In a Bipolar Junction Transistor (BJT), the three regions have different sizes and doping concentrations to ensure proper functioning:

1. Emitter: Heavily doped (to inject a large number of charge carriers) and moderate in size.
2. Base: Very thin and lightly doped (so that very few majority charge carriers recombine here, allowing most to pass through to the collector).
3. Collector: Moderately doped and largest in size (to efficiently collect charge carriers and dissipate heat). Additionally, for normal transistor action (active region), the emitter-base junction is forward-biased, and the collector-base junction is reverse-biased.