Why does power matter?

P = I D × V DS . This heat must be dissipated. Check the MOSFET’s maximum power rating and thermal resistance. A heatsink may be needed if P exceeds 1–2 W in a TO-220 package.

MOSFET Calculator

I_D = K(V_GS−V_th)²

P = I_D × V_DS

R_DS(on) = V_DS/I_D

MOSFET Calculator

V_GS Gate-Source Voltage

V_th Threshold Voltage

K Transconductance Parameter

A/V² (K = ½μC_oxW/L)

V_DS Drain-Source Voltage

Enter values

MOSFET Calculator Results

Drain Current ID
—
A

Operating Region

—

Power Dissipation

—

Overdrive V_ov

—

R_DS

—

g_m

—

Figure 1: MOSFET Calculator — enter values to calculate the result with step-by-step workings.

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Table of Contents

Fundamentals

How the MOSFET Controls Current
The Three Operating Regions

Worked Examples

Logic-Level Switch (5V Gate)
Power MOSFET in Saturation

Deep Dive

Finding the K Parameter
Power Dissipation and Thermal

Reference

Frequently Asked Questions
Related Calculators

How the MOSFET Controls Current

A MOSFET uses the gate-source voltage (V_GS) to control the drain current (I_D). Below the threshold voltage (V_th), no current flows — the device is in cutoff. Above V_th, current increases with the square of the overdrive voltage (V_GS − V_th). The calculator above determines I_D, the operating region, power dissipation, R_DS, and transconductance g_m from your input parameters.

The MOSFET Threshold Voltage Calculator computes V_th from device physics including the body effect. The MOSFET Gate Resistor Calculator handles the gate drive circuit that controls how fast V_GS transitions between off and on.

The Three Operating Regions

Cutoff: V_GS < V_th → I_D = 0 (switch off)
Linear (triode): V_DS < V_GS−V_th → I_D = K(2V_ovV_DS − V_DS²) (variable resistor)
Saturation: V_DS ≥ V_GS−V_th → I_D = K(V_GS−V_th)² (constant current source)

For switching applications, the MOSFET operates in cutoff (off) and deep linear (fully on, low R_DS(on)). For amplifier circuits, it operates in saturation where I_D depends on V_GS but not on V_DS.

Example: Logic-Level Switch (5V Gate)

V_GS = 5 V, V_th = 2 V, K = 0.01 A/V², V_DS = 0.5 V

V_ov = 5 − 2 = 3 V

V_DS (0.5) < V_ov (3) → Linear region

I_D = 0.01 × (2×3×0.5 − 0.25) = 0.01 × 2.75 = 27.5 mA

R_DS = 0.5 / 0.0275 = 18.2 Ω

Power = 27.5m × 0.5 = 13.75 mW

Example: Power MOSFET in Saturation

V_GS = 10 V, V_th = 4 V, K = 0.05 A/V², V_DS = 12 V

V_ov = 10 − 4 = 6 V

V_DS (12) ≥ V_ov (6) → Saturation

I_D = 0.05 × 36 = 1.8 A

g_m = 2 × 0.05 × 6 = 0.6 S

Power = 1.8 × 12 = 21.6 W — needs a large heatsink. The Transistor Biasing Calculator performs analogous Q-point analysis for BJT amplifiers.

Finding the K Parameter

K = ½μC_oxW/L combines carrier mobility, oxide capacitance, and gate geometry. Datasheets rarely state K directly. Instead, find it from a known operating point: if the datasheet says I_D = 10 A at V_GS = 10 V with V_th = 4 V, then K = I_D/(V_GS−V_th)² = 10/36 = 0.278 A/V².

Power Dissipation and Thermal

P = I_D × V_DS. In the linear region (switching), V_DS is small (millivolts to a few volts) and power is low. In saturation (amplifier), V_DS can be large and power is significant. Check the MOSFET’s maximum power rating and thermal resistance. A TO-220 package without a heatsink handles about 1–2 W. The LM317 Resistor Calculator faces identical thermal design constraints for its linear pass element.

Frequently Asked Questions

What are the MOSFET operating regions?

Cutoff (V_GS < V_th, no current), linear/triode (V_DS < V_ov, acts as variable resistor), and saturation (V_DS ≥ V_ov, constant current source). Switching uses cutoff and linear. Amplifiers use saturation.

What is the K parameter?

K = ½μC_oxW/L. Find it from the datasheet: K = I_D/(V_GS−V_th)² at a known operating point in saturation.

What is transconductance g_m?

g_m = 2K(V_GS−V_th) = 2I_D/V_ov. It measures how much I_D changes per volt of V_GS change. Higher g_m = more gain in amplifiers. The Op-Amp Gain Calculator shows how op-amps achieve much higher effective g_m through internal cascaded stages.

Why does power dissipation matter?

P = I_D × V_DS. This heat must be removed. Exceeding the thermal limit destroys the MOSFET. Check max power rating and use a heatsink if P > 1 W in TO-220. The Emitter Resistor Calculator helps with thermal stability analysis in BJT circuits.

Linear vs saturation for switching?

For switching, you want deep linear region (V_GS well above V_th, V_DS very low). The MOSFET acts as a low-value resistor (R_DS(on)). The Transistor Base Resistor Calculator achieves the BJT equivalent by driving into hard saturation.

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Last updated: March 2026