What is a diode?

• A. It stores energy magnetically
• B. It filters high-frequency signals
• C. It conducts current in both directions
• D. It conducts current in one direction

Answer: (D)

A diode is a device that allows current to flow in only one direction. This unidirectional behavior comes from the PN junction within the semiconductor: when the junction is forward biased, the barrier is lowered and carriers cross, letting current pass; when it is reverse biased, the depletion region widens and current is largely blocked, with only a tiny leakage until breakdown.

In practice, you also encounter a small forward voltage drop (about 0.7 V for silicon) before conduction starts, and a small leakage current in reverse. This combination—conducting readily in one direction and blocking in the opposite direction—defines what a diode does in circuits.

It’s not a device meant to store energy magnetically (that’s an inductor), nor is its primary role to filter signals (filters use capacitors and inductors). And while a perfect diode would block one direction, a real diode isn’t ideal, but the essential idea remains: it conducts current in one direction.