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Transistors & MOSFETs

Active switches and amplifiers. On digital boards they're mostly used as switches β€” turning power or loads on and off.

BJT vs MOSFET​

BJTMOSFET
Controlled byCurrent (base)Voltage (gate)
On-stateVce(sat) dropLow Rds(on) resistance
Use forSmall signal switching, level shiftPower switching, load switches

For switching power, MOSFETs win (low loss). N-channel for low-side, P-channel (or a high- side driver) for high-side.

Common MOSFET circuits​

  • Low-side switch β€” N-FET between load and GND; MCU drives the gate (add a gate resistor and a gate pull-down so it's off at boot).
  • High-side / load switch β€” P-FET (or a dedicated load-switch IC) to gate a rail cleanly, often with controlled inrush.
  • Reverse-polarity protection β€” a P-FET in the supply, lower loss than a diode.
  • Level shifting β€” a small N-FET + two pull-ups bidirectionally shifts IΒ²C between voltage domains.

Gate-drive basics​

  • The gate is capacitive β€” driving it fast needs current. For power switching use a proper gate driver; keep the gate loop short (see Power Electronics).
  • Always define the gate at power-on (pull-down/up) so the FET isn't randomly on.
Logic-level gates

A normal MOSFET may need 10 V on the gate to fully turn on. For 3.3 V/5 V MCU control, pick a logic-level MOSFET (full enhancement at ~2.5–4.5 V) or it'll run hot and partially on.

See also​