The three primary types of field effect transistor used in modern electronics are the Junction FET (JFET), the Depletion-mode MOSFET, and the Enhancement-mode MOSFET. While JFETs still hold a niche in low-noise audio preamplifiers, N-channel enhancement MOSFETs are the undisputed default for 95% of switching, power delivery, and logic-level translation tasks on the bench today.

Unlike Bipolar Junction Transistors (BJTs) which are current-controlled devices, FETs are voltage-controlled. You apply a voltage to the Gate to modulate the electric field, which in turn controls current flow between the Drain and Source. This results in near-infinite DC input impedance at the gate, making them incredibly easy to drive from high-impedance sources like microcontroller GPIO pins.

The Core FET Families and Safe Default Part Numbers

Before wiring anything, you need to read the symbol and pinout. All standard FETs share three main terminals: Gate (G), Drain (D), and Source (S). On a schematic, the JFET symbol features an arrow on the Gate pointing directly into the channel (for N-channel). The MOSFET symbol separates the Gate from the channel with a solid line (representing the silicon dioxide insulator). Crucially, power MOSFETs include an intrinsic body diode drawn between the Drain and Source, which conducts when the MOSFET is reverse-biased—a detail that dictates how you orient them in H-bridges and relay drivers.

When selecting a component, avoid guessing. Here are the safe, widely-available default part numbers for the most common bench scenarios, complete with their critical ratings.

Table 1: FET Family Comparison and Default Bench Part Numbers
Part Number Family / Type Vds (max) Id (max) Rds(on) / Idss Primary Application
2N5457 N-Channel JFET 25V 5mA Idss: 1-5mA Low-noise audio preamps, simple current sources
BSS138 N-Ch Enhancement (Small Signal) 50V 0.22A 3.5 Ω @ 4.5V I2C logic level shifting (3.3V to 5V), LED driving
IRLZ44N N-Ch Enhancement (Logic-Level) 55V 47A 0.022 Ω @ 5V Arduino/ESP32 motor control, relay driving, PWM
IRF520 N-Ch Enhancement (Standard) 100V 9.2A 0.27 Ω @ 10V 12V+ switching (Requires 10V+ gate drive, not 5V/3.3V)
Bench Warning: The IRF520 is heavily stocked in beginner kits, but it is not a logic-level MOSFET. Its Vgs(th) is up to 4V, meaning a 3.3V ESP32 pin will barely turn it on, causing it to overheat and fail at high currents. Always choose the IRLZ44N or IRLB8721 for 5V/3.3V microcontroller projects.

Operation Regions and Biasing Strategies

A massive source of confusion for makers transitioning from BJTs is FET terminology. In a BJT, 'saturation' means the switch is fully ON. In a FET, 'saturation' means the device is acting as a constant-current source (amplifier mode), while the fully-ON switch state is called the Linear or Ohmic region.

Table 2: N-Channel Enhancement MOSFET Operation Regions
Region Gate-Source Condition Drain-Source Condition Behavior & Use Case
Cutoff Vgs < Vgs(th) Any Switch is OFF. Only leakage current flows.
Linear (Ohmic) Vgs > Vgs(th) Vds < (Vgs - Vgs(th)) Switch is fully ON. Acts as a low-value resistor (Rds(on)). Used for PWM and power switching.
Saturation (Active) Vgs > Vgs(th) Vds > (Vgs - Vgs(th)) Current is constant regardless of Vds. Used for analog amplification and active loads.

How to Bias and Select for the Job

To use a MOSFET as a switch, you must drive the Gate voltage high enough to push the device deep into the Linear region. Do not design based on the Vgs(th) (Gate Threshold Voltage) listed on the first page of the datasheet. Vgs(th) is merely the voltage where the MOSFET begins to conduct a tiny leakage current (usually 250µA).

For an IRLZ44N, Vgs(th) might be 1V to 2V, but the Rds(on) is only guaranteed at Vgs = 5V or 10V. If you are switching 20A, you need Vgs = 10V to achieve the 0.022Ω resistance and keep power dissipation (I²R) under 9W. If you only drive it with 3.3V, Rds(on) might be 0.15Ω, resulting in 60W of heat and a melted TO-220 package.

