Q.42 Physics ⭐ 360° Option Elimination Active

NEET 2026 Physics Question 42 Solution

Topic: Semiconductor Electronics · Official NTA Booklet Code 50

Question

Three identical p-n junction diodes D₁, D₂ and D₃ are connected across a battery as shown in the figure. If the width of the depletion regions of D₁, D₂ and D₃ are W₁, W₂ and W₃, respectively, then the correct option is:

✓

Verified Against NTA Final Answer Key

Option (3) : W_3 > W_2 > W_1

📚 Semiconductor Electronics Medium

📋 Given Parameters & Problem Setup

Diodes Three identical p-n junction diodes D_1, D_2, D_3

Battery 5 V DC supply (left rail is positive +, right rail is negative -)

Branch 1 (D_1) Anode (p) connected to (+), cathode (n) connected to (-) ==> FORWARD BIASED

Branch 2 (D_2) Circuit has an open gap in the wire ==> UNBIASED / OPEN CIRCUIT

Branch 3 (D_3) Anode (p) connected to (-), cathode (n) connected to (+) ==> REVERSE BIASED

Depletion Widths W_1, W_2, W_3

⚡

The 30-Second Exam Hall Shortcut: ⚡ Bias-to-Depletion Width Master Rule

⏱️ Solves purely on qualitative inspection without equations.

Forward bias COMPRESSES depletion layer; Reverse bias EXPANDS it!

1 Inspect Diode D_1 (top): p-side to (+), n-side to (-). Forward biased! Forward bias opposes built-in potential, pushing majority carriers into the junction ==> W_1 decreases (W_1 < W_unbiased).

2 Inspect Diode D_2 (middle): Notice the OPEN GAP in the wire! No current flows, no external voltage appears across D_2. It is strictly UNBIASED ==> W_2 = W_unbiased.

3 Inspect Diode D_3 (bottom): Diode is reversed (p-side to (-), n-side to (+)). Reverse biased! Reverse bias pulls majority carriers away from the junction, exposing more immobile ions ==> W_3 increases (W_3 > W_unbiased).

4 Ordering is immediate: W_3 (reverse) > W_2 (unbiased) > W_1 (forward).

5 Matches Option (3) in 15 seconds!

🛡️

Why You Can 100% Trust This Shortcut: Depletion width formula: W = W_0 * sqrt(1 - V_applied / V_bi). For forward bias (V > 0), W shrinks. For zero bias (V = 0), W = W_0. For reverse bias (V < 0), W expands.

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Analyze Operating State of Diode D_1

Examine the connections of top branch containing diode D_1:

V_{\text{applied}} > 0 \implies \text{Forward Biased} \implies W_1 < W_0

• Positive terminal of 5 V battery is connected to p-type anode; negative terminal to n-type cathode.
• External electric field opposes the built-in barrier field, narrowing the depletion region.

Step 2

Analyze Operating State of Diode D_2

Examine the middle branch containing diode D_2:

V_{\text{applied}} = 0 \implies \text{Unbiased (Open Circuit)} \implies W_2 = W_0

• Careful inspection of the circuit crop reveals an open gap in the lead after D_2.
• Since the branch is open-circuited, zero current flows and zero voltage drops across the junction.
• Depletion width W_2 equals the natural thermal equilibrium width W_0.

Step 3

Analyze Operating State of Diode D_3

Examine the bottom branch containing diode D_3:

V_{\text{applied}} < 0 \implies \text{Reverse Biased} \implies W_3 > W_0

• Diode D_3 is connected with p-type anode to negative rail and n-type cathode to positive rail.
• Applied reverse bias aids the built-in barrier field, widening the space charge layer.

Step 4

Establish the Complete Inequality Ordering

Combine all three regimes:

W_1 < W_0 < W_3 \iff W_3 > W_2 > W_1

• Matches Option (3) exactly.

⭐

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 Option (1)

✗ Incorrect

Verdict & Scientific Reason:

Completely inverted relationship.

⚠️

Student Fallacy (Why candidates pick this trap): Assumes forward bias expands depletion width.

Note: Forward bias narrows the barrier; reverse bias widens it.

2 Option (2)

✗ Incorrect

Verdict & Scientific Reason:

Equates forward and reverse widths.

⚠️

Student Fallacy (Why candidates pick this trap): Believes any applied voltage widens the depletion layer equally.

Note: Forward and reverse biases produce opposite physical effects on space charge.

3 Option (3)

✓ 100% Correct

Verdict & Scientific Reason:

100% scientifically verified; confirmed by Official NTA Final Answer Key.

💡

Why this is airtight: Flawless identification of forward, open-circuit, and reverse bias regimes.

Note: Exact semiconductor physics.

4 Option (4)

✗ Incorrect

Verdict & Scientific Reason:

Unbiased maximum fallacy.

⚠️

Student Fallacy (Why candidates pick this trap): Believes unbiased diode has the widest depletion layer.

Note: Reverse bias creates a much wider depletion layer than equilibrium.

⚠️

NTA Trap Alert: Missing the Open Gap in Branch 2

Students who fail to notice the broken wire in branch 2 assume D_2 is forward biased just like D_1, marking W_1 = W_2 and getting confused.

🛡️ How to avoid the -1 mark: Always inspect schematic circuit wires closely for open breaks or gaps!

📖

Core NEET Syllabus Standard

NEET UG Physics Core Standards · Semiconductor Electronics: Materials, Devices and Simple Circuits Standard Benchmark

Core Scientific Principle Tested:

"Depletion layer width decreases under forward bias (W < W_0) and increases under reverse bias (W > W_0). For an unbiased/open diode, W = W_0."

Syllabus Focus: p-n Junction Diode Under Forward and Reverse Bias

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2026 Physics Question 42 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Bias-to-Depletion Width Master Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Analyze Operating State of Diode D_1

Analyze Operating State of Diode D_2

Analyze Operating State of Diode D_3

Establish the Complete Inequality Ordering

360° Option Elimination Engine Key Feature

NTA Trap Alert: Missing the Open Gap in Branch 2

Core NEET Syllabus Standard

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