Choosing the Right FET: A Practical Selection Checklist

FET vs. BJT — Which is best for your circuit?

Key differences

• Control: FET = voltage-controlled (gate). BJT = current-controlled (base).
• Carrier type: FET = unipolar (electrons or holes). BJT = bipolar (electrons + holes).
• Input impedance: FET very high → minimal drive current. BJT low → needs base drive.
• Switching speed: FETs (MOSFETs) typically faster for switching; BJTs can be faster in some RF linear modes.
• On-state losses: Power MOSFETs have low Rds(on) at low voltages → low conduction loss; BJTs have VCE(sat) which can be higher.
• Linearity & gain: BJTs generally better linearity and higher small-signal gain — preferred for analog amplifiers.
• Thermal behavior: MOSFETs have positive temperature coefficient (easier to parallel). BJTs have negative Vbe coeff — risk of thermal runaway.
• Noise: FETs usually lower noise (good for low-noise front-ends).
• Voltage/current range: BJTs and IGBTs can be preferable at very high voltages/currents; MOSFETs dominate low-to-medium-voltage power switching and ICs.
• Packaging & cost: MOSFETs scale well in ICs; discrete BJTs can be cheaper for some low-power analog uses.

Practical selection guidelines

• Choose a MOSFET when:

  • You need high input impedance and low drive power (logic-level gate drive).
  • Fast switching (DC–DC converters, synchronous rectification, PWM power stages).
  • Parallel devices for high current (positive temp. coeff.).
  • Low-noise, low-voltage signal switching.

• Choose a BJT when:

  • You need superior linearity and current gain for analog amplifiers or certain RF stages.
  • The circuit tolerates base-drive current and benefits from BJT gain characteristics.
  • Very specific legacy or discrete high-current designs where BJT characteristics match requirements.

• Consider IGBT or power BJTs for very high-voltage, high-current power electronics (motor drives, inverters) where MOSFET Rds(on) would be impractical.

Quick checklist (practical)

• Driving signal available: voltage → FET; limited current budget → FET.
• Need linear amplification/low distortion → BJT.
• Switching at >100 kHz and efficiency matters → MOSFET.
• Very high voltage (>600 V) or heavy current at lower switching freq → IGBT/BJT.
• Paralleling devices? Prefer MOSFETs for simpler current sharing.

Short example decisions

• Battery-powered DC–DC converter (100 kHz–1 MHz): MOSFET.
• Small-signal audio preamp: BJT (or JFET input) for linearity.
• Motor inverter at hundreds of volts: IGBT or high-voltage MOSFET depending on frequency.

If you want, I can produce a one-page comparison table with device examples and typical part numbers for your voltage/current and switching-frequency targets.