When a bipolar junction transistor is operating in the saturation mode...
Bipolar Junction Transistor in Saturation Mode
When a bipolar junction transistor (BJT) is operating in the saturation mode, the following statements about the state of its collector-base (CB) and base-emitter (BE) junctions are true:
Statement
Both the CB and BE junctions are forward biased.
Explanation
To understand why this statement is true, let's first review the operation of a BJT and the different modes in which it can operate.
Bipolar Junction Transistor (BJT)
A bipolar junction transistor is a three-layer semiconductor device that consists of two p-n junctions. It is commonly used as an amplifier or a switch in electronic circuits. The three layers are called the emitter (E), base (B), and collector (C).
BJT Operation Modes
A BJT can operate in three different modes: active mode, cutoff mode, and saturation mode.
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Active Mode: In the active mode, the emitter-base (EB) junction is forward biased, and the collector-base (CB) junction is reverse biased. This allows the BJT to amplify signals.
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Cutoff Mode: In the cutoff mode, both the EB and CB junctions are reverse biased, and no current flows through the transistor.
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Saturation Mode: In the saturation mode, both the EB and CB junctions are forward biased. This allows a maximum current to flow through the transistor.
Saturation Mode Operation
When a BJT is in saturation mode, it is fully on and acts like a closed switch. The forward biasing of both the EB and CB junctions allows a high current to flow from the collector to the emitter.
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Collector-Base (CB) Junction: In saturation mode, the CB junction is forward biased. This means that the collector terminal (C) is positive with respect to the base terminal (B), allowing current to flow freely through the CB junction.
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Base-Emitter (BE) Junction: In saturation mode, the BE junction is also forward biased. This means that the base terminal (B) is positive with respect to the emitter terminal (E), allowing current to flow freely through the BE junction.
Summary
In summary, when a bipolar junction transistor is operating in the saturation mode, both the collector-base (CB) and base-emitter (BE) junctions are forward biased. This allows a maximum current to flow through the transistor, making it act like a closed switch.