Transistors

For 5 V logic and currents under 300 mA: 2N7002 (60 V, 300 mA, 2.4 Ω). For 3.3 V or 1.8 V logic where gate threshold matters: BSS138 (50 V, 200 mA, Vgs(th) 0.8-1.5 V). For currents above 500 mA or PWM above 100 kHz: SI2302 (20 V, 2.8 A, 65 mΩ). All three are SOT-23 and pin-compatible in most circuits.

A logic-level MOSFET has a gate threshold voltage (Vgs(th)) low enough to fully turn on at 3.3 V or 5 V gate drive, which is what a microcontroller GPIO outputs. Standard-level MOSFETs like the IRF540N need 10 V gate drive to reach their datasheet Rds(on). Driving a standard-level MOSFET from a 3.3 V MCU leaves it partially on, causing excessive heat and failure. Always check the Rds(on) spec at your actual gate voltage, not just at Vgs=10 V.

Yes, for small relays with coil current under 100 mA. Wire the relay coil between +V and collector, add a 1N4148 flyback diode across the coil, and drive the base with 1-5 mA through a 2-10 kΩ resistor from your MCU. For larger relays, use MMBT2222A (600 mA) or a MOSFET like 2N7002 or SI2302.

Moderately. SOT-23 MOSFETs and BJTs are cheap, so counterfeiters usually sell Asian clones remarked as major-vendor parts rather than outright fakes. We verify gate threshold distribution and Rds(on) on 30-pc samples from every lot — genuine Nexperia/onsemi parts show tight parameter spreads; clones show wide distributions.