Application Circuit: Driving a 12V Relay from an ESP32

Let's build a robust, real-world switching circuit. We will use an ESP32 (3.3V logic) to switch a 12V automotive-style relay (coil resistance ~160Ω, drawing ~75mA) using an IRLZ44N logic-level MOSFET.

Bill of Materials & Component Values

  • Q1: IRLZ44N (N-Channel Logic-Level MOSFET)
  • R1: 100Ω (Gate series resistor, 1/4W)
  • R2: 10kΩ (Gate-to-Source pulldown resistor)
  • D1: 1N4148 or 1N4007 (Flyback diode)
  • K1: 12V SPDT Relay

Wiring Steps

  1. Gate Drive: Connect the ESP32 GPIO pin to the MOSFET Gate through the 100Ω resistor (R1). This resistor limits the inrush current into the MOSFET's gate capacitance (Ciss), preventing high-frequency ringing that can cause EMI or damage the ESP32 GPIO.
  2. Pulldown: Connect the 10kΩ resistor (R2) between the Gate and Source (GND). This ensures the MOSFET stays firmly OFF during ESP32 boot-up when GPIO pins are floating and high-impedance.
  3. Source to Ground: Connect the MOSFET Source pin directly to the shared system Ground (ESP32 GND and 12V supply GND must be bonded).
  4. Drain to Load: Connect the MOSFET Drain to the negative terminal of the relay coil.
  5. Power the Coil: Connect the positive terminal of the relay coil to the +12V supply.
  6. Flyback Protection: Place the 1N4148 diode (D1) in parallel with the relay coil. The cathode (striped end) must point toward +12V. When the MOSFET turns off, the collapsing magnetic field generates a massive reverse voltage spike; the diode safely recirculates this current. Without it, the MOSFET will experience avalanche breakdown and fail short-circuit.

Failure Modes and Bench Testing with a Multimeter

FETs generally fail in three ways: Gate oxide puncture from ESD (resulting in a permanent short from Gate to Source), thermal runaway from operating in the linear region with insufficient gate drive, and avalanche breakdown from inductive kickback without a flyback diode. When a MOSFET fails thermally or from avalanche, it almost always fails as a dead short between Drain and Source.

You can verify the health of an N-channel enhancement MOSFET on the bench using a standard digital multimeter (DMM) in Diode Test mode. According to Fluke's testing guidelines, the DMM's diode test outputs roughly 2.5V to 3.0V, which is enough to partially turn on a logic-level MOSFET.

Step-by-Step DMM Testing Procedure

  1. Discharge the Gate: With the MOSFET out of circuit, touch a piece of bare wire or your finger across the Gate and Source pins to discharge any stored capacitance.
  2. Test the Body Diode: Set DMM to Diode Test. Place the Black probe on Source and Red probe on Drain. You should read a standard diode drop (typically 0.4V to 0.6V). Reverse the probes (Red on S, Black on D); it should read 'OL' (Open Loop).
  3. Charge the Gate: Keep the Black probe on the Source. Momentarily touch the Red probe to the Gate. The DMM's internal voltage will charge the gate capacitance, turning the MOSFET on.
  4. Verify the Channel: Move the Red probe back to the Drain (Black still on Source). The meter should now read near 0.00V or emit a continuity beep, indicating the Drain-Source channel is conducting.
  5. Discharge and Re-verify: Touch Black to Gate and Red to Source to discharge it. Check Drain to Source again; it should revert to the 0.4V - 0.6V body diode reading.
Testing Gotcha: If you are testing a standard MOSFET like the IRF520 or IRF3205, the DMM's 3V output might not exceed the Vgs(th) enough to turn it on. If Step 4 still reads 'OL', use a 9V battery to briefly touch the Gate (positive) and Source (negative), then re-test the Drain-Source continuity.

Understanding the distinct behaviors of JFETs and MOSFETs—and specifically the difference between threshold voltage and full enhancement voltage—separates working prototypes from melted silicon. Stick to logic-level part numbers for microcontrollers, always use gate pulldowns, and never switch an inductive load without a flyback diode.