80 V, 1.5 A NPN — medium-voltage driver staple
The BD139-16 is an NPN epitaxial planar transistor in a TO-225AA (SOT-32-3) through-hole package, rated for 80 V collector-emitter breakdown and 1.5 A continuous collector current. The 1.25 W power dissipation ceiling in this package sets the thermal budget for linear regulator pass elements, relay drivers, and medium-speed switching up to a few hundred milliamps continuous.
Minimum DC current gain of 40 at Ic=150 mA, Vce=2 V means the transistor needs at least 3.75 mA base drive to saturate at that collector current. The Vce(sat) max of 500 mV at 500 mA gives a conduction loss of 250 mW at that point — within the 1.25 W Pd budget but requiring a heatsink if continuous. The 100 nA collector cutoff current is low enough for battery-powered circuits that need the load fully off in the off-state.