falling with temperature so a hot device turns on harder and shares current — whereas a hot bipolar takes more current and runs hotter still.

The bipolar result worth knowing is that voltage gain depends on the collector current and the load resistor, not on beta. Since beta varies threefold between nominally identical parts and drifts with temperature, that independence is what makes bipolar amplifiers designable at all. The fixed-base-resistor bias shown here sets base current rather than collector current, so it inherits all of beta's variability — which is why real circuits use an emitter resistor and a divider.

The thermal voltage kT/q scales with absolute temperature, so every one of these relations shifts as a device warms. Hard-coding 25 mV is a room-temperature approximation, not a constant.