In semiconductor devices (e.g., a silicon PN junction or each PN junction in a silicon BJT) at room temperature, what is the typical built-in barrier potential across the depletion region?

Introduction / Context:
The barrier potential (also called built-in potential) is the internal electric potential that forms across the depletion region of a PN junction when P-type and N-type materials are brought together. Knowing its approximate value in common materials such as silicon is essential for understanding diode conduction thresholds, BJT base-emitter voltage, and rectifier behavior in power and signal circuits.

Given Data / Assumptions:

• Material considered: silicon.
• Condition: room temperature (around 25 °C).
• Device examples: silicon diode, each PN junction in a silicon BJT (emitter–base and collector–base).

Concept / Approach:
The built-in potential arises from diffusion of majority carriers and the resulting space charge region that establishes an opposing electric field. Its value depends on temperature, doping levels, and material properties. For practical electronics education and design calculations, standard reference values are used: roughly 0.7 V for silicon and about 0.3 V for germanium at room temperature. This is consistent with the typical forward conduction “knee” seen in silicon diodes and the base-emitter voltage V_BE in BJTs biased in the forward-active region.

Step-by-Step Solution:
Identify the semiconductor material: silicon.Recall typical barrier potential at room temperature for silicon: approximately 0.7 V.Match this standard value to the closest listed option.Select 0.7 V as the correct answer.

Verification / Alternative check:
Design handbooks and device datasheets consistently use V_BE ≈ 0.7 V as a rule of thumb for silicon BJTs at moderate currents and room temperature. Similarly, silicon rectifier diodes exhibit about 0.7 V forward drop near their rated current, reflecting the same underlying physics of the PN junction barrier and series effects.

Why Other Options Are Wrong:
0 is impossible because a depletion region always has a built-in field in equilibrium. 0.8 V can occur under some doping/temperature conditions but the widely accepted nominal at room temperature is 0.7 V. 20 V is several orders of magnitude too large and pertains to breakdown or external bias, not the built-in potential. “None of the above” is invalid because 0.7 V is correct.

Common Pitfalls:
Confusing barrier potential with forward drop under load (which includes series resistance and current dependence); forgetting that temperature and current slightly shift V_BE; mixing germanium and silicon values.

Final Answer:
0.7 V