For transistor action, which of the following statements are correct?
A. Base, Emitter and Collector region have similar size and doping concentrations.
B. The base must be very thin and lightly doped.
C. The emitter junction is forward biased and the collector junction is reverse biased.
D. Both emitter and collector junction are forward biased.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B and C
For transistor action the emitter must inject a large number of charge carriers into the base, almost all of which must cross on to the collector.
(A) Base, Emitter and Collector region have similar size and doping concentrations. — Incorrect. The three regions differ: the emitter is heavily doped and of moderate size, the base is very thin and lightly doped, and the collector is moderately doped and larger.
(B) The base must be very thin and lightly doped. — Correct. Few carriers then recombine in the base, so most of them reach the collector.
(C) The emitter junction is forward biased and the collector junction is reverse biased. — Correct. This is the biasing of the active region, used for amplification.
(D) Both emitter and collector junction are forward biased. — Incorrect. With both junctions forward biased the transistor is in saturation and acts as a closed switch, not as an amplifier.
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 – “… 18. 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.”