A field effect transistor MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled switch. Unlike bipolar junction transistors (BJTs) that require continuous base current to stay on, a MOSFET requires almost zero steady-state gate current. You only need to supply current momentarily to charge the gate capacitance. This makes the MOSFET the undisputed king of high-efficiency power switching, motor control, and microcontroller GPIO interfacing.

However, picking the wrong part or driving the gate incorrectly will result in melted packages, bricked microcontrollers, or erratic switching. This guide cuts through the semiconductor physics and gives you the exact part numbers, biasing rules, and testing procedures you need on the bench.

Safe Default Part Numbers and Ratings

Do not waste time guessing which MOSFET to order for your next prototype. Keep these five safe defaults in your bin. They cover 95% of hobbyist and light-industrial applications, from I2C level shifting to 20A motor drives.

Part Number Type Vds (Max) Id (Cont) Rds(on) @ Vgs Package Best Application
IRLB8721 N-Channel 30V 62A 8.7mΩ @ 4.5V TO-220 Logic-level high-current switching (ESP32/Arduino to 12V motors)
IRFZ44N N-Channel 55V 49A 17.5mΩ @ 10V TO-220 Standard gate drive (requires 10V-12V gate driver IC, not direct GPIO)
BSS138 N-Channel 50V 200mA 3.5Ω @ 4.5V SOT-23 Signal switching, I2C level shifters, 3.3V to 5V logic translation
AO3400 N-Channel 30V 5.7A 35mΩ @ 4.5V SOT-23 SMD logic-level load switching (LED strips, relays up to 3A)
IRF4905 P-Channel -55V -74A 20mΩ @ -10V TO-220 High-side power switching and reverse polarity protection
Bench Tip: Notice the Rds(on) @ Vgs column. A 'logic-level' MOSFET like the IRLB8721 specifies its on-resistance at 4.5V, meaning it will fully turn on from a 5V Arduino or 3.3V ESP32 pin. The IRFZ44N specifies 10V; if you drive it with 3.3V, it will only partially turn on, overheat, and fail.

Pinout, Symbol, and Operation Regions

Every standard power MOSFET has three primary terminals: the Gate (G), Drain (D), and Source (S). In the schematic symbol, you will also see a fourth connection representing the Body Diode. This intrinsic diode points from Source to Drain in an N-channel MOSFET. It is not a manufacturing defect; it is a physical consequence of how the silicon is doped. You must account for this diode when designing high-side switches or H-bridges, as it will conduct if the Drain voltage drops below the Source voltage.

Understanding how a MOSFET transitions between states is critical. As detailed in standard semiconductor references like All About Circuits, the terminology for MOSFET regions is notoriously confusing because it is the exact opposite of BJT terminology.

Operation Region Condition (N-Channel) Typical Voltages / Currents Function
Cutoff Vgs < Vgs(th) Vgs = 0V, Vds = 12V, Id = 0A Switch is OFF. No current flows (except negligible leakage).
Linear (Ohmic / Triode) Vgs > Vgs(th) AND Vds < (Vgs - Vgs(th)) Vgs = 4.5V, Vds = 0.1V, Id = 10A Switch is fully ON. Acts as a low-value resistor (Rds(on)). This is where you want to be for power switching.
Saturation (Active) Vgs > Vgs(th) AND Vds ≥ (Vgs - Vgs(th)) Vgs = 3.0V, Vds = 5.0V, Id = 2A Acts as a voltage-controlled constant current source. Used for analog amplifiers, NOT for digital switching (it will overheat here).
Terminology Trap: In a BJT, 'Saturation' means the switch is fully ON. In a MOSFET, 'Saturation' means the device is acting as an amplifier (partially on, dropping high voltage). When using a MOSFET as a switch, you want to drive it deep into the Linear/Ohmic region.

How to Bias and Select a Field Effect Transistor MOSFET

Selecting the right MOSFET requires looking past the headline numbers on the datasheet. Here is the decision framework for biasing and selection:

  1. Ignore Vgs(th) for switching: The Gate-Source Threshold Voltage (Vgs(th)) is merely the voltage where the MOSFET begins to conduct (usually defined at a microscopic 250µA). If a datasheet lists Vgs(th) as 2.0V to 4.0V, applying 3.3V to the gate will not fully turn it on. Always look at the Rds(on) specification table to see what Vgs is required to achieve the rated low resistance.
  2. Calculate Conduction Loss: Power dissipated as heat is P = I² × Rds(on). If your load draws 15A and your MOSFET has an Rds(on) of 10mΩ (0.010Ω), the dissipation is 15² × 0.010 = 2.25W. A bare TO-220 package can only dissipate about 1W to 1.5W in free air before exceeding safe junction temperatures. At 2.25W, you must add a heatsink.
  3. Check Gate Charge (Qg): The gate acts like a capacitor. To switch the MOSFET on, you must charge this capacitor. The total gate charge (Qg) dictates how much current your driver must supply and how fast it can switch. If you are using PWM at 20kHz, a high Qg MOSFET will cause your driver IC to overheat unless you use a dedicated gate driver.

