Question

Which of the following is correct, about doping in a transistor?

• A. Emitter is lightly doped, collector is heavily doped and base is moderately doped
• B. Emitter is lightly doped, collector is moderately doped and base is heavily doped
• C. Emitter is heavily doped, collector is lightly doped and base is moderately doped
• D. Emitter is heavily doped, collector is moderately doped and base is lightly doped

Answer 1

The correct option is D Emitter is heavily doped, collector is moderately doped and base is lightly doped

Emitter is always forward biased with respect to base, so that it can supply a large number of majority carriers.
The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base. The base is lightly doped and very thin, it passes most of the emitter injected charge carriers to the collector. The collector is moderately doped.
Note: The emitter diode is always forward biased whereas collector diode is always reverse biased.