P–N junction behavior as a switch A p–n junction diode behaves like a nearly closed (ON) switch under which condition?

Introduction / Context:
A p–n junction diode can act like an electronic switch. Understanding when it mimics a closed (conducting) switch is foundational for rectifiers, clippers, logic clamps, and protection circuits. The ON condition corresponds to a very small dynamic resistance and a collapsed depletion region under forward bias.

Given Data / Assumptions:

• Standard silicon or germanium p–n junction.
• Room-temperature behavior, modest currents.
• Forward bias reduces junction resistance; reverse bias increases it drastically.

Concept / Approach:
In forward bias above the knee (approximately 0.7 V for silicon, 0.3 V for germanium), majority carriers cross the junction, the depletion region narrows, and the junction exhibits low incremental (dynamic) resistance. In reverse bias, the depletion region widens and current is essentially limited to leakage, so it behaves like an open switch.

Step-by-Step Solution:
Identify ON state: diode forward biased beyond barrier potential.Physical effect: depletion width shrinks, carrier injection rises.Electrical effect: junction dynamic resistance becomes low (small-signal r_d decreases as current increases).Conclusion: a low junction resistance corresponds to a closed-switch behavior.

Verification / Alternative check:
IV curve of a diode shows exponential increase in current with voltage in forward bias; slope dI/dV is large, so r_d = dV/dI is small. This is the signature of a low resistance path, analogous to a closed switch.

Why Other Options Are Wrong:
Is reverse biased: behaves like an open switch (very high resistance).Cannot overcome its barrier voltage: still OFF, not closed.Has a wide depletion region: indicates reverse or weak forward bias, hence high resistance.

Common Pitfalls:
Confusing the fixed barrier potential with a hard threshold; real diodes conduct gradually and have series resistance and temperature dependence.

Final Answer:
has a low junction resistance