Which device permits current to flow through it in only one direction?

• A. Capacitor
• B. Diode
• C. Inductor
• D. Resistor

Answer: (B)

A diode permits current to flow in only one direction because of its p-n junction. The depletion region at the junction acts as a barrier that blocks current when the device is reverse-biased (negative voltage on the anode side) and becomes easy for charge carriers to cross when it is forward-biased (positive voltage on the anode side). In forward bias, the barrier narrows, allowing carriers to pass and current to flow; in reverse bias, the barrier widens, preventing flow except for a tiny leakage current. This unidirectional behavior is why diodes are used for rectification, turning alternating signals into pulsating direct current. The other components don’t inherently enforce a single direction: capacitors pass current only while the voltage is changing and then block DC, inductors resist changes in current but don’t limit direction, and resistors conduct current in both directions proportional to the applied voltage.