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In a bipolar junction transistor (BJT), what is the approximate barrier potential across the emitter–base junction for silicon devices at room temperature?

Difficulty: Easy

Correct Answer: a total of 0.7 V

Explanation:


Introduction / Context:
The barrier potential (also called built-in potential) is critical for understanding forward biasing of BJTs and diodes. Knowing its approximate value for silicon and germanium helps predict conduction onset voltages and biasing requirements.


Given Data / Assumptions:

  • Material: silicon.
  • Temperature: approximately 300 K.
  • Junction type: emitter–base junction of a BJT.


Concept / Approach:
For silicon junctions at room temperature, the built-in potential is about 0.7 V. This is the voltage required to overcome the depletion region barrier and allow significant current flow under forward bias. Germanium devices, by contrast, exhibit ~0.3 V.


Step-by-Step Solution:

Identify junction: emitter–base p–n junction.Recall built-in potential for Si ≈ 0.7 V, Ge ≈ 0.3 V.Therefore, total barrier potential = 0.7 V.


Verification / Alternative check:

Practical measurement: Si diode forward conduction starts around 0.6–0.7 V, matching theoretical barrier.


Why Other Options Are Wrong:

0: unrealistic, ignores depletion region.0.7 V across each layer: misleading; the total across depletion is ~0.7 V, not per layer.0.35 V: corresponds to Ge approximations, not Si.1 V: too high for typical Si devices.


Common Pitfalls:

Confusing Ge with Si; misunderstanding that 0.7 V is approximate, not exact.


Final Answer:

a total of 0.7 V
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