Explanation:

Transistors are three-layered semiconductor devices that are used as amplifiers or switches in electronic circuits. An n-p-n transistor consists of two n-type semiconductors and one p-type semiconductor. The p-type semiconductor is sandwiched between two n-type semiconductors. The three layers are called emitter, base, and collector.

In an n-p-n transistor, the p-type crystal acts as the base. The emitter and collector are made up of n-type semiconductors. When a voltage is applied across the base-emitter junction, a current flows through the transistor. The amount of current that flows through the transistor depends on the voltage applied across the base-emitter junction.

The base region is very thin compared to the emitter and collector regions. The base region is lightly doped compared to the heavily doped emitter and collector regions. The base-emitter junction is forward-biased, and the base-collector junction is reverse-biased.

When a voltage is applied across the base-emitter junction, a large number of electrons are injected from the emitter into the base. These electrons combine with the holes in the base region. The base region is very thin, so the electrons and holes recombine quickly, producing a large number of minority carriers. These minority carriers diffuse towards the base-collector junction and are swept across the junction by the reverse-biased electric field. The majority carriers (electrons) flow from the collector to the emitter, producing a large current gain.

Conclusion:

Therefore, the p-type crystal acts as the base only in an n-p-n transistor. The emitter and collector are made up of n-type semiconductors. When a voltage is applied across the base-emitter junction, a large number of electrons are injected from the emitter into the base, which produces a large current gain.