Introduction / Context:
Emitter efficiency is critical to BJT performance. In a p–n–p transistor, the emitter is p-type and is heavily doped to inject holes efficiently into the base. Understanding which component of emitter current dominates clarifies why high emitter efficiency leads to high current gain and good transistor action.
Given Data / Assumptions:
- Device type: p–n–p BJT with p-type emitter, n-type base, p-type collector.
- Typical design: emitter heavily doped; base lightly doped and very thin.
- Active region biasing: emitter–base junction forward biased; collector–base junction reverse biased.
Concept / Approach:
In a p–n–p transistor, the main injection from the emitter into the base is of holes (majority carriers of the p-emitter). Some counter-injection of electrons from the n-base back into the emitter exists, but good design ensures it is much smaller. Hence IEp (holes) is much larger than IEn (electrons), making emitter efficiency close to unity.
Step-by-Step Solution:
Recognize emitter efficiency γ ≈ IEp / IE and is designed to be high.Heavily dope the emitter (p-type) to maximize hole injection; lightly dope the base to minimize electron back-injection.Thus IEp >> IEn under normal active operation.
Verification / Alternative check:
Mirror the logic for n–p–n devices: there electron injection dominates (from n-emitter), reinforcing the general rule that the emitter injects its majority carriers predominantly.
Why Other Options Are Wrong:
(a) “Almost equal” contradicts the purpose of heavy emitter doping and thin, lightly doped base.(c) Reverses the actual dominance.(d) Ambiguous; correct device physics picks a clear dominance.(e) IEp cannot be ≈ 0 because holes from the p-emitter are the very cause of conduction.
Common Pitfalls:
Confusing minority vs majority injection and ignoring doping profiles.
Final Answer:
IEp >> IEn
Discussion & Comments