In a bipolar junction transistor

In a Bipolar Junction Transistor (BJT), the three regions—emitter, base, and collector—are doped differently to optimize performance. The emitter is the most heavily doped region because its primary function is to inject a large number of majority charge carriers into the base [1]. The collector has an intermediate doping level, which is lower than the emitter but higher than the base [1]. The base is the middle region and is intentionally designed to be extremely thin and lightly doped [1]. This light doping in the base minimizes the recombination of charge carriers (electrons or holes) as they travel from the emitter to the collector, thereby ensuring a high current gain (beta) [1]. Consequently, the base has the least concentration of impurities among the three regions, making option 4 the correct choice.

Sources

1. [1] https://www.sathyabama.ac.in/sites/default/files/course-material/2020-10/UNIT-2_5.pdf