Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)

1. Definition

1.1 MOSFET

A MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) is an electronic switch or amplifier.
Unlike a regular transistor (BJT), it is controlled by voltage at its gate terminal instead of current.
This makes it very efficient ; it can switch large currents with almost no power used to control it.

It has three main pins:

  • Gate (G): Controls the switch.
  • Drain (D): Current flows out here in N-channel (or into here in P-channel).
  • Source (S): Current flows in here in N-channel (or out here in P-channel).

MOSFET examples

MOSFET Symbols

MOSFET symbols

1.2 N-Channel vs P-Channel MOSFETs

N-Channel MOSFET

  • Symbol: Arrow points in to the source.
  • Operation: Turns on when gate voltage is higher than source.
  • Current flow: Drain → Source.
  • Common use: Low-side switching (connect source to ground).

P-Channel MOSFET

  • Symbol: Arrow points out from the source.
  • Operation: Turns on when gate voltage is lower than source.
  • Current flow: Source → Drain.
  • Common use: High-side switching (connect source to positive supply).

Side-by-side comparison:

Feature N-Channel P-Channel
Arrow direction Into source Out from source
Gate voltage to turn on Higher than source Lower than source
Current flow Drain → Source Source → Drain
Common connection Source to GND Source to +V
Typical use case Switching ground Switching positive supply

2. Features

  • Type: N-channel or P-channel.
  • Logic-level or standard gate drive: Logic-level MOSFETs can be driven directly by microcontrollers (e.g., 5V or 3.3V).
  • RDS(on): Resistance when on , lower is better for efficiency.
  • Max current and voltage ratings.
  • Package: TO-220, TO-92, SMD types.

4. How to Use

Safety Note

MOSFET gates are sensitive to static electricity; always discharge yourself before touching pins.

4.1 Identify the pinout

  • Check the datasheet : pin order varies.
  • Common TO-220 pin order: Gate , Drain , Source.

MOSFET pinout

4.2 Switching example (N-channel)

To control an LED strip from an Arduino:

  1. Connect source to ground.
  2. Connect drain to LED strip negative.
  3. Connect LED strip positive to +12V.
  4. Connect Arduino pin to gate through a 100Ω resistor.
  5. Add a 10kΩ resistor from gate to ground (keeps it off when not driven).

4.3 Switching example (P-channel)

To switch +12V to a device:

  1. Connect source to +12V.
  2. Connect drain to device positive.
  3. Connect device negative to ground.
  4. Pull gate low (below source voltage) to turn on, high to turn off.

4.4 Testing with a multimeter

  • Set meter to diode mode.
  • For N-channel:
    • Check drain to source , should be open when gate is uncharged.
    • Briefly touch gate to source to discharge.
    • Apply small voltage between gate and source , drain-source path becomes conductive.
  • For P-channel: reverse polarities for the same test.

5. Applications

  • Driving motors, LEDs, relays.
  • High-efficiency power supplies.
  • Battery protection circuits.
  • Logic-level signal switching.

6. Video Explanation

video coming soon