Question

In case of a transistor, the central portion has to be :

Answer 1

Bipolar junction transistors (BJTs) are charge carriers that employ both electrons and electron holes. A unipolar transistor, such as a field-effect transistor, on the other hand, employs just one type of charge carrier. A bipolar transistor can control a much greater current running between two other terminals with a tiny current injected at one of its terminals, making it capable of amplification or switching.

Complete answer:
BJTs are made up of two junctions between two semiconductor kinds, n-type and p-type, which are areas inside a single crystal. The junctions may be created in a variety of ways, including altering the doping of the semiconductor material as it grows, depositing metal pellets to produce alloy junctions, and diffusing n-type and p-type doping chemicals into the crystal. Junction transistors quickly supplanted point-contact transistors because of their better predictability and performance. Diffused transistors, along with other components, are components of analogue and digital integrated circuits.
Based on the doping types of the three major terminal areas, BJTs are classified as PNP or NPN. Two semiconductor junctions share a narrow p-doped region in an NPN transistor, and two semiconductor junctions share a thin n-doped region in a PNP transistor. P-type means doped with impurities that give holes that readily receive electrons, whereas N-type means doped with impurities that provide mobile electrons. As a result, the centre part of a transistor can be N-type or P-type.
The diffusion of charge carriers across a junction between two areas of differing charge carrier concentration causes charge flow in a BJT. Emitter, base, and collector are the three areas of a BJT. To link to these areas, a discrete transistor needs three leads. In comparison to the other two layers, the emitter area is strongly doped, whereas the collector is doped less heavily than the base. BJTs are classed as minority-carrier devices because most of the collector current is due to the flow of charge carriers (electrons or holes) injected from a strongly doped emitter into the base, where they are minority carriers that diffuse toward the collector.

Note:
The base–emitter junction is forward-biased in normal operation, meaning the p-doped side of the junction is at a higher potential than the n-doped side, while the base–collector junction is reverse-biased. The equilibrium between thermally produced carriers and the repulsive electric field of the n-doped emitter depletion area is disrupted when forward bias is given to the base–emitter junction. Thermally excited electrons (in an NPN; holes in a PNP) can now inject into the base area from the emitter. These electrons spread through the base, from a high-concentration zone near the emitter to a low-concentration region near the collector.