The p-n junction diodes can act as rectifiers because they

Analyze the question: We're asked why a p-n junction diode can work as a rectifier. A rectifier converts alternating current (AC), which flows in both directions, into direct current (DC), which flows in one direction only.

Explain the concept: A p-n junction diode is made by joining p-type material (holes are the majority carriers) to n-type material (electrons are the majority carriers). At the junction a depletion layer forms.

• Forward bias (p-side connected to the positive terminal, n-side to the negative): the depletion layer narrows, and current flows easily.
• Reverse bias (p-side connected to the negative terminal): the depletion layer widens, and almost no current flows (only a tiny leakage current).

Step-by-step solution:

1. A rectifier must let current pass in one direction only.
2. When AC is applied to a diode, during one half-cycle the diode is forward-biased and conducts.
3. During the other half-cycle it is reverse-biased and blocks the current.
4. The output is therefore a one-directional (pulsating DC) current.

Checking the options:

• A. Conducts when forward-biased. This is correct and is the property that allows current to pass in one direction.
• B. Conducts when reverse-biased. Wrong, because a reverse-biased diode blocks current.
• C. Blocks current when forward-biased. Wrong, because it is the opposite of what happens.
• D. Conducts in both directions. Wrong, because then it could not rectify at all.

*Answer: A. Strictly, rectification depends on the diode conducting when forward-biased *and blocking when reverse-biased. Among the options given, only A states a true property of the diode, so it is the correct choice.

Exam tip: Remember "Forward = Flows, Reverse = Resists." If an option says a diode conducts both ways, or conducts in reverse, eliminate it immediately. Also remember that a diode is a one-way valve for current.