When a bipolar junction transistor is operating in the saturation mode, which one of the following statements is TRUE about the state of its collector-base (CB) and the base-emitter (BE) junctions?
Both the CB and BE junctions are forward biased.
BJT Saturation Mode: Junction Biasing Explained
A Bipolar Junction Transistor (BJT) is a fundamental semiconductor device used in electronic circuits for amplification and switching. It has three operating modes: Cut-off, Active, and Saturation. Understanding the biasing conditions of its two PN junctions, the Base-Emitter (BE) junction and the Collector-Base (CB) junction, is crucial for determining the mode of operation.
Understanding Saturation Mode
The saturation mode represents the state where the BJT is fully "ON". In this mode, the transistor acts like a closed switch, allowing the maximum possible current to flow between the collector and the emitter. This maximum current is typically limited by the external circuit components, such as the collector resistor and the supply voltage, rather than the transistor's characteristics.
Junction Biasing in Saturation
For a BJT to operate in the saturation mode, a specific biasing condition must be met for both the Base-Emitter (BE) junction and the Collector-Base (CB) junction:
- Base-Emitter (BE) Junction: This junction must be forward biased. This allows current to flow into the base terminal, enabling the transistor to turn on.
- Collector-Base (CB) Junction: This junction must also be forward biased. This is the key characteristic distinguishing saturation from the active mode.
When both the BE and CB junctions are forward biased, the transistor allows maximum current flow from collector to emitter. The collector current is no longer proportional to the base current (as it is in the active mode) but is instead limited by the external circuit.
Comparison of BJT Operating Modes
The biasing states of the junctions for the different operating modes are summarized below:
| Operating Mode | Base-Emitter (BE) Junction | Collector-Base (CB) Junction |
|---|---|---|
| Cut-off | Reverse Biased | Reverse Biased |
| Active | Forward Biased | Reverse Biased |
| Saturation | Forward Biased | Forward Biased |
Conclusion
Therefore, the statement that is TRUE when a BJT operates in saturation mode is that both the Collector-Base (CB) junction and the Base-Emitter (BE) junction are forward biased.
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