📘 ELECTRONIC DEVICES - THEORY CHEATSHEET (GATE 2026)
🎯 CONCEPTUAL ONE-LINERS FOR MCQs
1️⃣ SEMICONDUCTOR BASICS
✓ Conductivity: Conductor > Semiconductor > Insulator
✓ At 0 K: Semiconductor = Insulator (no free carriers)
✓ Temperature effect: ↑ Temp → ↑ Conductivity (negative temp coefficient)
✓ Examples:
Elemental: Si, Ge
Compound: GaAs, GaN, InP
2️⃣ ENERGY BANDS
Band Structure
✓ Valence Band (VB): Highest occupied energy band
✓ Conduction Band (CB): Electrons here conduct current
✓ Band Gap (Eg): Forbidden energy region between VB and CB
✓ Si band gap: ~1.12 eV at 300 K
✓ Temperature effect: ↑ Temp → ↓ Band gap
Direct vs Indirect Band Gap
Property Direct (GaAs) Indirect (Si)
Recombination Direct Via phonon
Energy release Photon (light) Heat
Used for LED, Laser General electronics
GATE Trap: Silicon CANNOT be used for LED (indirect band gap)
, 3️⃣ INTRINSIC SEMICONDUCTOR
✓ Definition: Pure semiconductor (no doping)
✓ Carrier balance: n = p = nᵢ
✓ Conductivity: Very low
✓ Fermi level: Exactly at mid-gap (Eᵢ)
✓ Temperature dependent: nᵢ increases exponentially with temp
4️⃣ EXTRINSIC SEMICONDUCTOR
N-type Semiconductor
Property Value
Dopant Pentavalent (P, As, Sb)
Majority carriers Electrons
Minority carriers Holes
Donor level Just below CB
Fermi level Shifts towards CB
P-type Semiconductor
Property Value
Dopant Trivalent (B, Al, Ga)
Majority carriers Holes
Minority carriers Electrons
Acceptor level Just above VB
Fermi level Shifts towards VB
Key Point: Higher doping → Fermi level moves closer to band edge
🎯 CONCEPTUAL ONE-LINERS FOR MCQs
1️⃣ SEMICONDUCTOR BASICS
✓ Conductivity: Conductor > Semiconductor > Insulator
✓ At 0 K: Semiconductor = Insulator (no free carriers)
✓ Temperature effect: ↑ Temp → ↑ Conductivity (negative temp coefficient)
✓ Examples:
Elemental: Si, Ge
Compound: GaAs, GaN, InP
2️⃣ ENERGY BANDS
Band Structure
✓ Valence Band (VB): Highest occupied energy band
✓ Conduction Band (CB): Electrons here conduct current
✓ Band Gap (Eg): Forbidden energy region between VB and CB
✓ Si band gap: ~1.12 eV at 300 K
✓ Temperature effect: ↑ Temp → ↓ Band gap
Direct vs Indirect Band Gap
Property Direct (GaAs) Indirect (Si)
Recombination Direct Via phonon
Energy release Photon (light) Heat
Used for LED, Laser General electronics
GATE Trap: Silicon CANNOT be used for LED (indirect band gap)
, 3️⃣ INTRINSIC SEMICONDUCTOR
✓ Definition: Pure semiconductor (no doping)
✓ Carrier balance: n = p = nᵢ
✓ Conductivity: Very low
✓ Fermi level: Exactly at mid-gap (Eᵢ)
✓ Temperature dependent: nᵢ increases exponentially with temp
4️⃣ EXTRINSIC SEMICONDUCTOR
N-type Semiconductor
Property Value
Dopant Pentavalent (P, As, Sb)
Majority carriers Electrons
Minority carriers Holes
Donor level Just below CB
Fermi level Shifts towards CB
P-type Semiconductor
Property Value
Dopant Trivalent (B, Al, Ga)
Majority carriers Holes
Minority carriers Electrons
Acceptor level Just above VB
Fermi level Shifts towards VB
Key Point: Higher doping → Fermi level moves closer to band edge
