What Is a FET Transistor? A Practical Guide to Selecting and Driving MOSFETs

The Direct Answer: What Is a FET Transistor?

A Field Effect Transistor (FET) is a voltage-controlled semiconductor device used to switch or amplify electrical signals. Unlike Bipolar Junction Transistors (BJTs), which require continuous base current to stay on, a FET uses an electric field applied to its Gate terminal to control current flow between the Drain and Source. This means once a FET is switched on, it draws virtually zero steady-state current from your control circuit.

If you are looking at a standard N-Channel Enhancement MOSFET (the most common type in DIY and power electronics), here is how to read its anatomy:

• Pinout: The three legs are the Gate (G), Drain (D), and Source (S). On a standard TO-220 package facing you with the tab up, the pins from left to right are Gate, Drain, Source.
• Schematic Symbol: Look for the broken vertical line between the Drain and Source (indicating 'enhancement mode'—it is normally OFF). The arrow on the Source terminal points inward toward the channel for an N-Channel device, and outward for a P-Channel device.

Because the Gate is insulated from the channel by a microscopic layer of silicon dioxide, the DC input impedance is astronomically high. You are not driving a current load; you are charging a tiny capacitor (the Gate capacitance). This distinction is the root of both the FET's incredible efficiency and its most common failure modes.

The Safe Defaults: FET Part Numbers You Should Keep in Stock

Walking into a semiconductor distributor's catalog can be paralyzing. Here are the four benchmark FETs you should keep in your bench drawer, complete with the ratings that actually matter:

Operation Regions and Biasing for the Job

To select and bias a FET correctly, you must understand its three operating regions. Note that MOSFET terminology is notoriously confusing because 'Saturation' in a FET means something entirely different than it does in a BJT.

How to bias for switching: If you are using the FET as a switch (which is 95% of DIY applications), you want to drive it deep into the Triode region. You do this by applying a Gate-Source voltage (V_GS) that is significantly higher than the threshold voltage (V_GS(th)), ensuring the voltage drop across the Drain-Source (V_DS) remains minimal.

How to select it: Ignore V_GS(th) when calculating power dissipation. V_GS(th) is merely the voltage where the FET begins to conduct a trivial 250µA. Instead, look at the datasheet's R_DS(on) chart. Find the R_DS(on) value specified at your exact available Gate drive voltage (e.g., 4.5V for an Arduino), and use P = I²R to calculate your heat dissipation.

A Complete Application Circuit: Driving a 12V Solenoid

Let's build a reliable driver for a 12V, 500mA water valve solenoid using an ESP32 (3.3V logic). We will use the IRLZ44N because its R_DS(on) is fully characterized at V_GS = 4.5V, meaning 3.3V will still turn it on sufficiently for a 500mA load.

Bill of Materials:

• Q1: IRLZ44N N-Channel MOSFET
• R1 (Gate Resistor): 100Ω (Limits inrush current to the gate capacitance, protecting the ESP32 GPIO)
• R2 (Pull-down Resistor): 10kΩ (Gate to GND, ensures FET stays off during MCU boot)
• D1 (Flyback Diode): 1N4007 (Cathode to 12V, Anode to Drain)

1. Wire the Load: Connect the solenoid's positive terminal to your 12V power supply. Connect the solenoid's negative terminal to the Drain pin of the IRLZ44N.
2. Install the Flyback Diode: Place the 1N4007 diode in reverse bias across the solenoid coil (stripe/cathode pointing toward the 12V positive side). Never skip this; inductive kickback will instantly punch through the FET's oxide layer.
3. Wire the Gate Network: Connect the ESP32 GPIO pin to one leg of the 100Ω resistor. Connect the other leg to the FET's Gate pin.
4. Add the Pull-down: Connect the 10kΩ resistor between the Gate pin and the common Ground. This bleeds off static charge and prevents the solenoid from chattering while the ESP32 boots and its pins are high-impedance.
5. Complete the Circuit: Connect the FET's Source pin to the common Ground (shared between the 12V supply and the ESP32).

