A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled semiconductor device used to switch or amplify electronic signals. Unlike Bipolar Junction Transistors (BJTs) that require continuous base current to stay on, a MOSFET draws virtually zero steady-state gate current. Once the gate capacitance is charged, it holds its state using only an electric field. This makes the MOSFET the undisputed king of modern power switching, motor control, and logic-level interfacing.

If you are asking what is a MOSFET in a practical bench context: it is an electronic valve. You apply a voltage to the Gate to open the channel between the Drain and Source, allowing current to flow. Below, we break down the exact part numbers to keep in your bin, how to read their datasheets without getting tripped up by physics terminology, and how to wire and test them safely.

MOSFET Pinout, Symbols, and Safe Default Part Numbers

Every standard enhancement-mode MOSFET has three terminals: Gate (G), Drain (D), and Source (S). In an N-channel device, current flows from Drain to Source when a positive voltage is applied to the Gate relative to the Source. In a P-channel device, current flows from Source to Drain when the Gate is pulled negative relative to the Source.

For the ubiquitous TO-220 through-hole package, hold the component with the legs pointing down and the metal tab facing away from you. The pins from left to right are Gate, Drain, Source. The metal heatsink tab is almost universally tied internally to the Drain pin.

Pro-Tip: Schematic symbols include an intrinsic "body diode" pointing from Source to Drain on N-channel MOSFETs (and Drain to Source on P-channel). This diode will conduct if you wire the MOSFET backward in a DC circuit, often leading to catastrophic failure if the current exceeds the diode's rating.

When prototyping, you do not need to hunt for obscure silicon. Keep these five safe default part numbers stocked. They cover 95% of hobbyist and bench power-switching needs.

Part Number Type Vds (Max) Id (Max) Rds(on) @ Vgs Package Best Use Case
IRLZ44N N-Ch 55V 47A 0.022Ω @ 5V TO-220 High power, 5V/3.3V MCU logic
IRF520 N-Ch 100V 9.2A 0.27Ω @ 10V TO-220 Standard gate drive (10V+ op-amps)
2N7000 N-Ch 60V 200mA 1.2Ω @ 10V TO-92 Low power signal/LED switching
IRF9540N P-Ch -100V -23A 0.117Ω @ -10V TO-220 High-side P-channel power switching
AO3400 N-Ch 30V 5.7A 0.035Ω @ 4.5V SOT-23 Compact SMD logic-level switching

Operation Regions and Biasing for the Job

Selecting the right MOSFET requires understanding its operation regions. There is a massive terminology trap here that catches many engineers transitioning from BJTs. In a BJT, "saturation" means the switch is fully ON. In a MOSFET, "saturation" means the amplifier region, while the fully ON switch state is called the Linear, Triode, or Ohmic region. According to Electronics Tutorials, confusing these regions on a datasheet will result in a melted component.

Operation Region Bias Condition (N-Ch) Behavior Typical Application
Cutoff Vgs < Vgs(th) Channel is closed. Id = 0. Switch OFF state.
Triode / Ohmic Vgs > Vgs(th) AND Vds < (Vgs - Vgs(th)) Acts as a voltage-controlled resistor. Lowest Rds(on). Switch ON state (Power switching).
Saturation / Active Vgs > Vgs(th) AND Vds ≥ (Vgs - Vgs(th)) Acts as a constant current source. High power dissipation. Amplifiers, linear regulators, electronic loads.

How to Bias and Select for Switching

When using a MOSFET as a switch, you want it deep in the Triode/Ohmic region to minimize heat (I²R losses). To get there, you must look past the Vgs(th) (Gate Threshold Voltage) on the datasheet.

Vgs(th) is merely the voltage where the MOSFET barely starts to conduct (usually defined at a tiny 250µA drain current). If your microcontroller outputs 3.3V and the datasheet lists a Vgs(th) of 3.0V, the MOSFET will turn on, but it will have a massive internal resistance and overheat. You must look at the Static Characteristics table for the Rds(on) test conditions. If Rds(on) is specified at Vgs = 4.5V or Vgs = 10V, you must drive the gate with that exact voltage to achieve the low resistance promised by the manufacturer. For 3.3V or 5V microcontrollers like the ESP32 or Arduino Uno, always select a "Logic-Level" MOSFET (like the IRLZ44N) where Rds(on) is guaranteed at 4.5V.