For a deeper dive into calculating gate drive currents and managing the Miller plateau, the Texas Instruments Gate Drive Guide remains the industry-standard reference.

Complete Application Circuit: 12V Low-Side PWM Switch

Below is a robust, battle-tested circuit for driving a 12V DC motor (or high-power LED strip) using an ESP32 (3.3V logic) or Arduino (5V logic). This is a low-side switch configuration, meaning the load is connected directly to the positive supply, and the MOSFET switches the ground path.

Bill of Materials

  • Q1: IRLB8721 (N-Channel Logic-Level MOSFET)
  • R1 (Gate Series): 100Ω 1/4W Resistor
  • R2 (Pull-down): 10kΩ 1/4W Resistor
  • D1 (Flyback): 1N5819 Schottky Diode
  • Load: 12V DC Motor (up to 20A continuous)

Wiring Steps

  1. Connect the Source (S) pin of Q1 directly to the system Ground (GND).
  2. Connect the Drain (D) pin of Q1 to the negative terminal of the 12V Motor.
  3. Connect the positive terminal of the 12V Motor to the 12V Power Supply positive rail.
  4. Place the 1N5819 Schottky diode across the motor terminals. The cathode (stripe) must face the 12V positive rail, and the anode must face the MOSFET Drain. This clamps inductive voltage spikes when the MOSFET turns off.
  5. Connect the 10kΩ pull-down resistor (R2) between the Gate (G) and Source (S). This ensures the MOSFET stays off if the microcontroller pin is floating (e.g., during boot-up).
  6. Connect the 100Ω gate series resistor (R1) between the microcontroller PWM GPIO pin and the Gate (G). This limits the inrush current into the gate capacitance, protecting the microcontroller's internal GPIO traces from burning out.
Why the 100Ω Gate Resistor? A MOSFET gate looks like a dead short to a microcontroller pin the exact microsecond the pin goes HIGH. The 100Ω resistor limits this spike current to roughly 33mA (3.3V / 100Ω), keeping your ESP32 safe while still allowing the gate to charge fast enough for standard PWM frequencies.

Failure Modes and Multimeter Testing

MOSFETs rarely fail gracefully. When they exceed their limits, they typically fail short-circuit, melting the silicon die and sometimes taking the gate driver with it. The three most common failure modes are:

  • Thermal Runaway: Rds(on) has a positive temperature coefficient. As the die gets hot, resistance increases, which generates more heat (I²R), leading to a runaway thermal loop until the silicon melts.
  • Vds Avalanche Breakdown: Switching off an inductive load without a flyback diode causes a massive voltage spike. If this exceeds the Vds rating (e.g., 30V on an IRLB8721), the junction breaks down and punches through.
  • ESD / Gate Oxide Punch-through: The gate is separated from the channel by a microscopic layer of silicon dioxide. A static shock exceeding ±20V can permanently puncture this oxide layer, shorting the gate to the source.

How to Test a MOSFET with a Digital Multimeter

You can verify the health of an N-channel MOSFET on your bench using the Diode Test mode on a standard digital multimeter. This tests both the intrinsic body diode and the gate's ability to hold a charge.

  1. Set your multimeter to Diode Test mode (the symbol with an arrow and a line).
  2. Test 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 0.6V.
  3. Reverse the probes (Red on Drain, Black on Source). The meter should read 'OL' (Open Loop), confirming the diode blocks reverse current.
  4. Test Gate Turn-On: While keeping the Red probe on the Drain and Black on the Source (reading OL), briefly touch the Red probe to the Gate pin to charge it, then move it back to the Drain. The reading should drop to near 0.0V (or beep on continuity), indicating the MOSFET has turned ON.
  5. Test Gate Turn-Off: Briefly touch the Black probe to the Gate pin to discharge it, then move it back to the Source. The Drain-Source reading should return to 'OL', confirming the MOSFET turns OFF.

If the Drain-Source reads 0.0V in both directions before you even touch the gate, or if the Gate-Source reads as a short circuit, the MOSFET is dead and must be desoldered and replaced.