Introduction:
A Bipolar Junction Transistor (BJT) can be used as a switch in various electronic circuits. When operating as a switch, the BJT needs to transition between two distinct states: on and off. To achieve this, it must operate in either the cut-off mode or the saturation mode.

Explanation:
The operation of a BJT as a switch depends on the biasing conditions applied to its base-emitter junction. Let's discuss the two modes of operation:

1. Cut-off Mode:
In the cut-off mode, the base-emitter junction is reverse-biased, and no current flows through the junction. As a result, the transistor is in the off state, and there is no current flowing from the collector to the emitter. This state is represented as "0" or "off" in digital circuits.

2. Saturation Mode:
In the saturation mode, the base-emitter junction is forward-biased, allowing current to flow through the junction. This forward biasing causes a large amount of charge carriers to be injected into the base region, resulting in a significant collector current. The transistor is in the on state, and there is a low resistance path between the collector and the emitter. This state is represented as "1" or "on" in digital circuits.

3. Active Mode:
The active mode is a region between the cut-off and saturation modes. In this mode, the base-emitter junction is forward-biased, but the transistor is not fully saturated. It operates as an amplifier, where a small change in the base current leads to a larger change in the collector current. However, when used as a switch, it is desirable for the transistor to be either fully on or fully off. Therefore, the active mode is not suitable for switch applications.

Conclusion:
To use a BJT as a switch, it must operate in either the cut-off mode or the saturation mode. The cut-off mode ensures that the transistor is completely off, while the saturation mode ensures that it is fully on. The active mode, although an amplifying region, is not suitable for switch applications. By choosing the appropriate biasing conditions, the BJT can be effectively used as a switch in various electronic circuits.