What is a MOSFET and How to Choose the Right One for Switching

A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a voltage-controlled electronic switch. Unlike a Bipolar Junction Transistor (BJT), which requires a continuous base current to stay on, a MOSFET uses an electric field generated by voltage at the Gate pin to control current flow between the Drain and Source pins. Once the internal gate capacitance is charged, it draws virtually zero steady-state current, making it the undisputed king of power switching, motor control, and high-efficiency DC-DC conversion.

What is a MOSFET? The Workbench Definition

On the workbench, you will almost exclusively deal with Enhancement-mode N-channel MOSFETs for low-side switching. Here is the physical pinout and what you need to know about each terminal:

• Gate (G): The control terminal. It is electrically isolated from the rest of the device by a microscopic layer of silicon dioxide. Because it acts like a capacitor, it draws current only during the microsecond it takes to switch on or off.
• Drain (D): The terminal where conventional current enters the MOSFET from your load. In a low-side switch, the load connects between your positive supply and the Drain.
• Source (S): The terminal where current exits to ground.

The Three Regions of Operation (And When to Use Them)

The biggest trap for makers transitioning from BJTs to MOSFETs is the terminology. In a BJT, 'saturation' means fully ON. In a MOSFET, saturation means the constant-current amplifier region, while the triode (or ohmic) region means fully ON. All About Circuits provides an excellent deep dive into this semiconductor physics quirk.

How to Select and Bias a MOSFET (Decision Tree)

Selecting a MOSFET is not about finding the part with the highest voltage rating; it is about matching the gate drive voltage to your microcontroller and minimizing $R_{DS(on)}$ (Drain-Source On-Resistance). Use this decision path to pick your topology:

A Complete Switching Circuit: Driving a 12V DC Motor

Here is a battle-tested, complete application circuit for driving a 12V brushed DC motor drawing up to 10A using an ESP32 or Arduino 5V GPIO. According to Electronics Tutorials, proper flyback protection is non-negotiable for inductive loads.

Bill of Materials (BOM)

• Q1: IRLZ44N (N-Channel Logic-Level MOSFET, TO-220 package)
• R1 (Gate Series): 100Ω 1/4W Resistor
• R2 (Gate Pulldown): 10kΩ 1/4W Resistor
• D1 (Flyback): 1N5819 Schottky Diode (or 1N4007 if PWM frequency is below 1kHz)
• M1: 12V DC Motor

Wiring Steps

1. Connect the motor's positive terminal to your 12V power supply positive rail.
2. Connect the motor's negative terminal to the Drain (pin 2) of the IRLZ44N.
3. Connect the Source (pin 3) of the IRLZ44N to the system Ground (shared between the 12V supply and the microcontroller).
4. Place the 1N5819 diode in parallel with the motor: the cathode (stripe) goes to the 12V positive rail, and the anode goes to the Drain pin. This clamps the inductive voltage spike when the MOSFET turns off.
5. Connect the 10kΩ pulldown resistor (R2) between the Gate (pin 1) and Ground.
6. Connect the 100Ω gate resistor (R1) between your microcontroller's PWM GPIO pin and the Gate (pin 1).

Thermal Check: The IRLZ44N has an $R_{DS(on)}$ of roughly 22mΩ at $V_{GS}$ = 5V. At 10A, power dissipation is $P = I^2 \times R = 100 \times 0.022 = 2.2W$. A bare TO-220 package dissipates about 1W to 2W in free air before reaching critical junction temperatures. For continuous 10A operation, clip a small $15 \text{ mm}^2$ heatsink onto the tab, or drop the continuous load to 6A.

How MOSFETs Fail and How to Test Them with a Multimeter

MOSFETs rarely die of old age; they die from abuse. The three most common failure modes are:

1. Gate Punch-Through: Exceeding the maximum $V_{GS}$ (usually ±20V). Even a static shock from your finger can blow the microscopic silicon dioxide gate insulator, resulting in a dead short between Gate and Source.
2. Avalanche Breakdown: Exceeding the $V_{DS}$ rating (e.g., hitting a 60V MOSFET with a 70V inductive spike). The device breaks down and usually shorts Drain to Source.
3. Thermal Runaway: Operating in the linear region too long, or switching high currents without a heatsink, causing the silicon to melt and internally short all three pins.

Testing a Suspect N-Channel MOSFET with a DMM

Set your digital multimeter to Diode Test mode. Remove the MOSFET from the circuit before testing.

1. Discharge the Gate: Touch the meter's black probe to the Source and the red probe to the Gate. This shorts any residual gate charge.
2. Check the Body Diode: Place the red probe on the Source and the black probe on the Drain. You should read a forward voltage drop of roughly 0.4V to 0.7V. Reverse the probes (red on Drain, black on Source); the meter should read 'OL' (Open Loop). If it reads 0.00V or beeps continuously in both directions, the MOSFET is shorted and dead.
3. Charge the Gate: Keep the black probe on the Source. Move the red probe to the Gate. The meter's internal battery (usually 3V to 9V) will charge the gate capacitance, turning the MOSFET on.
4. Verify Channel Conduction: Move the red probe back to the Drain (keeping black on Source). The meter should now read a very low voltage drop (often 0.00V to 0.05V) or beep, indicating the channel is fully enhanced. Touch your finger across Gate and Source to discharge it; the reading should instantly snap back to 'OL'. If it does, your MOSFET is healthy.

The 'Safe Default' MOSFET Part Numbers for Your Bin

Stop wasting time calculating marginal specs for one-off hobby builds. Stock these four safe defaults to cover 95% of your workbench needs. Prices reflect typical 2026 distributor rates for single units.

When you need a switch, buy the IRLZ44N for through-hole logic-level projects and the IRF3205 for heavy 12V/24V power distribution. Keep your gate resistors at 100Ω, your pulldowns at 10kΩ, and never forget the flyback diode. Your microcontroller will thank you, and your MOSFETs will stay cool.