A transistor will remain on as long as the base current is applied

A transistor is a three-terminal semiconductor device that amplifies or switches electronic signals and electrical power. It is composed of three layers of semiconductor material, namely the emitter, base, and collector. The base current is responsible for controlling the transistor's operation, and it plays a crucial role in determining whether the transistor is on or off.

Understanding transistor operation:
To understand why option A is the correct statement, let's first discuss the basic operation of a bipolar junction transistor (BJT). There are two types of BJTs: NPN (Negative-Positive-Negative) and PNP (Positive-Negative-Positive). For simplicity, we will focus on the NPN transistor.

1. Emitter-Base Junction: The emitter-base junction is forward-biased, meaning the voltage at the emitter is higher than the voltage at the base. This biasing condition allows current to flow from the emitter to the base.

2. Base-Collector Junction: The base-collector junction is reverse-biased, meaning the voltage at the collector is higher than the voltage at the base. This biasing condition prevents significant current flow from the collector to the base.

Transistor operating modes:
There are three operating modes of a transistor: cutoff, active, and saturation. The operating mode is determined by the biasing conditions and the current flowing through the transistor.

1. Cutoff Mode: In cutoff mode, the transistor is off, and there is no current flowing through the collector-emitter path. This occurs when the base-emitter junction is not forward-biased, and the base current is below a certain threshold.

2. Active Mode: In active mode, the transistor is on and can amplify signals. The base-emitter junction is forward-biased, and there is a sufficient base current to allow current flow from the emitter to the collector.

3. Saturation Mode: In saturation mode, the transistor is also on, but it is operating at its maximum current-carrying capability. Both the base-emitter and base-collector junctions are forward-biased, allowing maximum current flow from the emitter to the collector.

Base current and transistor operation:
The base current is crucial in controlling the transistor's operation. Without a sufficient base current, the transistor will not turn on, and it will remain in cutoff mode.

When a base current is applied to an NPN transistor, it causes the base-emitter junction to become forward-biased. This allows current to flow from the emitter to the base and triggers the transistor to enter active mode. As long as the base current is present, the transistor will remain on and allow current flow from the collector to the emitter.

If the base current is removed or falls below the threshold level, the transistor will turn off and enter cutoff mode. In this state, no current flows through the collector-emitter path.

Conclusion:
In summary, the correct statement is option A: "A transistor will remain on as long as the base current is applied." The base current is essential for turning on the transistor and allowing current flow from the collector to the emitter. Without a sufficient base current, the transistor will remain off in cutoff mode.