For a bipolar junction transistor, which of the following statements are true?
Options:
(A) Doping concentration of emitter region is more than that in collector and base
region
(B) Only electrons participate in current conduction
(C) The current gain \beta depends on temperature
(D) Collector current is less than the emitter current
Official Answer Key:
A, C, D
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE PH 2022 Q.28)**
**1. Semiconductor Device Physics (BJT)**:
- A bipolar junction transistor consists of three doped semiconductor regions: Emitter, Base, and Collector.
**2. Step-by-Step Verification of BJT Properties**:
- **(A) [TRUE]**: To maximize emitter injection efficiency, the emitter region is the most heavily doped:
$N_E \gg N_B > N_C$
- **(B) [FALSE]**: The transistor is called *bipolar* precisely because **both electrons and holes** participate in the conduction mechanism.
- **(C) [TRUE]**: Current gain $\beta = I_C / I_B$ is strongly temperature-dependent due to temperature dependence of carrier lifetime, diffusion length, and thermal generation.
- **(D) [TRUE]**: By Kirchhoff's Current Law, $I_E = I_B + I_C$. Since base current $I_B > 0$, collector current is always slightly less than emitter current:
$I_C = \alpha I_E < I_E \quad (\alpha \approx 0.98 - 0.998)$
**3. Official Verification**:
The verified official IIT Master Answer is **Options (A, C, D)**.
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