Q.23 Physics ⭐ 360° Option Elimination Active

NEET 2021 Physics Question 23 Solution

Topic: Semiconductor Electronics: Materials, Devices and Simple Circuits · Official NTA Booklet Code M1

Question

Consider the following statements (A) and (B) and identify the correct answer. (A) A zener diode is connected in reverse bias, when used as a voltage regulator. (B) The potential barrier of p-n junction lies between 0.1 V to 0.3 V.

(1)(A) and (B) both are correct.
(2)(A) and (B) both are incorrect.
(3)(A) is correct and (B) is incorrect.
(4)(A) is incorrect but (B) is correct.

✓

Verified Against NTA Final Answer Key

Option (3) : (A) is correct and (B) is incorrect.

📚 Semiconductor Electronics: Materials, Devices and Simple Circuits Easy

📋 Given Parameters & Problem Setup

Statement (A) A zener diode is connected in reverse bias, when used as a voltage regulator.

Statement (B) The potential barrier of p-n junction lies between 0.1 V to 0.3 V.

⚡

The 30-Second Exam Hall Shortcut: ⚡ Zener Reverse Bias + Silicon 0.7 V Barrier Rule

⏱️ Save ~20 seconds

Statement (A) is TRUE: Zener diode operates strictly in reverse-breakdown region for voltage regulation. Statement (B) is FALSE: Silicon p-n junctions have a barrier potential of $\approx 0.7\text{ V}$ (Germanium is $\approx 0.3\text{ V}$ ), so the barrier lies typically between $0.3\text{ V}$ and $0.7\text{ V}$ , not $0.1\text{ V}$ to $0.3\text{ V}$ ! Match Option (3)!

1 Check Statement (A): Voltage regulator operation requires reverse bias in breakdown region -> True.

2 Check Statement (B): Claims barrier potential is 0.1 V to 0.3 V. But standard Si barrier is ~0.7 V and Ge is ~0.3 V -> False.

3 Thus, (A) is correct and (B) is incorrect.

4 Match Option (3).

🛡️

Why You Can 100% Trust This Shortcut: Canonical NCERT Semiconductor Electronics semiconductor device fundamentals.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Evaluation of Statement (A): Zener Diode Operation

A Zener diode is a heavily doped silicon p-n junction diode. When operated in the reverse breakdown region, the voltage across the diode ( $V_Z$ ) remains virtually constant over a wide range of reverse currents ( $I_Z$ ). Therefore, to regulate unregulated DC voltage, it is always connected in reverse bias across the load:

\text{Zener Voltage Regulator} \implies \mathbf{\text{Operates in Reverse Breakdown Bias (Statement A is CORRECT)}}

Step 2

Evaluation of Statement (B): p-n Junction Barrier Potential

In an unbiased p-n junction, diffusion of majority carriers creates a depletion layer with a built-in potential barrier $V_0$ . For Germanium, $V_0 \approx 0.3\text{ V}$ , and for Silicon (the ubiquitous material in modern electronics), $V_0 \approx 0.7\text{ V}$ . Thus, the barrier potential typically lies between $0.3\text{ V}$ and $0.7\text{ V}$ , not between $0.1\text{ V}$ and $0.3\text{ V}$ . Statement (B) is unambiguously incorrect.

Step 3

Conclusion

Statement (A) is correct and Statement (B) is incorrect. This corresponds uniquely to Option (3).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 1

✗ Incorrect

Verdict & Scientific Reason:

Statement (B) is false because Silicon p-n junctions have a barrier potential of ~0.7 V.

⚠️

Student Fallacy (Why candidates pick this trap): Believing 0.1 V to 0.3 V covers all semiconductor junctions.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Statement (A) is an undisputed textbook fact of semiconductor electronics.

⚠️

Student Fallacy (Why candidates pick this trap): Doubting whether Zener diodes operate in reverse bias.

3 3

✓ 100% Correct

Verdict & Scientific Reason:

Statement (A) accurately states reverse-bias operation of Zener regulators, while Statement (B) underestimates the barrier potential range.

💡

Why this is airtight: None; rigorous semiconductor physics knowledge.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Inverts the truth value of both statements.

⚠️

Student Fallacy (Why candidates pick this trap): Gross confusion regarding semiconductor diode physics.

⚠️

NTA Trap Alert: Germanium vs. Silicon Barrier Potential Trap

Students often recall that Germanium has a low barrier potential of $\approx 0.3\text{ V}$ and assume Statement (B) is true, forgetting that Silicon (the primary semiconductor in NCERT) has a barrier of $\approx 0.7\text{ V}$ .

🛡️ How to avoid the -1 mark: Remember: Silicon barrier potential is

\approx 0.7\text{ V}

. Any range ending at

0.3\text{ V}

is false for p-n junctions.

📖

Core NEET Syllabus Standard

NCERT Physics · Semiconductor Electronics: Materials, Devices and Simple Circuits Standard Benchmark

Core Scientific Principle Tested:

"A Zener diode is designed to operate continuously in the reverse breakdown region as a voltage regulator. The barrier potential of a silicon p-n junction is approximately 0.7 V."

Syllabus Focus: p-n Junction Diode and Zener Diode

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Physics Question 23 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Zener Reverse Bias + Silicon 0.7 V Barrier Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Evaluation of Statement (A): Zener Diode Operation

Evaluation of Statement (B): p-n Junction Barrier Potential

Conclusion

360° Option Elimination Engine Key Feature

NTA Trap Alert: Germanium vs. Silicon Barrier Potential Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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