Question

In a uniformly doped BJT, assume that N_E, N_B and N_C are the emitter, base and collector doping in atoms/cm³, respectively. If the emitter injection efficiency of the BJT is close to unity, which one of the following conditions is TRUE?
1. N_E = N_B = N_C
2. N_E ≫ N_B and N_B < N_C
3. N_E = N_B and N_B < N_C
4. N_E< N_B< N_C

Answer 1

Correct Answer - Option 2 : N_E ≫ N_B and N_B < N_C

The emitter injection efficiency is a measure of how efficient the emitter is in injecting electrons into the base.

It is important because the entire transistor action is due to the electrons injected into the base.

The holes, which are simultaneously injected into the emitter, are useless from this point of view.

In BJT the doping profile are related as-

N_E> N_C> N_B

Now, as emitter injection efficiency of the BJT is close to unity we can say

N_E≫ N_B

Thus, N_E≫ N_B and N_B < N_C