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 three regions of a transistor differ in size and doping because each has a different job.
Emitter: moderate size, heavily doped
Base: very thin, lightly doped
Collector: largest, moderately doped
The emitter must supply a large number of majority carriers, so it is heavily doped. The base must let almost all of these carriers pass through to the collector without recombining, so it is kept thin and lightly doped. Hence the heavily doped region is the emitter and the lightly doped region is the base.
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.”