Obsolete NPN general-purpose switch
The STMicroelectronics MMBT2222A is an NPN general-purpose switching transistor in the SOT-23-3 surface-mount package, rated for 40 V collector-emitter breakdown and 600 mA continuous collector current.
The 270 MHz transition frequency (fT) means it handles switching speeds into the low-MHz range without excessive storage time, though the 350 mW power dissipation ceiling in the SOT-23 package limits continuous conduction to well below the 600 mA peak rating — derate for ambient temperature and duty cycle. Vce saturation is specified at 1 V maximum with 50 mA base drive and 500 mA collector current — a useful figure for estimating on-state voltage drop in a saturated switch.
Package and compliance
The SOT-23-3 footprint (also known as TO-236-3 or SC-59) is a standard small-signal transistor land pattern — the same footprint used by hundreds of NPN and PNP devices, so board spins for alternative parts are minimal.
Sourcing an obsolete part
Because STMicroelectronics has marked the MMBT2222A obsolete, the primary supply channel is independent distribution — surplus and new-old-stock inventory held by brokers and specialty houses.