MOSFETs

What is a MOSFET?

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled switch. Unlike a BJT, which needs continuous base current, a MOSFET is controlled by voltage at the gate terminal — it draws virtually no steady-state current. This makes MOSFETs ideal for power switching, motor control, and anything driven by microcontrollers.

The Three Terminals

• •Gate (G) — the control terminal. Voltage here turns the MOSFET on or off. Draws almost no current (just charges the gate capacitance).
• •Drain (D) — where current enters (N-channel) or exits (P-channel). Connects to the load.
• •Source (S) — output terminal. Usually connected to ground (N-ch) or V_CC (P-ch).

N-Channel vs P-Channel

• •N-channel (most common) — turns ON when V_GS > V_th (gate is positive relative to source). Used for low-side switching (load between V_CC and drain, source to ground).
• •P-channel — turns ON when V_GS < −V_th (gate is more negative than source). Used for high-side switching (source to V_CC, load between drain and ground).
• •N-channel MOSFETs generally have lower R_DS(on) and are cheaper, so they're preferred whenever possible.

Threshold Voltage (V_th)

The threshold voltage is the minimum gate-to-source voltage needed to start turning the MOSFET on. For standard MOSFETs, V_th is typically 2–4 V. For logic-level MOSFETs, V_th is lower (~1–2 V), allowing them to be driven directly by 3.3 V or 5 V microcontroller pins.

On-Resistance: R_DS(on)

When fully on, the MOSFET acts like a small resistance between drain and source called R_DS(on). Lower R_DS(on) means less power wasted as heat. For a logic-level MOSFET like the IRLZ44N, R_DS(on) is about 22 mΩ at V_GS = 10 V. The power dissipated is P = I_D² × R_DS(on).

Using a MOSFET as a Switch

• •Connect the load between V_CC and the drain.
• •Connect the source to ground.
• •Apply a voltage > V_th to the gate to turn on. Connect gate to ground to turn off.
• •Add a 10 kΩ pull-down resistor from gate to source to ensure the MOSFET stays off when no signal is applied.
• •For fast switching (PWM), add a gate driver or low-value gate resistor (100–470 Ω) to charge/discharge the gate capacitance quickly.

MOSFET vs BJT — When to Use Which

• •MOSFETs: high-current switching (motors, LED strips, heaters), PWM control, battery-powered devices (no gate current wasted).
• •BJTs: small-signal amplification, simple low-current switches, when you need a linear amplifier.
• •MOSFETs scale better: switching 10 A with a BJT requires a large base current from the controller. A MOSFET does it with almost zero steady-state current.
• •MOSFETs have a parasitic body diode (drain → source in N-ch), which can be useful or problematic depending on the application.

Popular MOSFET Types

• •IRLZ44N — logic-level N-ch, 55V / 47A, R_DS(on) = 22 mΩ. Great for 5 V Arduino projects.
• •IRLB8721 — logic-level N-ch, 30V / 62A, R_DS(on) = 8.7 mΩ. Excellent for high-current LED strips.
• •IRF540N — standard N-ch, 100V / 33A. Needs ~10 V gate drive, not suitable for direct 3.3 V/5 V driving.
• •AO3401 — small P-ch SOT-23, 30V / 4A. Popular for high-side switching in battery circuits.