Question

For a transistor to act as a switch, it must be operated in

• A. Active region
• B. Saturation state only
• C. Cut-off state only
• D. Saturation and cut-off state

Answer 1

The Correct Option is D

To understand how a transistor operates as a switch, we need to examine its behavior in different regions of operation. A transistor, specifically a Bipolar Junction Transistor (BJT), has three primary modes of operation: active, saturation, and cut-off.

1. Active Region: This is where the transistor operates as an amplifier. In this region, the base-emitter junction is forward-biased, and the collector-base junction is reverse-biased. This is not suitable for switching applications as the transistor is partially on, regulating a continuous range of output instead of switching.
2. Saturation State: In the saturation state, both the base-emitter and base-collector junctions are forward-biased. The transistor behaves like a closed switch, allowing maximum current to flow through it. Hence, this state is essential for conducting current from collector to emitter without restrictions.
3. Cut-off State: In the cut-off state, both the base-emitter and base-collector junctions are reverse-biased. The transistor acts like an open switch, effectively stopping current flow from collector to emitter, similar to an off state.

Conclusion: For a transistor to function as a switch, it needs to alternately move between the saturation and cut-off states. In the saturation state, the transistor is "on," allowing current to pass through, representing a closed circuit state. Conversely, in the cut-off state, the transistor is "off," preventing current flow, simulating an open circuit. Therefore, the correct answer is the Saturation and cut-off state.

• The options "Active region," "Saturation state only," and "Cut-off state only" are incorrect because they do not allow the transistor to switch between on and off states effectively.