Complete Application Circuit: 12V DC Motor Switch

Let us build a robust low-side switch to control a 12V, 2A DC motor using an ESP32 (3.3V logic) and an IRLZ44N MOSFET. This circuit includes the necessary protection components that generic tutorials often omit.

Bill of Materials

  • Q1: IRLZ44N N-Channel MOSFET
  • M1: 12V DC Motor (2A nominal)
  • D1: 1N4007 Flyback Diode
  • R1: 100Ω Gate Resistor (1/4W)
  • R2: 10kΩ Pull-down Resistor (1/4W)

Wiring Steps

  1. Source to Ground: Connect the Source pin of Q1 directly to your main system ground (GND).
  2. Pull-down Resistor: Solder R2 (10kΩ) between the Gate and Source pins. This ensures the MOSFET stays firmly OFF if the microcontroller pin goes high-impedance during boot-up or resets.
  3. Gate Resistor: Connect R1 (100Ω) between the ESP32 GPIO pin and the Gate pin. This limits the inrush current required to charge the MOSFET's internal gate capacitance, protecting the microcontroller's GPIO trace from current spikes and preventing high-frequency ringing.
  4. Load Connection: Connect the negative terminal of the motor to the Drain pin of Q1. Connect the positive terminal of the motor to the +12V supply.
  5. Flyback Diode: Place D1 (1N4007) in reverse bias across the motor terminals. The cathode (stripe) must point toward +12V, and the anode toward the Drain. This safely clamps the inductive voltage spike when the MOSFET switches off, preventing avalanche breakdown.
Safety Caveat: If you are switching loads above 30V or currents exceeding 10A, ensure your PCB traces are sized for the current (e.g., 2oz copper, 40 mils wide for 5A) and mount the TO-220 package to an appropriate heatsink using thermal paste and an insulating mica pad if the heatsink is grounded.

Failure Modes and Multimeter Testing

MOSFETs are rugged but have specific Achilles' heels. Understanding how they fail informs how you test them. According to Texas Instruments, the most common failure modes include:

  • Gate Oxide Puncture: The insulating layer between the Gate and the channel is microscopically thin. An ESD strike or exceeding the maximum Vgs (typically ±20V) will punch a hole through the oxide, shorting the Gate to the Source or Drain.
  • Thermal Runaway: Unlike BJTs, MOSFETs have a positive temperature coefficient for Rds(on). As they get hot, their resistance increases, which generates more heat (I²R), leading to a runaway thermal loop if not heatsunk properly.
  • Avalanche Breakdown: Switching off an inductive load (like a motor or relay coil) without a flyback diode causes a massive voltage spike that exceeds the Vds rating, destroying the silicon die.

How to Test an N-Channel MOSFET with a Multimeter

You can verify a MOSFET's health on the bench using a standard digital multimeter (DMM). Set your DMM to Diode Test Mode.

  1. Discharge the Gate: Briefly short the Gate pin to the Source pin with a piece of wire or your finger to bleed off any stored gate charge.
  2. Check the Body Diode (Forward): Place the Red probe on the Source and the Black probe on the Drain. You should read a standard silicon diode drop, typically between 0.45V and 0.65V.
  3. Check the Body Diode (Reverse): Swap the probes (Red on Drain, Black on Source). The meter should read OL (Open Loop), confirming the diode blocks reverse current.
  4. Charge the Gate: Keep the Black probe on the Source. Briefly touch the Red probe to the Gate pin. (Note: Many DMMs output 2V-3V in diode mode. If your MOSFET has a high Vgs(th), this might not fully turn it on. For a definitive test, use a 9V battery: touch the negative terminal to Source and briefly tap the positive terminal to the Gate).
  5. Verify the Channel (Triode State): Move the Red probe back to the Drain (Black remains on Source). The meter should now read a very low voltage drop, typically < 0.1V, indicating the channel is fully enhanced and conducting.
  6. Discharge and Re-verify: Short the Gate to the Source again to turn it off. Measure Drain to Source once more; it should return to OL.

If the MOSFET reads OL in both directions, the internal bond wire has blown open. If it reads near 0V in both directions, the drain-source channel has shorted due to thermal or avalanche failure. If the Gate-to-Source step reads a short or fails to hold a charge, the gate oxide is punctured, and the part belongs in the bin.

For deeper design parameters, including thermal impedance curves and safe operating area (SOA) graphs, always consult the manufacturer's specific datasheet via suppliers like Infineon or Vishay before committing a design to a printed circuit board.