Lesson 3.2 · Module 3: Semiconductors & Active Switching

MOSFET as a Switch: Gate Voltage & Heat

A MOSFET used as an electronic switch, so that a small microcontroller pin can turn a big load (a motor or LED strip) on and off.

Think of it like this: A MOSFET is like a tap: a light touch on the handle (the gate voltage) controls a big flow of water.

The live, interactive simulation for this lesson opens here when JavaScript is on.

Key ideas

A MOSFET turns on when its gate voltage is high enough above its source.
Half-on is bad: the MOSFET then acts like a resistor and gets very hot.
“Logic-level” MOSFETs switch fully on from a 3.3 V pin; standard ones need about 10 V.

Try this in the simulation

With a standard MOSFET and 3.3 V gate drive, watch it overheat. Then pick a logic-level part.
Lower the duty cycle to dim the load with PWM.
Hover any wire or part in the circuit to see its live voltage and current. Click to pin the readout.

How is a MOSFET used as a switch?

A MOSFET is a switch controlled by voltage. When the gate-to-source voltage is above the threshold, a channel forms and the MOSFET conducts between drain and source with a very small on-resistance. A logic-level MOSFET switches fully on from a 3.3 V microcontroller pin, so a tiny signal can control a motor or LED strip. If the gate voltage is too low the MOSFET is only half on, acts like a resistor and gets hot.

What you’ll learn

What the gate, drain and source do.
What the gate threshold voltage means, and why a logic-level part matters.
How on-resistance turns into heat, P = I² × R_DS(on).
How PWM dims a load and what extra heat switching adds.

The formula, letter by letter

\begin{aligned}\htmlClass{mv mv-i_d}{\htmlStyle{font-size:1.00em}{\htmlData{sym=I_D}{I_{\mathrm{D}}}}} &= \htmlData{sym=k}{k}\left[(\htmlClass{mv mv-v_g}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_GS}{U_{\mathrm{GS}}}}}-\htmlData{sym=U_th}{U_{\mathrm{th}}})\htmlClass{mv mv-v_ds}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_DS}{U_{\mathrm{DS}}}}} - \tfrac{\htmlData{sym=U_DS}{U_{\mathrm{DS}}}^2}{2}\right]\quad \text{(ohmic)}\\ \htmlData{sym=P}{P} &= \htmlClass{mv mv-v_ds}{\htmlStyle{font-size:1.00em}{\htmlData{sym=U_DS}{U_{\mathrm{DS}}}}}\cdot \htmlClass{mv mv-i_d}{\htmlStyle{font-size:1.00em}{\htmlData{sym=I_D}{I_{\mathrm{D}}}}} = \htmlData{funit=W,v=3.260086304304187e-35}{\text{0.00 W}},\quad \htmlData{sym=U_th}{U_{\mathrm{th}}}=3\,\htmlData{funit=V}{\text{V}},\; \htmlData{sym=Q_g}{Q_{\mathrm{g}}}=\htmlData{funit=nC,v=7.1e-8}{\text{71.0 nC}}\end{aligned}

Shown with this lesson’s starting values. In the simulation the numbers change as you move the sliders.

$I_{\mathrm{D}}$: Drain current (A · ampere) : The current through a MOSFET from drain to source.
$k$: Transistor strength k (A/V²) : How much current a MOSFET passes for a given gate overdrive. Bigger k means a stronger transistor.
$U_{\mathrm{GS}}$: Gate-source voltage (V · volt) : The voltage on a MOSFET’s gate, measured from its source. It is what turns the MOSFET on. Datasheets usually write it V_GS.
$U_{\mathrm{th}}$: Threshold voltage (V · volt) : The gate voltage where a MOSFET just starts to turn on. Datasheets usually write it V_th.
$U_{\mathrm{DS}}$: Drain-source voltage (V · volt) : The voltage across a MOSFET’s switch path, from drain to source. Datasheets usually write it V_DS.
$P$: Power (W · watt) : How fast energy is used or turned into heat.
$Q_{\mathrm{g}}$: Gate charge (C · coulomb) : How much charge must be pushed into a MOSFET gate to turn it on.

Common questions

What is a logic-level MOSFET?

A MOSFET that turns fully on with a gate voltage of about 3.3 V or 5 V, the levels a microcontroller can supply. Standard MOSFETs need roughly 10 V, so on a 3.3 V pin they stay half on and overheat.

Why does my MOSFET get hot?

Either the gate voltage is too low, so the on-resistance is high, or the current is large and I² × R_DS(on) adds up, or it is switching so fast and so often that the transitions waste energy.

What is the difference between a MOSFET and a BJT?

A MOSFET is controlled by gate voltage and draws almost no gate current. A bipolar junction transistor (BJT) is controlled by a base current and needs a resistor to set it. MOSFETs are usually the better switch for larger loads.