What Is a MOSFET? When to Use One vs a BJT Transistor

If you've ever built a motor driver, power supply, or anything that switches more than a few hundred milliamps, you've probably encountered a MOSFET. But what exactly is a MOSFET, and when should you choose one over a traditional BJT transistor?

MOSFET Basics

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. Unlike a BJT (Bipolar Junction Transistor) which is current-controlled, a MOSFET is voltage-controlled. This fundamental difference makes MOSFETs easier to drive and more efficient in many applications.

A MOSFET has three terminals:

• Gate (G) — Controls whether the device is on or off
• Drain (D) — Where current flows in
• Source (S) — Where current flows out (for N-channel)

How a MOSFET Works

When you apply a voltage to the gate (relative to the source), it creates an electric field that either allows or blocks current flow between the drain and source. The key parameter is the gate threshold voltage (Vgs(th)) — the minimum gate-to-source voltage needed to turn the MOSFET on.

For a typical logic-level N-channel MOSFET like the IRLZ44N:

• Below ~2V Vgs: MOSFET is off, no current flows
• Above ~4V Vgs: MOSFET is fully on, acts like a low-resistance switch
• The "on resistance" (Rds(on)) can be as low as a few milliohms

N-Channel vs P-Channel

N-Channel MOSFETs are turned on by a positive gate voltage. They're placed on the low side (between the load and ground) in most simple circuits. They are more efficient and more commonly available.

P-Channel MOSFETs are turned on by a negative gate voltage (gate is pulled below the source). They're used for high-side switching (between the power supply and load). They typically have higher on-resistance than equivalent N-channel parts.

Enhancement vs Depletion Mode

Enhancement mode MOSFETs (the vast majority) are normally off — they need a gate voltage to turn on. This is what you'll use 99% of the time.

Depletion mode MOSFETs are normally on and require a gate voltage to turn off. These are rare and used in specialized applications like constant-current sources.

MOSFET vs BJT: When to Use Which

Choose a MOSFET when:

• Switching high currents (>1A) — MOSFETs can handle tens or hundreds of amps with minimal heat
• Efficiency matters — No base current means less wasted power in the control circuit
• PWM control — MOSFETs switch faster and more efficiently at high frequencies
• Driving from a microcontroller — Logic-level MOSFETs can be driven directly from 3.3V or 5V GPIO pins
• Battery-powered designs — Lower power consumption in the driver circuit

Choose a BJT when:

• Switching small signals (<100 mA) — BJTs like the 2N3904 are cheaper and simpler
• Linear amplification — BJTs have more predictable gain characteristics for analog circuits
• Cost is critical — BJTs are still cheaper than equivalent MOSFETs for small-signal work
• Low gate capacitance matters — BJTs respond faster in some small-signal applications
• You need a simple current source — BJT current mirrors are simpler to implement

Common Beginner Mistakes with MOSFETs

1. Using a non-logic-level MOSFET with a microcontroller — Standard MOSFETs need 10V+ on the gate. Use "logic level" variants (look for "IRL" prefix or "logic level" in the description).

1. Forgetting a gate resistor — A 100Ω–1kΩ resistor between the driver and gate prevents oscillation and limits peak current.

1. No gate pulldown resistor — A 10kΩ–100kΩ resistor from gate to source ensures the MOSFET stays off when the control signal is disconnected.

1. Exceeding Vgs maximum — Most MOSFETs have a maximum gate voltage of ±20V. Exceeding this destroys the gate oxide permanently.

1. Ignoring thermal management — Even with low Rds(on), high-current MOSFETs can generate significant heat. Calculate P = I² × Rds(on) and add heatsinking if needed.

Popular MOSFETs for Hobbyists

• IRLZ44N — Logic level, 55V, 47A, great for Arduino projects
• IRF540N — Standard level, 100V, 33A, needs a gate driver
• AO3400 — SMD logic level, 30V, 5.7A, good for compact designs
• IRF9540 — P-channel, 100V, 23A, for high-side switching
• 2N7000 — Small signal N-channel, 60V, 200 mA, TO-92 package

Browse our component database to find the right MOSFET for your project, with full specifications and equivalent parts.