Question

When NPN transistor is used as an amplifier then (A) electron moves from base to collector (B) hole travels from emitter to base (C) hole goes to emitter from base (D) electron goes to base from collector

Short answer

The correct answer is (A) electron moves from base to collector.

Step-by-step solution

Understand NPN Transistor Functionality

An NPN transistor is made up of two n-type (higher electron concentration) semiconductor layers separated by a thin p-type (higher hole concentration) semiconductor layer. The three regions of the transistor are called the emitter (n-type layer), base (p-type layer), and collector (n-type layer). When a small input signal is applied between the base-emitter junction, a current is formed, which in turn amplifies the output signal between the collector and emitter.

Analyze Option A

(A) electron moves from base to collector. In an NPN transistor, the majority charge carriers in the base are holes, and the majority charge carriers in the collector are electrons. When the transistor is being used as an amplifier, electrons from the emitter enter the base and are attracted by the positive potential at the collector. The base region is very thin, so most of these electrons travel across the base and are collected in the collector region. Therefore, Option A is correct.

Analyze Option B

(B) hole travels from emitter to base. In the NPN transistor, the emitter region is n-type material, which means it has more electrons than holes. The majority charge carrier movement in the emitter region is of electrons and not holes. So, this option is incorrect.

Analyze Option C

(C) hole goes to emitter from base. The majority charge carriers in the base region of an NPN transistor are holes, and the majority charge carriers in the emitter region are electrons. However, when the transistor is biased as an amplifier, the electrons from the emitter diffuse into the base and then move towards the collector. The movement of holes is negligible compared to the electron flow. So, this option is incorrect.

Analyze Option D

(D) electron goes to base from collector. This statement is contradictory to the correct functionality of an NPN transistor, as explained in the analysis of Option A. Electrons move from the emitter to the base, get attracted by the positive potential at the collector, and then move to the collector. Thus, this option is incorrect. Based on the above analysis, we can conclude: The correct answer is (A) electron moves from base to collector.