GATE EC 2017 Set 1 — Question 11
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Electronic Devices → BJT → Biasing Regions
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Question
For a narrow base PNP BJT, the excess minority carrier concentrations ( for emitter, for base, for collector) normalized to equilibrium minority carrier concentrations ( for emitter, for base, for collector) in the quasi-neutral emitter, base and collector regions are shown below. Which one of the following biasing modes is the transistor operating in?
Correct answer
(C) Inverse active
Solution
Let's analyze the given graph for a PNP BJT:
1.Emitter Region (P-type): The minority carriers are electrons (). The graph shows is negative at the emitter-base junction. A negative excess minority carrier concentration implies reverse bias at the emitter-base (EB) junction. In a PNP transistor, for the EB junction to be reverse biased, the emitter is positive with respect to the base.
2.Collector Region (P-type): The minority carriers are electrons (). The graph shows is positive at the collector-base junction. A positive excess minority carrier concentration implies forward bias at the collector-base (CB) junction. In a PNP transistor, for the CB junction to be forward biased, the collector is negative with respect to the base.
Let's summarize the junction biases:- Emitter-Base (EB) junction: Reverse biased
- Collector-Base (CB) junction: Forward biased
- Forward Active: EB junction forward biased, CB junction reverse biased.
- Saturation: EB junction forward biased, CB junction forward biased.
- Inverse Active (Reverse Active): EB junction reverse biased, CB junction forward biased.
- Cutoff: EB junction reverse biased, CB junction reverse biased.
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