Common base current gain ideal value
bipolar transistors operate by injecting carriers from the external base lead (a gold? bond wire) to the large-base contact region that shrinks down and down to become the very narrow BASE region that is sandwiched between collector and emitter.

The EMITTER injects carriers of opposite polarity, these carriers attracted to the BASE carriers and trying to intercept and annihilate them.

Fortunately most of the EMITTED carriers MISS and can gathered up in the COLLECTOR.

The hit/miss ratio is crucial to useful bipolar behavior.