For transistor action, which of the following statements are correctA…

The correct answer is Option (4) → B and D only

In transistor action the emitter injects a large number of carriers into the base, almost all of them cross the thin base, and the collector gathers them.

(A) Both the emitter junction as well as the collector junction are forward biased. — Incorrect. With both junctions forward biased the collector would also push carriers into the base, and the transistor would be in saturation, not in its active (amplifying) state.

(B) The base region must be very thin and lightly doped. — Correct. A thin, lightly doped base lets very few injected carriers recombine there, so most of them reach the collector.

(C) Base, emitter and collector should have same doping concentration. — Incorrect. The emitter is heavily doped, the base lightly doped and the collector moderately doped; equal doping would spoil the injection and collection of carriers.

(D) The emitter junction is forward and collector junction is reverse bias. — Correct. This is the active state: the forward-biased emitter junction injects carriers, and the reverse-biased collector junction sweeps them into the collector.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Summary – “… Transistor is an n-p-n or p-n-p junction device. The central block (thin and lightly doped) is called ‘Base’ while the other electrodes are ‘Emitter’ and ‘Collectors’. The emitter-base junction is forward biased while collector-base junction is reverse biased.”