MOSFETs & Transistors

Discrete switches: gate-drive requirements at logic levels, SOA limits, and the classic substitution mistakes.

Written and verified by John Merton · verified 2026-07-10

MOSFETs are voltage-controlled switches, and the category splits between signal FETs and power FETs. The BSS138 is a signal FET: 50 V, 220 mA, RDS(on) measured in ohms, used in the bidirectional logic-level shifter circuit that made it famous. The IRLZ44N and AO3400A are power FETs: RDS(on) measured in milliohms, rated for amps of continuous current, used for load switching and motor drive.

The classic substitution mistake is using a signal FET for a power job because the package fits. A BSS138 switching a 200 mA LED strip drops over a volt across the FET and dissipates a quarter watt in a SOT-23 — and that is before gate drive headroom. The gotchas page for each part covers its actual thermal and gate-drive limits from the datasheet, not the optimistic front-page summary.

How to choose

The axes below are the criteria you actually weigh when selecting a part in this category.

Signal FET vs power FET: RDS(on) tells the story: RDS(on) in ohms means a signal FET (level shifting, small-signal switching). RDS(on) in milliohms means a power FET (load switching, motor drive). Using the wrong one cooks the part.; See:BSS138 AO3400A IRLZ44N
Gate threshold voltage vs your logic level: A MOSFET with VGS(th) of 2–4 V will not fully enhance on a 3.3 V GPIO. Check the RDS(on) test condition: it is specified at a gate voltage, and if your GPIO cannot reach it, you get ohms instead of milliohms.; See:BSS138 AO3400A IRLZ44N
Package thermal resistance and continuous current derating: The headline current rating is often at 25 °C case temperature with infinite heatsink. Real current capability in still air at 70 °C ambient is a fraction of that number. Check θJA and do the math.; See:AO3400A IRLZ44N BSS138
Gate charge and switching speed: Total gate charge determines how fast your gate driver can turn the FET on and off. A big power FET with 50 nC of gate charge switching at 100 kHz needs a gate driver, not a GPIO.; See:IRLZ44N AO3400A

Standout parts

A small curated group of parts in this category that represent different positions on the selection axes. Start here if you are new to the category.

BSS138

50 V 220 mA N-channel MOSFET; standard bidirectional level-shifter part

Active

AO3400A

30 V 5.7 A N-channel trench MOSFET in SOT-23 for load switching and PWM at logic-level gate drive

Active

IRLZ44N

55 V 47 A logic-level N-channel MOSFET for hobby power switching

Active

All parts in this category

Compare specs

Parts in this guide verified against manufacturer datasheets; latest 2026-07-10.

MPN	Package	Lifecycle	Headline spec
BSS138	SOT-23-3 (Case 318-08)	Active	VDS: 50 V (VDSS, absolute maximum)
IRLZ44N	TO-220AB	Active	VDS: 55 V (VDSS; V(BR)DSS min 55 V at VGS = 0 V, ID = 250 µA)
AO3400A	SOT-23	Active	VDS (drain-source voltage): 30 V

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