What is a MOSFET in Electronics: Selection, Testing, and Default Part Numbers

The Short Answer: What is a MOSFET in Electronics?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled semiconductor switch used to route power or amplify signals. Unlike Bipolar Junction Transistors (BJTs) that require continuous base current to stay on, a MOSFET requires almost zero steady-state gate current. You apply a voltage to the Gate pin to create an electric field across a silicon dioxide insulator, which forms a conductive channel between the Drain and Source pins. Once the channel is formed, current flows. Because the gate is physically insulated, the input impedance is practically infinite (often >10^9 ohms), making MOSFETs the undisputed standard for microcontroller-driven power switching and high-efficiency power supplies.

Pinout and Symbol Identification

Every standard discrete MOSFET has three terminals: Gate (G), Drain (D), and Source (S). For an N-Channel MOSFET (the most common type for low-side switching), the schematic symbol features a vertical channel line with the Drain at the top and Source at the bottom. The Source pin has an arrow pointing inward toward the channel. The Gate pin is drawn as a separate line parallel to the channel, disconnected by a small gap representing the metal-oxide dielectric layer. In physical TO-220 packages (facing you with the tab on the back and pins pointing down), the standard pinout from left to right is Gate, Drain, Source. Always verify this against the specific manufacturer's datasheet, as SMD packages like SOT-23 vary wildly.

MOSFET Operation Regions and Biasing

To select and bias a MOSFET correctly, you must understand its three operating regions. The behavior of the transistor changes entirely depending on the relationship between Gate-to-Source voltage (Vgs), Drain-to-Source voltage (Vds), and the threshold voltage (Vth).

Biasing for Switching: When using a MOSFET as a switch (which is 90% of DIY and embedded projects), you only care about Cutoff and the extreme end of the Linear region (fully enhanced). You want to drive Vgs well above the threshold voltage (Vth) to minimize Rds(on) — the on-state resistance. If a datasheet lists Vth as 2.0V to 4.0V, applying exactly 3.3V might leave it partially on, causing massive heat dissipation. Always look at the 'Rds(on) vs Vgs' graph in the datasheet, not just the Vth spec.

The Bench Test: How MOSFETs Fail and How to Test Them

MOSFETs typically fail in three ways: Gate punch-through (exceeding the +/- 20V Vgs max limit, puncturing the oxide layer), Thermal runaway (insufficient heat sinking causing Rds(on) to rise, which creates more heat until the silicon melts), and Avalanche breakdown (inductive voltage spikes exceeding the Vds rating). When they fail, they almost always fail 'short' — Drain and Source become a dead short, destroying the load or the power supply.

Testing an N-Channel MOSFET with a Multimeter

You do not need a semiconductor curve tracer to verify a MOSFET. A standard digital multimeter (DMM) in Diode Test mode is sufficient to check the internal body diode and the gate capacitance.

1. Discharge the Gate: Touch the Gate and Source pins together with your finger or a piece of wire to bleed off any stored static charge.
2. Check the Body Diode (Forward): Set your DMM to Diode Test mode. 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. This is the intrinsic body diode conducting.
3. Check the Body Diode (Reverse): Swap the probes (Red on Drain, Black on Source). The meter should read 'OL' (Open Loop). If it reads near 0.00V or buzzes, the Drain-Source channel is shorted. The MOSFET is dead.
4. Charge the Gate: Keep the Black probe on the Source. Briefly touch the Red probe to the Gate pin. This uses the multimeter's internal 3V battery to charge the gate capacitance, turning the MOSFET on.
5. Verify the Channel: Move the Red probe back to the Drain (Black still on Source). The meter should now read near 0.00V (or buzz), indicating the channel is fully enhanced and conducting.
6. Discharge and Re-verify: Touch Gate to Source again to discharge. Re-test Drain to Source; it should revert to 'OL'. If it stays shorted after discharging, the gate oxide is punctured.

Application Circuit: Driving a 12V DC Load from a 3.3V MCU

A classic beginner mistake is trying to drive a 12V, 5A DC motor or LED strip using an IRF520 or IRFZ44N directly from an ESP32 or Arduino. Those are standard-level MOSFETs requiring 10V on the gate to fully turn on. At 3.3V, they operate in the linear region, acting as expensive heaters and dropping massive voltage. For 3.3V or 5V logic, you must use a Logic-Level MOSFET, denoted by an 'L' in the part number (e.g., IRLZ44N).

Circuit Schematic and Component Values

This circuit safely switches a 12V, 5A DC motor using an ESP32 (3.3V GPIO).

• Q1: IRLZ44N (Logic-Level N-Channel, TO-220 package, Rds(on) = 22mΩ at Vgs=5V)
• R1 (Gate Resistor): 100Ω. Limits the inrush current required to charge the gate capacitance, protecting the ESP32 GPIO from exceeding its 40mA absolute max rating during PWM switching.
• R2 (Pull-down Resistor): 10kΩ between Gate and Ground. Ensures the MOSFET stays off during ESP32 boot-up when GPIO pins are floating and high-impedance.
• D1 (Flyback Diode): 1N4007 or Schottky (e.g., 1N5819). Placed in reverse bias across the motor (Cathode to 12V, Anode to Drain). Clamps inductive voltage spikes when the motor switches off, preventing avalanche breakdown of the MOSFET.
• Motor: Connected between the +12V power supply and the Drain pin.
• Source: Connected directly to system Ground.

How it works: When the ESP32 drives the GPIO HIGH (3.3V), the voltage passes through the 100Ω resistor to the Gate. Because the IRLZ44N is fully enhanced at Vgs = 3.3V (yielding an Rds(on) of roughly 25mΩ), the Drain-Source path acts like a 25-milliohm wire. The motor receives nearly the full 12V. At 5A, the MOSFET dissipates only P = I²R = 25 * 0.025 = 0.625 Watts, which a standard TO-220 package can handle in free air without a heatsink. When the GPIO goes LOW, the 10kΩ resistor pulls the gate charge to ground, snapping the MOSFET off. The motor's collapsing magnetic field forward-biases D1, safely recirculating the current.

The Decision Tree: Which MOSFET Should You Buy?

Stop guessing part numbers. Use this decision matrix to select the exact MOSFET for your project based on your voltage, current, and logic-level requirements. These are the proven, high-availability defaults that stock every serious electronics workbench.

Final Purchasing Advice

If you are building a low-side switch for an Arduino or ESP32 project operating at 12V or 24V, buy a 10-pack of IRLZ44N or FQP30N06L transistors. They are logic-level, forgiving, and handle massive current without heatsinks. If you are doing surface-mount design for signal routing, stock up on BSS138 reels. Avoid the classic IRF520 module boards sold for Arduinos; they are not true logic-level devices and will overheat at 5A on a 5V GPIO. Always cross-reference the 'L' in the part number, verify the Rds(on) at your specific gate drive voltage, and never forget the flyback diode on inductive loads.

References: For deeper reading on MOSFET capacitance and switching losses, consult the All About Circuits MOSFET Guide and the Electronics Tutorials MOSFET primer.