MOSFET Calculator

MOSFET Operating Regions

A MOSFET (metal-oxide-semiconductor field-effect transistor) controls drain current with gate voltage. Enhancement-mode n-channel devices have three main regions: cut-off, triode (linear), and saturation.

Drain Current Formulas

Cut-off (device off):

$$I_D = 0 \quad \text{when } V_{gs} < V_T$$

Triode (linear) region:

$$I_D = 2K\left(V_{gs} - \frac{V_{ds}}{2} - V_T\right) V_{ds}$$

Saturation region:

$$I_D = K\left(V_{gs} - V_T\right)^2$$

$K$ combines channel geometry and mobility: $K = \frac{1}{2}\frac{W}{L}\mu_N C_{ox}$. $V_T$ is the threshold voltage.

Choosing the Region

When $V_{gs} \geq V_T$ and $V_{ds} < V_{gs} - V_T$, the device is in the triode region and acts like a voltage-controlled resistor. When $V_{ds} \geq V_{gs} - V_T$, current saturates and depends mainly on gate voltage.

Frequently Asked Questions

What is the K parameter?

K is a device constant combining channel width-to-length ratio, electron mobility, and oxide capacitance. Datasheets often let you derive it from saturation current measurements.

When is a MOSFET in saturation?

Saturation occurs when Vds is at least Vgs minus the threshold voltage. In this region, drain current is nearly independent of Vds.

Why is triode region called linear?

Drain current increases roughly linearly with Vds, similar to an ohmic resistor whose resistance is set by gate voltage.

Can drain current be negative?

This calculator assumes conventional positive current from drain to source in an n-channel device. Reverse bias would require different sign conventions.

How do I find K from a datasheet?

Measure Id in saturation at known Vgs and VT, then rearrange Id = K(Vgs - VT)² to solve for K.

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MOSFET Operating Regions

Drain Current Formulas

Choosing the Region

Frequently Asked Questions

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