MOSFET Calculator: Operating Region, Drain Current, Transconductance and Output Resistance

Square-law model of an n-channel enhancement MOSFET.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the gate-source voltage, the threshold voltage and the drain-source voltage.

Enter the process transconductance, the width to length ratio and lambda.

The calculator gives the region, the drain current and the small-signal parameters.

Formulas used

  • I_D = \frac{k_n\prime}{2}\frac{W}{L}V_{ov}^2(1 + \lambda V_{DS})
  • I_D = k_n\prime\frac{W}{L}\left[V_{ov}V_{DS} - \frac{V_{DS}^2}{2}\right]
  • g_m = k_n\prime\frac{W}{L}V_{ov}

Worked example

With the default numbers in the calculator above:

  1. Overdrive voltage: 2 V
  2. Region of operation: Saturation (the amplifier region)
  3. Drain current: 2 mA
  4. Transconductance: 2 mS

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Drain-source voltage 1 V.

  1. Overdrive voltage: 2 V
  2. Region of operation: Triode (linear) region
  3. Drain current: 1.5 mA
  4. Resistance of the channel for small drain voltage: 500 ohms
  5. Transconductance: 1 mS

Common questions

How do I tell the region of a MOSFET?

If the gate-source voltage is below the threshold it is off (cut-off). Otherwise compare the drain-source voltage with the overdrive voltage: below it the device is in triode, above it in saturation.

What is the overdrive voltage?

The gate-source voltage minus the threshold voltage.

Which region is used for amplifiers?

Saturation. In triode the device acts like a voltage-controlled resistor, which is how it works as a closed switch.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any microelectronics text (the MOSFET square-law model).

Need a calculator we do not have?

Tell us what you would like to calculate. We read every request and build the most asked-for ones.

We store your request (and your email if you give it) to decide what to build. We do not store the numbers you type into calculators.

Embed this calculator on your own page

Copy this code into your page. The calculator runs inside the frame and links back to Drafting Central.

Need this drawn, designed or checked?

Drafting Central is a mechanical engineering and drafting business working with customers across Australia and around the world. A calculator gives you the numbers; we can turn them into drawings and designs.

Need help with a real project?

Talk to Drafting Central →

← All engineering calculators