What is the threshold voltage?

Vth is the minimum gate-source voltage needed to create a conducting channel. Below Vth, the MOSFET is off (cutoff region).

Rds(on) is the drain-source resistance when the MOSFET is fully on (linear region). Lower Rds(on) means lower conduction losses and less heat.

kn = μnCox(W/L)/2 for enhancement-mode MOSFETs. It is often derived from datasheet curves (Id vs Vgs at a known Vds in saturation).

Why does the junction temperature matter?

Exceeding the maximum junction temperature (typically 150-175°C) damages the device. High power dissipation requires heatsinking.

When is a MOSFET in saturation vs linear?

Saturation occurs when Vds ≥ Vgs−Vth (the channel is pinched off). Linear mode is when Vds < Vgs−Vth (the channel is fully formed).

What is transconductance?

gm = ∂Id/∂Vgs measures how effectively the gate voltage controls the drain current. Higher gm means higher gain in amplifier circuits.

MOSFET Calculator

Calculate MOSFET drain current, operating region, power dissipation, transconductance, and thermal estimates for N-channel and P-channel devices.

MOSFET Type

Gate-Source Voltage Vgs (V)

Drain-Source Voltage Vds (V)

Threshold Voltage Vth (V)

Transconductance Parameter kn (A/V²)

Rds(on) (Ω)

Load Resistance (Ω)

Operating Region

Linear (Triode)

Vov = Vgs−Vth = 7.000 V

Drain Current Id

9.0000 A

Id = kn[2(Vov)Vds−Vds²]

Power Dissipation

45.0000 W

Pd = Id × Vds

Conduction Loss

3.5640 W

Pcond = Id² × Rds(on)

Transconductance gm

2.8000 S

gm = 2·kn·(Vgs−Vth)

Load Voltage

90.000 V

VL = Id × RL

Est. Junction Temp

2,815.0 °C

ΔT ≈ 2,790.0°C (Rθja=62°C/W)

Operating Region Indicator

Cutoff

Linear (Triode)

Saturation

Thermal Estimate

2,815°C / 175°C max

MOSFET Operating Regions

Region	Condition	Behavior
Cutoff	Vgs < Vth	Id ≈ 0, switch OFF
Linear (Triode)	Vgs > Vth & Vds < Vgs−Vth	Id = kn[2(Vgs−Vth)Vds − Vds²]
Saturation	Vgs > Vth & Vds ≥ Vgs−Vth	Id = kn(Vgs−Vth)²
Breakdown	Vds > BVdss	Destructive current flow

Planning notes, formulas, and examples

About the MOSFET Calculator

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is one of the core switching and amplification devices in modern electronics. Its operating point depends on the gate-source voltage Vgs relative to the threshold voltage Vth and on the drain-source voltage Vds. That makes it easy to describe, but the real operating region still depends on all three values together. Small changes in gate drive can move the device between cutoff, linear, and saturation.

This calculator determines whether the device is in cutoff, linear (triode), or saturation, then estimates drain current, power dissipation, transconductance, and junction temperature using a simple square-law model. It supports both N-channel and P-channel devices and gives a quick first-pass bias check before you move to full simulation or datasheet validation.

Preset buttons load parameters for common MOSFET families such as the IRF540N, 2N7000, and BS170. The thermal estimate uses a typical junction-to-ambient resistance so you can spot cases where conduction losses are likely to require better layout, lower Rds(on), or a heatsink.

When This Page Helps

MOSFET biasing calculations are easy to get wrong when you are moving between threshold voltage, overdrive, current, and dissipation by hand.

Use this calculator for quick bench estimates, design reviews, and teaching examples where you want the operating region and thermal impact explained from the same set of inputs. It is a fast way to see whether a gate drive is likely to be safe and effective before checking the datasheet curves. That makes it useful for both switching and linear-signal checks.

How to Use the Inputs

Select N-channel or P-channel MOSFET type.
Enter gate-source voltage (Vgs) and drain-source voltage (Vds).
Enter the threshold voltage (Vth) from the datasheet.
Enter the transconductance parameter kn (A/V²) and Rds(on).
Optionally enter a load resistance to compute load voltage.
Read the operating region, drain current, power dissipation, and thermal estimate.

Formula used

Cutoff: Vgs < Vth → Id = 0.
Linear: Vgs > Vth, Vds < (Vgs−Vth) → Id = kn[2(Vgs−Vth)Vds − Vds²].
Saturation: Vgs > Vth, Vds ≥ (Vgs−Vth) → Id = kn(Vgs−Vth)².
gm = 2·kn·(Vgs−Vth). Pd = Id·Vds.

Example Calculation

Result: Linear region, Id = 9 A, Pd = 45 W

Vov = 10−3 = 7 V. Since Vds (5 V) < Vov (7 V), the MOSFET is in the linear region. Id = 0.2[2(7)(5)−25] = 0.2[70−25] = 9 A, so Pd = 9 × 5 = 45 W.

Tips & Best Practices

For switching applications, drive Vgs well above Vth to minimize Rds(on) and conduction losses.
Check the SOA (Safe Operating Area) chart on the datasheet — sustained operation in the linear region produces the most heat.
Gate charge (Qg) determines switching speed and driver requirements — lower Qg means faster switching.
P-channel MOSFETs are useful for high-side switching but typically have higher Rds(on) than equivalent N-channel devices.
Always add a gate resistor (10-100 Ω) to prevent oscillation from parasitic inductance.

Practical Guidance

Treat the calculator as a first-order DC model, not as a substitute for the datasheet curves. It is most useful for checking whether a chosen gate drive is comfortably above threshold, whether the expected dissipation is plausible, and whether the device is likely to spend time in a hot linear region.

Common Pitfalls

Threshold voltage is not the same thing as a recommended drive voltage. Many MOSFETs need a substantially higher Vgs than Vth to reach a low Rds(on), and switching applications also depend on gate charge, switching speed, and safe operating area. If the result is close to the device limits, validate it against the datasheet before treating it as design-ready.

Sources & Methodology

Last updated: January 15, 2025

Frequently Asked Questions

Vth is the minimum gate-source voltage needed to create a conducting channel. Below Vth, the MOSFET is off (cutoff region).