The first junction on transistor consisting of two back to back P-N junction was invented in 1951 by William Shockley. Later new Bipolar Junction Transistor (BJT) were invented. Transistor has three doped regions forming two P-N junction between them. There are two types of transistors (i) P-N-P and (ii) N-P-N. Each transistor has three segments (i) Emitter (ii) Base (iii) Collector. The size and level of doping is different of all three segments. For connecting in the circuit three terminals of a transistor are available - emitter, base and collector and it can be connected in three configuration Common Emitter (CE), Common Base (CB) and Common Collector (CC). The transistor can be used as a device application depending on configuration used, the biasing done to the segments and operate in the region namely cut off, Active and Saturation. It can work as an amplifier, a switch and an oscillator too. Based on transistor knowledge give answer.
The heavily and lightly doped regions of a junction transistor are respectively:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Emitter and Base
The emitter has to inject a large number of majority carriers into the base, so it is heavily doped.
The base is very thin and lightly doped, so that only a few of the injected carriers recombine there and most of them reach the collector.
The collector is moderately doped and is the largest region, because it has to collect the carriers and dissipate heat.
So the heavily and lightly doped regions are the emitter and the base respectively.
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):
Transistor: Structure and Action – “… All the three segments of a transistor have different thickness and their doping levels are also different. … Emitter: This is the segment on one side of the transistor shown in Fig. 14.27(a). It is of moderate size and heavily doped. … Base: This is the central segment. It is very thin and lightly doped.”