Electronic materials are the backbone of modern electrical and electronic systems. From simple connecting wires to complex microprocessors, the choice of material depends mainly on its electrical behavior. This electrical behavior could be best understood using energy band theory.
According to energy band theory, the ability of a material to conduct electricity depends on:
- The arrangement of electrons in energy bands
- The availability of free electrons
- The size of the forbidden energy gap between bands
Based on this theory, electronic materials are broadly classified into:
- Conductors
- Semiconductors
- Insulators
This classification provides a clear physical explanation for why different materials behave differently when an electric field is applied.
Energy Band Theory: A Brief Review
In solids, a large number of atoms come together and their discrete atomic energy levels split to form energy bands. The most important bands are:
- Valence Band (VB):
The highest energy band that is completely or partially filled with electrons. - Conduction Band (CB):
The energy band above the valence band where electrons are free to move and conduct electricity. - Forbidden Energy Gap (Eg):
The energy gap between the valence band and the conduction band where no electron states are allowed.
The width of the forbidden energy gap plays a decisive role in determining the electrical nature of a material.
Conductors
Definition
Conductors are materials that allow electric current to flow easily through them.
Energy Band Structure of Conductors
In conductors:
- The valence band and conduction band overlap.
- The conduction band is partially filled
As a result:
- No forbidden energy gap exists
- Electrons can move freely even at very low temperatures
Eg = 0 eV
Because a large number of free electrons are already present, conductors exhibit very high electrical conductivity.
Electrical Behaviour
- High conductivity at room temperature.
- Conductivity slightly decreases with increase in temperature due to increased lattice vibrations.
- Current flow is due to free electrons.
Examples
Copper, Silver, Aluminium, Gold, etc.
Semiconductors
Definition
Semiconductors are materials whose electrical conductivity lies between that of conductors and insulators.
Energy Band Structure of Semiconductors
In semiconductors:
- The valence band is completely filled at absolute zero
- The conduction band is completely empty at absolute zero
- A small forbidden energy gap exists between VB and CB
0 < Eg < 3 eV
At room temperature, thermal energy excites some electrons from the valence band to the conduction band, making conduction possible.
Electrical Behaviour
- Moderate electrical conductivity
- Conductivity increases with increase in temperature
- Both electrons and holes contribute to conduction
Examples and Band Gap Values
- Silicon (Si): Eg = 1.1 eV
- Germanium (Ge): Eg = 0.66eV
Importance
Semiconductors form the basis of:
- Diodes
- Transistors
- Integrated circuits
- Solar cells
Insulators
Definition
Insulators are materials that strongly oppose the flow of electric current.
Energy Band Structure of Semiconductors
In insulators:
- The valence band is completely filled
- The conduction band is completely empty
- A large forbidden energy gap exists
Eg > 3 eV
Due to this large energy gap, electrons cannot reach the conduction band under normal conditions.
Electrical Behaviour
- Very low electrical conductivity
- Conductivity almost independent of temperature
- Used mainly for electrical insulation
Examples
Glass, Rubber, Mica, Diamond, Porcelain.
Comparison of Conductors, Semiconductors, and Insulators
| S. No. | Conductotrs | Semiconductors | Insulators |
|---|---|---|---|
|
1 |
Easily conduct the electrical current |
Conduct the electrical current greater than insulator but less than the conductor. |
Does not conduct any current |
|
2 |
Has only one valence electron in its outermost orbit. |
Has four valence electron in its outermost orbit. |
Has eight valence electrons in its outermost orbit. |
|
3 |
Conductor formed using metallic bonding. |
Semiconductors are formed due to covalent bonding. |
Insulators are formed due to ionic bonding. |
|
4 |
Valence and conduction bands are overlapped. |
Valence and conduction bands are separated by forbidden energy gap of 1.1 eV. |
Valence and conduction bands are separated by forbidden energy gap of 6 to 10 eV. |
|
5 |
Resistance is very small. |
Resistance is high. |
Resistance is very high. |
|
6 |
It has a positive temperature coefficient. |
It has a negative temperature coefficient. |
It has a negative temperature coefficient. |
|
7 |
Ex: copper, aluminium, etc. |
Ex: silicon, germanium, etc |
Ex: Mica, Paper, etc. |
Importance of Energy Band Classification
- Explains electrical conduction in solids
- Helps in selection of materials for electronic devices
- Forms the foundation of semiconductor technology
- Essential for understanding modern electronics
Important Examination Questions
Short Answer Questions
- What is an electronic material?
- Define the forbidden energy gap.
- Why are metals good conductors of electricity?
Long Answer Questions
- Explain the classification of electronic materials using energy band theory with neat diagrams.
- Describe the energy band structures of conductors, semiconductors, and insulators.
Conceptual Questions
- Why does the conductivity of semiconductors increase with temperature?
- Why does an insulator not conduct electricity even at room temperature?
FAQs
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1. What are conductors according to energy band theory?
Conductors are materials in which the valence band and conduction band overlap, allowing electrons to move freely and conduct electricity easily.
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2. Why do conductors have high conductivity?
Because there is no energy gap between bands, electrons can move without needing extra energy.
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3. What are examples of conductors?
Common examples include Copper, Aluminum, and Silver.
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4. What are semiconductors in energy band theory?
Semiconductors are materials with a small energy gap between the valence band and conduction band.
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5. Why is conductivity moderate in semiconductors?
Because electrons require a small amount of energy to jump across the band gap.
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6. Why do insulators not conduct electricity?
Because electrons cannot easily jump across the large band gap, even at higher temperatures.
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7. What are examples of insulators?
Examples include glass, rubber, and materials like carbon.