An NPN transistor is configured to amplify an audio signal. Which of the following statements is correct?

Explanation:

An NPN transistor is a type of bipolar junction transistor (BJT) that is widely used in amplification applications, including audio signal amplification. It consists of three regions: the emitter, base, and collector. In the active region of operation, the transistor amplifies input signals by controlling the flow of charge carriers between the emitter and collector, modulated by the base region.

To understand why option 3 is correct, let us analyze the functioning of the NPN transistor:

Structure and Working Principle:

• Emitter: The emitter is heavily doped and serves as the source of charge carriers (electrons in the case of NPN transistors). Its primary function is to inject electrons into the base region.
• Base: The base is lightly doped and thin. It serves as the control region, regulating the flow of electrons between the emitter and collector. Only a small fraction of electrons recombine with holes in the base.
• Collector: The collector is moderately doped and larger in area compared to the base and emitter. It collects the electrons that successfully pass through the base region, allowing the transistor to amplify the signal.

Amplification Process:

• When the emitter junction (base-emitter) is forward biased, electrons flow from the emitter into the base region. Since the base is thin and lightly doped, only a few electrons recombine with holes in the base.
• The majority of electrons in the base region diffuse into the collector due to the reverse bias of the collector junction (base-collector). This movement generates a larger current in the collector circuit compared to the base current.
• The ratio of collector current (Ic) to base current (Ib) is known as the current gain (β) of the transistor, which is typically high, enabling amplification.

Why Option 3 is Correct:

The statement "Very few charge carriers from the emitter recombine in the base region" accurately describes the operation of an NPN transistor in the active mode. The base region is intentionally designed to be thin and lightly doped so that only a small fraction of electrons injected by the emitter recombine with holes in the base. This design ensures that most of the electrons pass through to the collector, enabling efficient amplification of the input signal.

The minimal recombination in the base is a critical feature of the transistor's operation, as it allows the device to achieve high current gain and effectively amplify weak signals, such as audio signals.