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Semiconductors & Logic Gates – JEE Mains 2026 Revision Series

Semiconductors & Logic Gates – JEE Mains 2026 Revision Series

This revision-friendly guide covers Semiconductors, Diodes, and Logic Gates with all high-yield formulae, shortcuts, learning traps, and JEE-style reasoning points. Designed for rapid yet complete revision.

Semiconductors – Core Concepts

Definition: Materials with conductivity between conductors and insulators. Silicon (Si) and Germanium (Ge) are most common.

Energy Band Concept

Semiconductors have a small band gap (~1 eV) compared to insulators. Temperature increases free electron concentration.

JEE Point: Intrinsic carrier concentration increases exponentially with temperature.

Types of Semiconductors

1. Intrinsic Semiconductor

Pure Si/Ge. Carriers generated by thermal excitation.

2. Extrinsic Semiconductor

Formed by doping:

  • N-type: doped with pentavalent impurity → electrons majority carriers.
  • P-type: doped with trivalent impurity → holes majority carriers.
Formula:
Mass action law: np = ni2
JEE's favorite conceptual trap.

Diodes

A diode allows current only in one direction. It is a P–N junction with unique I–V characteristics.

Forward Bias

Potential applied to reduce depletion width → current flows.

Reverse Bias

Current nearly zero (leakage) until breakdown voltage.

JEE Trap: Students confuse Zener breakdown (tunneling) with avalanche breakdown (collision-based).

Important Diodes & JEE Uses

1. Zener Diode

Works in reverse breakdown. Used for voltage regulation.

2. LED (Light Emitting Diode)

Emits photons when electrons recombine with holes in forward bias.

3. Photodiode

Reverse biased light-sensing device → photocurrent ∝ light intensity.

4. Tunnel Diode

Shows negative resistance due to quantum tunneling.

5. Schottky Diode

Fastest switching, low voltage drop (~0.2 V). Used in logic circuits.

Logic Gates – High Yield Revision

Logic gates perform basic boolean operations. They are the foundation of digital electronics and carry heavy JEE weightage.

JEE Insight: NAND & NOR are universal gates — any gate can be built from them.

1. AND Gate

ABOutput (A·B)
000
010
100
111

2. OR Gate

ABOutput (A+B)
000
011
101
111

3. NOT Gate

AOutput (~A)
01
10

4. NAND Gate

AB(AB)'
001
011
101
110

5. NOR Gate

AB(A+B)'
001
010
100
110

6. XOR Gate

ABA ⊕ B
000
011
101
110

JEE Patterns to Expect

  • Finding output of combined logic circuits
  • Simplifying boolean expressions
  • Truth table based questions
  • Zener voltage regulation numericals
  • LED wavelength & energy gap relation
  • PN junction I–V curve interpretation

Quick Conclusion

Semiconductors and logic gates are scoring topics in JEE. Memorize truth tables, understand diode behavior thoroughly, and practice boolean simplification for guaranteed marks.

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