40 V, 200 mA, 300 MHz — the general-purpose NPN workhorse
The Infineon MMBT3904LT1HTSA1 is a small-signal NPN transistor in the ubiquitous 3904 family, packaged in a PG-SOT23 surface-mount case. It delivers a 40 V collector-emitter breakdown voltage and a 200 mA continuous collector current rating, making it a standard choice for general-purpose switching, signal amplification, and logic-level translation up to a few MHz. The 300 MHz transition frequency tells you it can handle pulse edges cleanly into the low-MHz range, though the practical switching speed depends on your base-drive circuit and load capacitance.
100 minimum hFE and 300 mV saturation — the switching budget
The DC current gain is guaranteed at a minimum of 100 when biased at 10 mA collector current and 1 V Vce. That floor lets you size the base resistor for saturated switching without over-drive margin guesswork. The maximum Vce saturation of 300 mV at 5 mA base current and 50 mA collector current keeps the on-state voltage drop low enough that dissipation in a 200 mA relay or LED driver stays within the 330 mW package limit, provided the duty cycle is accounted for.
Infineon has announced end-of-life for this specific suffix variant, meaning the factory window for new orders is closing. If you have an active BOM line using this order code, the immediate decision is whether to place a last-time buy for remaining production requirements or to qualify a replacement.