The GATE Grind

GATE 2017 EC – Question 21

Electronic Devices · P-N Junction and Semiconductor Devices · 1 mark · Multiple choice

For a narrow base PNP BJT, the excess minority carrier concentrations ($\Delta n_E$ for emitter, $\Delta p_B$ for base, $\Delta n_C$ for collector) normalized to equilibrium minority carrier concentrations ($n_{E0}$ for emitter, $p_{B0}$ for base, $n_{C0}$ 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?

Normalized excess carrier concentration against position. In the emitter (P) the excess is slightly negative. In the base (N) the excess hole concentration is $-1$ at the emitter side and rises to about $10^5$ at the collector side. In the collector (P) the excess electron concentration is positive near the junction and decays away.
  1. Forward active
  2. Saturation
  3. Inverse active
  4. Cutoff

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Show answer and explanation

Correct answer: (C) Inverse active

Explanation

At the emitter side of the base the excess is $-1$, which means the minority carrier concentration there is zero. This happens when the emitter-base junction is reverse biased. At the collector side the excess is very large and positive, which happens when the collector-base junction is forward biased. With the emitter junction reversed and the collector junction forward, the transistor is in the inverse active mode.