Bipolar transistors

What is the main reason for making the emitter of an NPN transistor more heavily doped than the base?

  1. ATo increase emitter injection efficiency and the current gain
  2. BTo raise the collector breakdown voltage
  3. CTo increase the base resistance
Show answer and explanation

Answer: A. To increase emitter injection efficiency and the current gain

With a heavily doped emitter, almost all of the current across the base-emitter junction consists of electrons injected from the emitter into the base (high injection efficiency). The hole current flowing from the base into the emitter does not contribute to the collector current, so reducing it increases the current gain.

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More Bipolar transistors questions

  1. What is the approximate common-emitter current gain β of a bipolar transistor whose common-base current gain α is 0.99?
  2. Which correctly describes the Early effect in a bipolar transistor?
  3. Which is a characteristic of a Darlington connection of two transistors?
  4. In a PNP transistor, which carriers are injected from the emitter into the base and mainly carry the current?
  5. Which is a characteristic of a common-collector amplifier (emitter follower)?