Describe the forward conduction threshold of a silicon diode and its I-V behavior in forward and reverse bias.

• A. In forward bias, current increases rapidly after approximately 0.6-0.7 V; exponential I-V; reverse bias yields small leakage until breakdown.
• B. In forward bias, current remains negligible until about 1.2 V; reverse leakage dominates.
• C. In forward bias, current rises linearly with voltage; reverse bias yields high leakage.
• D. In forward bias, current increases rapidly after approximately 0.2 V; reverse bias yields breakdown at 0.5 V.

Answer: (D)

Forward conduction in a silicon diode turns on around 0.6 to 0.7 volts. Below that, the depletion region blocks most current, so the current is essentially negligible. Once the forward voltage crosses that threshold, current rises very quickly because the diode follows an exponential relationship between current and voltage (I ≈ I_s(e^(V/(nV_T)) − 1)), which makes the device switch from off to on in a short span.

In reverse bias, the current is dominated by a small leakage current and remains tiny over a wide range of voltages. Only when the reverse voltage reaches the breakdown level does the current surge dramatically. The exact breakdown voltage depends on the diode type, but for ordinary silicon diodes it is typically well above 0.5 V, far higher than the simple small-leakage assumption.

The option describing a forward threshold of about 0.2 V and reverse breakdown at 0.5 V does not align with typical silicon diode behavior (0.2 V is more characteristic of germanium or some Schottky devices, and silicon diodes normally breakdown well above 0.5 V). The most accurate portrayal is that forward conduction starts around 0.6–0.7 V with exponential growth in forward current, and reverse bias yields only small leakage until a relatively high breakdown voltage.