I don't completely understand this but it does answer your question
Bipolar transistors are so named because they conduct by using both majority and
minority carriers. The three terminals of the BJT are named emitter, base and
collector. Two p-n junctions exist inside a BJT: the base/emitter junction and
base/collector junction. "The [BJT] is useful in amplifiers because the currents
at the emitter and collector are controllable by the relatively small base
current."
In an NPN transistor operating in the active region, the emitter-base junction
is forward biased, and electrons are injected into the base region. Because the
base is narrow, most of these electrons will diffuse into the reverse-biased
base-collector junction and be swept into the collector; perhaps one-hundredth
of the electrons will recombine in the base, which is the dominant mechanism in
the base current. By controlling the number of electrons that can leave the
base, the number of electrons entering the collector can be controlled.