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MOSFET vs BJT

MOSFET and BJT (NPN / PNP) compared — what each is better at, and which one the job actually calls for.

A BJT is current-driven: hold base current and it conducts, with a roughly fixed saturation drop that does not improve however big the device is. A MOSFET is voltage-driven: push charge into the gate and it then behaves like a resistor, so at high current it dissipates far less than a BJT ever can. BJTs keep their place on cost, at very low currents, and wherever generating a clean gate drive would be more trouble than it is worth.

What each one is

MOSFET

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

BJT (NPN / PNP)

A current-controlled switch and amplifier with three pins. A small base current controls a much larger collector current. The classic transistor.

Key specs

MOSFET

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.

BJT (NPN / PNP)

Current gain (hFE/β)(-)
Collector current ÷ base current; often 100-300.
Collector current (Ic max)(A)
Max continuous current through the device.
VCEO(V)
Max collector-emitter voltage before breakdown.
Power dissipation(W)
Max heat; needs a heatsink for power types.
Transition frequency (fT)(MHz)
Upper frequency limit for useful gain.

Types you will meet

MOSFET

  • N-channel
  • P-channel
  • Enhancement mode
  • Depletion mode
  • Power MOSFET
  • Logic-level (drives from 3.3/5V)

BJT (NPN / PNP)

  • NPN (e.g. BC547, 2N2222, 2N3904)
  • PNP (e.g. BC557, 2N3906)
  • Darlington (huge gain)
  • Power BJT (TIP/TO-220)

How to choose

MOSFET

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.

BJT (NPN / PNP)

Switching: pick Ic > load, VCEO > supply, and drive the base hard (base resistor for ~Ic/10). Amplifying: choose for gain and fT. Add a heatsink for power use.

How each one fails

MOSFET

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

BJT (NPN / PNP)

Shorts C-E from overcurrent/overvoltage, or opens from overheating. Often fails shorted.

Full reference for each part

MOSFETBJT (NPN / PNP)

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