Ch 12: Semiconductor | Exam Fusion Prep

💻 Semiconductor — At a Glance

📘 Definition

Solid substances whose electrical conductivity is less than conductors but more than insulators. Conductivity increases with temperature.

🚫 Insulator
Rubber, Glass

⚙️ Semiconductor
Si, Ge

✅ Conductor
Cu, Ag, Al

Si & Ge

Pure Semiconductors

Doping

Process to Make Extrinsic

n & p

Types of Extrinsic SC

Key Semiconductor Devices

📊 Semiconductor vs Conductor vs Insulator

Property	Conductor	Semiconductor	Insulator
Conductivity	High	Medium	Very Low
Band Gap	Zero (overlap)	Small (~1 eV)	Large (>5 eV)
Temp Effect	Resistance ↑	Resistance ↓	Minimal change
Examples	Ag, Cu, Au, Al	Si, Ge	Glass, Rubber, Diamond
Current Carriers	Free electrons	Electrons & Holes	None

⚠️ Exam Trap — Temperature Effect

Semiconductors: Conductivity increases with temperature (resistance ↓)
Conductors: Conductivity decreases with temperature (resistance ↑)
This is the opposite behaviour — most asked trap!

🧪 Types of Semiconductors

Type 1

Intrinsic (Pure) Semiconductor

No impurity added
Conductivity depends on temperature
Obtained in pure form in nature
Substances: Germanium (Ge) and Silicon (Si)
At room temp: very few free electrons & holes
Equal number of electrons and holes

Type 2

Extrinsic (Impure) Semiconductor

Impurities added to increase conductivity
Process of adding impurity = Doping
Added impurities = Dopants
Two types: n-Type and p-Type
Much higher conductivity than intrinsic

⚠️ Exam Trap — Intrinsic SC

Only Silicon (Si) and Germanium (Ge) are intrinsic semiconductors (most asked)
Carbon (diamond form) is an insulator, NOT a semiconductor
At absolute zero (0 K), intrinsic semiconductor behaves like a perfect insulator

⚗️ Doping — n-Type & p-Type

📘 Doping

Adding a small amount of trivalent or pentavalent impurity to an intrinsic semiconductor to increase its conductivity. The added substance is called a dopant.

n-Type Semiconductor

Negative carriers (Electrons)

Impurity added: Pentavalent (5 valence electrons)
Dopants: Arsenic (As), Antimony (Sb), Phosphorus (P)
Current flow by: Electrons (majority carriers)
Holes = minority carriers
Extra electron from dopant becomes free

5 valence electrons → 4 bonds + 1 free electron

p-Type Semiconductor

Positive carriers (Holes)

Impurity added: Trivalent (3 valence electrons)
Dopants: Aluminium (Al), Boron (B), Gallium (Ga), Indium (In)
Current flow by: Holes (majority carriers)
Electrons = minority carriers
Missing electron creates a "hole"

3 valence electrons → 4 bonds − 1 = 1 hole created

📊 n-Type vs p-Type — Quick Comparison

Property	n-Type	p-Type
Dopant valency	Pentavalent (5)	Trivalent (3)
Dopant examples	As, Sb, P	Al, B, Ga, In
Majority carriers	Electrons (−)	Holes (+)
Minority carriers	Holes	Electrons
"n" stands for	Negative (electrons)	—
"p" stands for	—	Positive (holes)

⚠️ Exam Trap — Doping

n-type → Pentavalent (5) dopant → extra electron → current by electrons
p-type → Trivalent (3) dopant → hole created → current by holes
Aluminium dopant → p-type (NOT n-type, even though Al is a conductor)
Phosphorus dopant → n-type
Overall, both n-type and p-type are electrically neutral (no net charge)

📱 Semiconductor Devices — Applications

📷

Photodiode

Optical Detection

Flow of optical signals
Principle: Photoelectric effect
Uses: Detecting intensity of optical signals in light-operated keys

⚡

Zener Diode

Voltage Regulation

Designed for reverse bias in breakdown region
Use: Voltage control / regulation
Maintains constant voltage across load

💡

LED

Light Emitting Diode

Converts electrical energy → radiant (light) energy
Principle: Electroluminescence
Inventor: Nick Holonyak
Uses: Remote control, burglar alarm, optical communication

☀️

Solar Cell

Photovoltaic Effect

Converts solar energy → electrical energy
Principle: Photovoltaic effect
Uses: Remote areas power generation, artificial satellite batteries

📋 Semiconductor Devices — Summary Table

Device	Energy Conversion	Principle	Key Use
Photodiode	Light → Electrical signal	Photoelectric Effect	Light-operated keys, intensity detection
Zener Diode	Electrical → Electrical (regulated)	Reverse breakdown	Voltage regulation/control
LED	Electrical → Light	Electroluminescence	Remote control, burglar alarm, optical comm.
Solar Cell	Solar (Light) → Electrical	Photovoltaic Effect	Satellite batteries, remote power

⚠️ Exam Trap — Device Principles

Photodiode → Photoelectric Effect | Solar Cell → Photovoltaic Effect (different!)
LED inventor = Nick Holonyak (not Edison, not Tesla)
LED principle = Electroluminescence (NOT photoelectric effect)
Zener diode works in reverse bias (NOT forward bias like normal diode)
Solar cell = Photovoltaic; Photodiode = Photoelectric — easy to confuse!

🎯 High-Frequency BPSC/BSSC Exam Points

Semiconductor conductivity: between conductor and insulator
Intrinsic semiconductors: Silicon (Si) and Germanium (Ge)
Doping = adding impurity to increase conductivity
n-Type → Pentavalent dopant (As, Sb, P) → current by electrons
p-Type → Trivalent dopant (Al, B, Ga) → current by holes
Semiconductor conductivity increases with temperature (opposite of conductors)
Photodiode principle = Photoelectric effect
LED principle = Electroluminescence | Inventor = Nick Holonyak
Solar cell principle = Photovoltaic effect
Zener diode = voltage regulation, works in reverse bias
LED uses: Remote control, burglar alarm, optical communication
Solar cell uses: Remote areas, satellite batteries, power generation

📋 Quick Reference Table

Concept	Key Fact
Intrinsic SC	Si, Ge — no impurity
Extrinsic SC	Doped Si or Ge
n-Type dopant	Pentavalent (As, Sb, P)
p-Type dopant	Trivalent (Al, B, Ga)
n-Type carriers	Electrons (majority)
p-Type carriers	Holes (majority)
LED principle	Electroluminescence
LED inventor	Nick Holonyak
Solar cell	Photovoltaic effect
Zener diode	Reverse bias, voltage control
Photodiode	Photoelectric effect

⚠️ Most Common Exam Traps

SC conductivity ↑ with temp (NOT decreases like conductors)
n-type = Pentavalent (5); p-type = Trivalent (3)
Aluminium → p-type dopant (though Al is a good conductor)
LED = Electroluminescence; Solar = Photovoltaic; Photodiode = Photoelectric — 3 different principles!
LED invented by Nick Holonyak (not a common name — easy to miss)
Zener diode = reverse bias operation (opposite of normal diode)
At 0 K, intrinsic SC = perfect insulator
Carbon (diamond) = insulator, NOT semiconductor
Both n-type and p-type are electrically neutral overall

💻 Chapter 12: Semiconductor