Transistor Action

NCERT explains that in an npn transistor operating in the active region the emitter-base junction is forward biased and the collector-base junction is reverse biased. The emitter injects a large number of electrons into the base, and because the base is made very thin and lightly doped, very few of these electrons find holes to recombine with there. Consequently the vast majority diffuse across the narrow base and are swept into the collector by its reverse-biased field, making the collector current nearly equal to the emitter current and the base current very small. The option that the base carries the largest share of emitter current is wrong because a thin lightly doped base deliberately keeps the base current tiny. The option that the collector junction becomes forward biased is wrong because active-region amplification requires the collector junction to be reverse biased. The option that emitter and collector currents become exactly equal is wrong because they differ by the small base current, so they are nearly but not exactly equal. As stated in NCERT Class 12, Chapter 14, the thin base ensures efficient carrier transfer to the collector. A consistency check confirms $I_E=I_B+I_C$ with $I_C\approx I_E$.