Question

The pn junction diode acts as a rectifier because:

• A. it conducts in both directions
• B. it blocks current in reverse bias
• C. it allows ac to pass
• D. it amplifies the signal
• E. it offers a phase difference between voltage and current

Hint:

A diode acts like a one-way valve for current: forward → ON, reverse → OFF.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The key property needed for rectification is allowing current to flow easily in one direction while blocking it in the opposite direction.
Step 2: Detailed Explanation:
A pn junction diode is a semiconductor device that exhibits this one-way conduction property.

Forward Bias: When the positive terminal of a voltage source is connected to the p-type material and the negative terminal to the n-type material, the diode is forward-biased. The depletion region narrows, the potential barrier is overcome, and a large current flows through the diode with very low resistance.

Reverse Bias: When the polarity is reversed (negative terminal to p-type, positive to n-type), the diode is reverse-biased. The depletion region widens, increasing the potential barrier. This results in a very high resistance, and only a very small leakage current can flow. Essentially, the diode blocks the current in the reverse direction.

This behavior of conducting in one direction (forward bias) and blocking in the other (reverse bias) is precisely what is required for rectification.
Let's analyze the options:
(A) it conducts in both directions: This describes a resistor, not a diode for rectification.
(B) it blocks current in reverse bias: This, combined with its ability to conduct in forward bias, allows it to act as a rectifier. This is the correct reason.
(C) it allows ac to pass: A rectifier is meant to block one half of the AC cycle, not allow the entire AC to pass.
(D) it amplifies the signal: Amplification is the function of devices like transistors, not diodes.
(E) it offers a phase difference between voltage and current: This describes reactive components like capacitors or inductors.
Step 3: Final Answer:
The pn junction diode's ability to block current under reverse bias while allowing it under forward bias enables it to function as a rectifier.