Semiconductors in NEET Physics 2025: PN Junction, Logic Gates & Most Asked Questions

Semiconductors form a small but scoring chapter in the NEET Physics syllabus. Introduced at the end of NCERT Class 12 Physics (Chapter 14), it often gets ignored—yet, 1–2 direct and factual questions are almost guaranteed every year.

Why it matters:

Questions are straightforward—mostly memory-based
Includes theory, logic gates, and diode-based MCQs
Requires very little calculation
Scoring potential: 4 marks in under 2 minutes

In this blog, you’ll learn:

Band theory of conductors, insulators & semiconductors
Working and behavior of a PN junction diode
Forward and reverse biasing logic
Truth tables and logic gates asked in NEET

Previous year NEET papers reveal key trends.

Explore Deeksha Vedantu Long Term NEET Coaching to boost your NEET Scores today.

NEET Weightage: Semiconductors & Logic Gates

Based on NEET trends from 2020 to 2024, you can expect 1 question from this chapter in almost every paper.

How questions are framed:

Logic Gate Questions: Identify output from a combination or truth table
PN Junction Diode: Current flow based on forward/reverse bias
Application-Based MCQs: Use of diodes in circuits or identifying n/p-type semiconductors

Why it’s High ROI:

Entire chapter can be revised in 1–2 hours
Most questions come directly from NCERT
No complex derivations or numericals

Mock tests boost speed and accuracy.

Conductors, Insulators & Semiconductors: Band Theory

Understanding the energy band theory is key to distinguishing between different types of materials.

Energy Bands Basics

Valence Band (VB): Highest occupied energy band
Conduction Band (CB): Next higher band where electrons can move freely
Band Gap (Eg): Energy difference between VB and CB

Material	Band Gap Behavior	Conduction Nature
Conductor	No gap or overlapping VB & CB	Electrons move freely
Insulator	Very large gap (Eg > 5 eV)	No conduction at room temp
Semiconductor	Small gap (~1.1 eV for Si)	Limited conduction (increases with temp)

Intrinsic & Extrinsic Semiconductors

Intrinsic Semiconductors

Pure elements like Silicon (Si) and Germanium (Ge)
Conductivity increases with temperature
Equal number of electrons and holes

Extrinsic Semiconductors

Doped with impurities to increase conductivity
n-type:
- Donor atoms (like P, As)
- Provide extra electrons → majority carriers = electrons
p-type:
- Acceptor atoms (like B, Al)
- Create holes → majority carriers = holes

NEET often asks: “Which dopant creates an n-type semiconductor?”

PN Junction Diode & Its Behavior

Formation of a PN Junction

A PN junction is created by combining p-type and n-type semiconductors.
At the junction, electrons from the n-side recombine with holes from the p-side, forming a depletion region.
A potential barrier is created that opposes further diffusion.

Biasing in Diodes

Forward Bias:

p-side connected to +ve terminal
Reduces potential barrier → current flows easily
Diode is ON

Reverse Bias:

p-side connected to –ve terminal
Increases potential barrier → current doesn’t flow (only small leakage current)
Diode is OFF

Diode Characteristics

VI curve shows that:
- No current flows in reverse bias
- In forward bias, current shoots up sharply after a certain voltage (threshold)
Cut-in Voltage (Silicon): ~0.7 V
Cut-in Voltage (Germanium): ~0.3 V

NEET Focus Areas

Current direction through a PN junction in different biasing
Identifying forward vs reverse bias in a circuit
Diagram-based MCQs with battery and diode placement

Typical NEET Question:
“If the p-side is connected to –ve terminal of a battery, is the diode forward or reverse biased?”

Logic Gates: AND, OR, NOT, NAND, NOR

Logic gates form the digital backbone of modern electronics and are a guaranteed part of NEET Physics, often tested via truth tables or symbolic circuits.

Basic Logic Gates

These are the fundamental building blocks used to design digital circuits:

AND Gate:
- Output is 1 only when both inputs are 1
- Logic expression: A·B (multiplication logic)
OR Gate:
- Output is 1 if any one input is 1
- Logic expression: A + B (addition logic)
NOT Gate:
- Inverts the input
- Logic expression: ¬A or A̅

These three are the primary gates from which complex logical circuits are built.

Universal Gates: NAND & NOR

NAND and NOR gates are called universal because you can construct all other gates using just one of them.

NAND Gate:
Output is inverted AND → $boldsymbol{lnot (A cdot B)}$
NOR Gate:
Output is inverted OR → $boldsymbol{lnot (A + B)}$

NEET frequently includes logic circuits using NAND/NOR gates—so knowing their truth tables is essential.

Truth Tables You Must Know

Gate	Inputs (A, B)	Output	Logic Expression
AND	0,0 → 0 1,1 → 1	1 only if both 1	A·B
OR	0,1 → 1 1,1 → 1	1 if any input 1	A + B
NOT	A = 0 → 1 A = 1 → 0	Inverts input	¬A or A̅
NAND	A·B → Inverted	0 if both 1	¬(A·B)
NOR	A + B → Inverted	1 only if both 0	¬(A + B)

Most Expected Questions from Semiconductors in NEET

Based on NEET trends, here are the most predictable question types:

Identify diode biasing
- Given circuit → tell if the diode is forward or reverse biased
Find the output of a logic gate circuit
- Combine truth tables to compute final result
Which impurity leads to n-type or p-type semiconductor?
- Memory-based question on dopants (Boron, Phosphorus, etc.)
Truth table-based MCQ
- Match the output to the correct gate logic
Matching type (gate ⇌ function)
- Match gates like AND, NAND, etc. with their truth table behavior or symbol

High chances of 1 direct MCQ from this set in NEET Physics.

NEET PYQs: Solved Examples

Q1. A diode is connected in a circuit with the p-side towards the battery’s positive terminal. What is the biasing condition?

Solution:
p-side to +ve → Forward Bias

Answer: Forward Bias

Q2. Which of the following will result in an n-type semiconductor?

A) Doping Si with Boron
B) Doping Si with Phosphorus
C) Doping Ge with Gallium
D) Doping Ge with Aluminium

Solution:
Phosphorus = Donor → n-type

Answer: B) Doping Si with Phosphorus

Q3. What is the output of the logic gate for A = 1, B = 0 in an AND gate?

Solution:
AND gate = A·B = 1×0 = 0

Answer: 0

Q4. Which gate gives output 1 only when both inputs are 0?

Solution:
NOR gate → Only gives 1 when A = 0 and B = 0

Answer: NOR Gate

Revision Tips for Semiconductors

📌 Stick to NCERT language
NEET rarely goes beyond it for this chapter.

📌 Create a mini flashcard with all 5 logic gate truth tables
Revising this alone helps you crack any logic-based MCQ.

📌 Practice 10+ diode-based MCQs
Especially for forward/reverse bias identification in circuits.

📌 Do NOT skip this chapter during final revision
Takes less than 2 hours to fully master—and it’s a free 4 marks.

FAQs on Semiconductors for NEET

Q: Is Semiconductors an easy chapter for NEET?

A: Absolutely. It’s one of the most scoring chapters in NEET Physics. Questions are primarily theory-based, sourced directly from NCERT Class 12, and rarely require complex calculations. With a bit of focused preparation, full marks are very achievable.