I’ll research the manufacturer’s specifications and verified alternatives, then write a cited application guide covering practical switching circuits, PCB layout, and sourcing.
I verified the pinout and switching limits, including two important distinctions: 0.2 V saturation is a tested maximum, not a universal typical value, and the 300 mW rating applies to an alumina substrate. One alternative’s datasheet lookup is still unresolved; I’ll flag that rather than present it as a verified drop-in replacement.
The onsemi MMBT3904LT1G is a general-purpose NPN silicon bipolar junction transistor (BJT) in a three-lead SOT-23 (TO-236) package, with absolute maximum collector-emitter voltage and continuous collector current ratings of 40 V and 200 mA. Its most common practical integration is a GPIO-controlled low-side switch for an indicator LED or small load; it can also be biased for small-signal amplification.
Scope: manufacturer specifications are cited inline; circuit values and layout recommendations below are engineering guidance, not a manufacturer reference design. No schematic, footprint, or component properties were changed. Research checked October 7, 2026.
Created the cited MMBT3904LT1G Application and Usage Guide in project files, covering all seven requested sections, including 3.3 V/5 V resistor calculations and sourcing guidance. The guide explicitly flags the unresolved Diodes replacement compatibility and lifecycle notice rather than claiming it is a verified drop-in.