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ComponentsActive / Semiconductor

MOSFET

Transistor

A voltage-controlled switch/amplifier. Used everywhere for switching power efficiently (motors, LEDs, power supplies). Tiny gate current controls big load current.

Schematic symbol

MOSFET (N-channel)

How mosfet appears in a circuit diagram.

Types & variants

N-channelP-channelEnhancement modeDepletion modePower MOSFETLogic-level (drives from 3.3/5V)

Key specs

Vds (drain-source voltage)

V

Max voltage it can block when off.

Id (drain current)

A

Max continuous current it can carry.

Rds(on)

mOhm

On-resistance - lower means less heat/loss when conducting.

Vgs(th)

V

Gate threshold voltage. For 3.3/5V control, use a logic-level MOSFET.

Laws that govern it

  • Ohm's lawV = I × R

    Fully on, the MOSFET is just a resistor: the drop is I × Rds(on), which is the number you compare parts on.

  • Watt's lawP = V × I = I²R = V²/R

    Conduction loss is I²·Rds(on), and it is why a 50 A part still needs a heatsink at 20 A if its Rds(on) is poor.

  • Law of capacitance — capacitance formula Q = CVQ = C × V, i = C·dv/dt, E = ½CV²

    The gate is a capacitor — Q = CV means turning it on fast needs a big current pulse, and a slow gate drive means expensive switching losses.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

How it compares

  • MOSFET vs BJT

    Side by side with BJT (NPN / PNP) — specs, variants and which one the job calls for.

  • MOSFET vs JFET

    Side by side with JFET — specs, variants and which one the job calls for.

Markings

Part number printed on body (e.g. IRLZ44N, AO3400). Look up pinout per part.

Standard values

Popular: IRLZ44N (logic-level N-ch), AO3400 (SMD), IRF540N.

How to choose

1) N-ch for low-side switching. 2) Vds >= 2x supply. 3) Id >> load current. 4) Vgs(th) below your control voltage (use logic-level for MCUs). 5) Low Rds(on) for high current.

Pinout & package

GDS123

TO-220 — front view, tab at back

Check first: Typical for many N-channel TO-220 MOSFETs (e.g. IRFZ44). Order varies by part — verify with the datasheet.

3 pins: Gate, Drain, Source (order varies - check datasheet). TO-220, TO-92, SOT-23, DPAK.

Example circuits

  • Low-side switch for a motor/LED strip from an MCU pin
  • PWM speed control
  • Buck converter switch

Common failures

Gate destroyed by static (ESD) or overvoltage; overheats if Rds(on) too high or no heatsink; shorts D-S when blown.

How to test

Diode mode S->D shows body diode; with gate charged it should conduct D-S. (Dedicated transistor testers are easiest.)

Substitutes

Match channel type, equal/higher Vds & Id, equal/lower Rds(on), compatible Vgs(th) and package.

Where to buy

DigiKeyMouserLCSCRobuAmazon