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MOSFET vs JFET

MOSFET and JFET compared — what each is better at, and which one the job actually calls for.

A JFET's gate is a reverse-biased junction, so it conducts with zero gate voltage and is pinched off by driving the gate the other way — it is normally-on, and the gate must never be forward-biased. A MOSFET's gate is insulated, so it draws no steady current at all, and the common enhancement types are normally-off. JFETs survive where their low noise matters: amplifier front ends and analogue switches, not power switching.

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.

JFET

A junction FET: a normally-ON transistor whose channel is pinched off by reverse-biasing the gate. Prized for very high input impedance and low noise in analog front-ends.

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.

JFET

Idss(mA)
Drain current with gate shorted to source — the 'fully on' current.
Vgs(off) / pinch-off(V)
Gate voltage that turns the channel off (negative for N-ch).
Transconductance (gm)(mS)
Gain: how much Id changes per volt of Vgs.
Input capacitance(pF)
Matters at RF.

Types you will meet

MOSFET

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

JFET

  • N-channel
  • P-channel
  • Low-noise audio (J201, 2SK170 class)
  • RF (BF245 class)
  • Dual matched pairs

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.

JFET

Low-noise high-gm parts for audio preamps; check Idss range fits your bias plan. For a simple switch, a MOSFET is almost always easier.

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.

JFET

Gate junction blown by static or forward-biasing it hard; parameter drift makes circuits that relied on a specific Idss stop biasing correctly.

Full reference for each part

MOSFETJFET

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