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.

Key concepts

Transistor Amplifier

An NPN transistor is a fundamental building block in electronics. It acts as an amplifier, meaning it can increase the power of a weak signal. This occurs when a small current in its base-emitter junction allows a much larger current to flow between its collector and emitter.

When used as an amplifier, NPN transistors amplify input signals by utilizing the movement of charge carriers. These tiny semiconductor devices can amplify electrical signals, which is essential in applications like audio devices and communication systems.

Here’s how it typically works:

• A small input current at the base is applied.
• This opens the path for a larger current to pass from the collector to the emitter.
• Resulting in a much larger output signal compared to the input.

By mastering how transistors amplify signals, you can unlock numerous possibilities in electronic circuit design.

Charge Carriers

Charge carriers are pivotal in the operation of NPN transistors as amplifiers. In solid-state physics, charge carriers refer to particles that can carry electric charge within a material. These can be electrons or holes.

For an NPN transistor:

• Electrons: These are the primary charge carriers, especially in the n-type regions like the emitter and collector.
• Holes: These are minority carriers in the base region, which is a thin p-type layer.

The flow of these charge carriers results in the amplification of the input signal. The base region, although thin, allows electrons to pass through with minimal resistance, enabling efficient charge flow from the emitter to the collector.

Emitter-Base Junction

The emitter-base junction is a critical area in an NPN transistor. This juncture is where the input signal is applied, activating the transistor's amplifying action.

The junction consists of:

• Emitter: An n-type layer rich in electrons.
• Base: A p-type layer, less than one-tenth the thickness of the emitter.

In operation, the emitter-base junction is forward-biased, meaning electrons flow from the emitter into the base. This flow is essential for creating the base current, a small current that initiates the larger collector current.

Once an external input voltage surpasses a certain threshold, electrons from the emitter are injected into the base, moving towards the collector, defining the transistor's amplifying behavior.

Collector Region

The collector region of an NPN transistor plays a crucial role in signal amplification. It is designed to gather the major flow of electrons that have crossed through the base from the emitter.

This region is constructed as follows:

• Collector: It is an n-type layer, like the emitter, but it has a much larger area and can handle high voltages.

In the amplification process, the collector region is reverse-biased. This setup creates an electrical field that attracts electrons from the base. These attracted electrons move swiftly due to the collector’s positive potential, enhancing the signal's strength.

The collector region's function is to effectively channel this electron flow, which results in the transistor output, crucial for driving various electronic circuits.