MOS Transistor Guide: Pinouts, Biasing, and Safe Default Part Numbers

If you need a default N-channel logic-level MOS transistor for 5V or 3.3V microcontrollers switching up to 30A, buy the IRLZ44N (approx. $1.50 in TO-220 package). For low-current signal switching under 200mA on a breadboard, use the 2N7000 ($0.20 in TO-92). Skip the standard IRF520; its gate threshold voltage is too high to fully saturate from a 5V GPIO pin, which will cause it to overheat and fail at high currents.

The Quick-Answer: Which MOS Transistor Should You Buy?

Choosing the right MOS transistor comes down to your load current and your gate drive voltage. The most common mistake makers make is confusing the gate-source threshold voltage ($V_{GS(th)}$) with the voltage required to fully turn the device on. $V_{GS(th)}$ is merely the voltage where the device begins to conduct a tiny trickle of current (usually 250µA). To achieve the low $R_{DS(on)}$ (on-resistance) listed on the datasheet, you must drive the gate to the specific $V_{GS}$ listed in the $R_{DS(on)}$ test conditions—usually 4.5V, 5V, or 10V.

Pinout, Symbol, and the Three Operating Regions

The IEC standard symbol for an N-channel MOS transistor features three main terminals: Gate (G), Drain (D), and Source (S). The Gate is drawn separated from the channel by a line representing the silicon dioxide insulator. An arrow on the Source terminal points inward for N-channel (and outward for P-channel). Crucially, the symbol includes an intrinsic body diode connected between the Source and Drain. For an N-channel device, the cathode is at the Drain and the anode is at the Source. This diode will conduct if the Drain voltage drops below the Source voltage, which is a frequent trap in H-bridge and reverse-polarity circuits.

For the ubiquitous TO-220 through-hole package (like the IRLZ44N), hold the component facing you with the pins pointing down and the metal tab facing away. The pins from left to right are Gate, Drain, Source. The metal tab is internally connected to the Drain.

Note on terminology: Do not confuse MOSFET "Saturation" with BJT "Saturation". In a BJT, saturation means fully ON (closed switch). In a MOSFET, the fully ON closed-switch state is the Triode/Ohmic region. The MOSFET Saturation region is actually the active amplification region. (Source: All About Circuits Semiconductor Textbook).

How to Bias and Drive a MOS Transistor (With a Real Circuit)

Driving a MOS transistor directly from a microcontroller GPIO pin requires three passive components to ensure reliability and protect your silicon. Below is a complete, bench-tested application circuit for driving a 12V DC motor drawing 5A using an Arduino Uno and an IRLZ44N.

The Component List and Wiring

• Q1: IRLZ44N (N-Channel Logic-Level MOSFET)
• R1 (Gate Series Resistor): 100Ω, 1/4W. Placed between Arduino Pin 9 and the Gate.
• R2 (Gate Pulldown Resistor): 10kΩ, 1/4W. Placed between Gate and Source (GND).
• D1 (Flyback Diode): 1N4007 or 1N5819 (Schottky). Placed in parallel with the motor, cathode (stripe) to 12V, anode to Drain.
• Load: 12V DC Motor connected between 12V supply and the Drain pin.
• Source: Connected directly to the common Ground of the Arduino and 12V supply.

Why These Specific Component Values?

The 100Ω Gate Resistor (R1): A MOSFET gate acts like a capacitor (the IRLZ44N has an input capacitance of roughly 2600pF). When the Arduino pin goes HIGH, it dumps current into this capacitor. Without a resistor, the inrush current can exceed the ESP32/Arduino GPIO absolute maximum rating (typically 20mA-40mA), degrading the microcontroller over time. Furthermore, the 100Ω resistor damps high-frequency LC ringing caused by trace inductance and gate capacitance, preventing the MOSFET from oscillating and generating EMI.

The 10kΩ Pulldown Resistor (R2): During microcontroller boot-up, GPIO pins are high-impedance (floating). Because the MOSFET gate is highly sensitive, stray electromagnetic noise or capacitive coupling from your hand can induce enough voltage to partially turn on the device. If the MOSFET turns on in the linear region while the motor is connected, it will dissipate massive heat and destroy itself before the Arduino finishes its boot sequence. The 10kΩ resistor safely bleeds away any stray gate charge to ground. (Source: Vishay Application Note 70745).

The Flyback Diode (D1): When you turn off the MOSFET, the motor's collapsing magnetic field generates a high-voltage reverse spike (inductive kickback). Without D1 to provide a recirculation path, this spike will exceed the IRLZ44N's 55V $V_{DSS}$ rating, causing avalanche breakdown and instantly shorting the Drain to the Source.

Failure Modes and How to Test with a Multimeter

MOS transistors generally fail in three distinct ways:

1. Gate Oxide Puncture (ESD): The silicon dioxide layer between the gate and channel is incredibly thin. A static shock exceeding the $V_{GS}$ max rating (usually ±20V) punches a microscopic hole through the oxide, permanently shorting the Gate to the Source or Drain.
2. Thermal Runaway: Caused by under-driving the gate. If you drive an IRF520 with 5V, it never fully enters the Triode region. $R_{DS(on)}$ remains high, generating $I^2R$ heat. As the silicon heats up, its threshold voltage drops, altering conduction and often leading to a catastrophic thermal short.
3. Avalanche Breakdown: Exceeding the Drain-Source voltage rating ($V_{DSS}$) via inductive kickback or voltage transients, causing the internal junction to punch through and short Drain to Source.

Numbered Steps: Testing a MOS Transistor with a DMM

You can verify the health of an N-channel MOSFET using a standard digital multimeter in Diode Test Mode. This test checks the intrinsic body diode and the gate's ability to hold a charge.

The 2026 Buyer's Decision Matrix: BJT vs. Logic-Level MOS vs. Standard MOS

When designing a switching circuit, you must choose between Bipolar Junction Transistors (BJTs), standard MOSFETs, and logic-level MOSFETs. Here is how they stack up for modern maker projects.