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GATE 2017 EC (EC2) – Question 20

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

An npn bipolar junction transistor is operating in the active region. If the reverse bias across the base–collector junction is increased, then

  1. the effective base width increases and common-emitter current gain increases
  2. the effective base width increases and common-emitter current gain decreases
  3. the effective base width decreases and common-emitter current gain increases
  4. the effective base width decreases and common-emitter current gain decreases

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Correct answer: (C) the effective base width decreases and common-emitter current gain increases

Explanation

Increasing base–collector reverse bias widens the depletion region. Its extension into the base reduces the effective neutral-base width: the Early effect.

A narrower neutral base allows a larger fraction of injected carriers to reach the collector instead of recombining in the base. Base current falls relative to collector current, so common-emitter current gain increases.

Thus the effective base width decreases while the gain increases, **C**.

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