Step 1: Purpose of the emitter
The emitter injects charge carriers (electrons or holes) into the base region.
Step 2: Doping rationale
To ensure a high injection efficiency, the emitter is heavily doped so that it can provide a large number of charge carriers.
Step 3: Comparison
Base is lightly doped, collector is moderately doped. The doping gradient supports unidirectional carrier flow.
Conclusion: The emitter is heavily doped to efficiently inject carriers into the base.