Bench War Story: When the Gate Charge Bites Back

Theory is clean; the workbench is not. A few years ago, I was tasked with switching a 10A LED strip array at 20kHz PWM for a high-speed camera lighting rig. I grabbed an IRF520 from the bin and drove it directly from an Arduino Uno's 5V pin.

The Setup: The IRF520 datasheet lists V_GS(th) as 2.0V to 4.0V. I assumed 5V from the Arduino would easily turn it on. I wired it up, uploaded the PWM sketch, and hit reset.

The Numbers: At V_GS = 5V, the IRF520 is barely out of its linear region. Its R_DS(on) at 5V isn't the 0.27Ω listed on the front page (which is spec'd at 10V); it's closer to 0.5Ω. Passing 10A through 0.5Ω yields a 5V drop and dissipates 50W of heat (P = 10² × 0.5). Furthermore, at 20kHz, the Arduino's weak GPIO couldn't source enough current to charge the IRF520's gate capacitance quickly. The FET spent microseconds lingering in the high-resistance linear region on every single PWM edge, adding massive switching losses.

The Outcome: The TO-220 package hit 150°C in about four seconds. The solder on the breadboard wires melted, the FET shifted, shorted the 12V rail directly to ground, and back-fed the Arduino's USB regulator, killing the board.

What Went Wrong & The Fix: I confused threshold voltage with full enhancement voltage. I swapped the IRF520 for an IRLB8721 (a true logic-level FET with an R_DS(on) of 11mΩ at 4.5V) and added a TC4427 dedicated gate driver IC to supply the peak amps needed to slam the gate capacitance in and out of the Miller plateau at 20kHz. The new setup ran at 35°C ambient without a heatsink.

How FETs Fail and How to Test Them With a Multimeter

FETs generally fail in three ways: Gate punch-through (ESD destroys the microscopic silicon dioxide layer, shorting Gate to Source), Thermal runaway (excessive I²R heat melts the silicon die), or Avalanche breakdown (inductive voltage spikes exceed V_DS max, punching a hole through the Drain-Source channel).

When a FET fails, it almost always fails short (Drain to Source), which is why a blown FET often takes your load or power supply down with it. Here is how to test an N-Channel MOSFET using a standard digital multimeter (DMM) in Diode Test mode.

1. Discharge the Gate: Touch your bare finger across the Gate and Source pins to bleed off any stored capacitance. The FET is now fully OFF.
2. Test the Body Diode: Place your DMM's Red lead on the Source and Black lead on the Drain. You should read a standard diode drop (typically 0.4V to 0.6V). Reverse the leads (Red on Drain, Black on Source); the meter should read 'OL' (Open Loop). If it reads short (0.00V) in both directions, the FET is dead.
3. Charge the Gate: Move the Red lead from the Source to the Gate pin, keeping the Black lead on the Source. The DMM's internal battery will charge the gate capacitor, turning the FET ON.
4. Test Conduction: Move the Red lead back to the Drain (Black remains on Source). Because the FET is now turned ON, the meter should read a very low voltage drop (usually 0.01V to 0.05V), indicating the channel is conducting.
5. Verify Turn-Off: Touch your finger across the Gate and Source again to discharge it. Repeat Step 4. The meter should now read 'OL' again, proving the FET can successfully turn off.

Mastering the FET means respecting the gate capacitance and the difference between threshold and full enhancement. Keep a few logic-level IRLZ44Ns and small-signal 2N7000s on hand, always use a pull-down resistor, and never trust a V_GS(th) spec sheet value without checking the R_DS(on) transfer curves. For deeper reading on MOSFET gate charge characteristics and switching losses, refer to the All About Circuits semiconductor textbook and Texas Instruments' MOSFET design resources.