For transistor action, which of the following statements are correct:
(A) The base region must be very thin and lightly doped
(B) Base emitter and collector regions should have same size but different doping concentration
(C) The collector junction is reverse biased and emitter junction is forward biased
(D) Base, emitter and collector regions should have different sizes, but same doping concentration
(E) Both the emitter junction as well as the collector junction are forward biased
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) only
Transistor action needs a thin, lightly doped base and the right biasing of the two junctions.
(A) The base region must be very thin and lightly doped — Correct. A thin, lightly doped base lets very few carriers recombine there, so most of them reach the collector.
(B) Base emitter and collector regions should have same size but different doping concentration — Incorrect. The three regions have different sizes: the base is the thinnest, the emitter is of moderate size and the collector is the largest.
(C) The collector junction is reverse biased and emitter junction is forward biased — Correct. This is the active region: the forward-biased emitter junction injects carriers and the reverse-biased collector junction collects them.
(D) Base, emitter and collector regions should have different sizes, but same doping concentration — Incorrect. The doping is different: the emitter is heavily doped, the base lightly doped and the collector moderately doped.
(E) Both the emitter junction as well as the collector junction are forward biased — Incorrect. With both junctions forward biased the transistor is in saturation, not in the active region.
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):
Exercises – …
