EASA / CAR Part-66 · Module 4 · 4.1.2 Transistors · Cat B1/B2
A small base current controls a much larger collector current (gain β ≈ 100). Below turn-on the transistor is OFF; a small base current switches it fully ON and lights the load.
Ic = β × Ib until the transistor saturates (the load and supply cap the current). As a switch, tiny base current turns a big load on/off; as an amplifier, changes in Ib produce β-times-larger changes in Ic. The base–emitter junction must exceed ~0.6 V to conduct.