Introduction
Semiconductors are the foundation of modern electronics. Every electronic device, such as smartphones, computers, televisions, LED lights, solar panels, and integrated circuits, uses semiconductor materials.
The most commonly used semiconductor materials are Silicon (Si) and Germanium (Ge). Their electrical conductivity lies between conductors and insulators.
Semiconductors are broadly classified into:
- Intrinsic Semiconductors
- Extrinsic Semiconductors
The electrical conductivity of semiconductors can be controlled by a process called doping.
What is a Semiconductor?
A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator.
Examples:
- Silicon (Si)
- Germanium (Ge)
Characteristics
- Moderate electrical conductivity.
- Small forbidden energy gap (about 1 eV).
- Conductivity increases with temperature.
- Used in almost all electronic devices.
Intrinsic Semiconductor
Definition
An Intrinsic Semiconductor is a pure semiconductor that contains no intentionally added impurities.
Pure silicon and pure germanium are intrinsic semiconductors.
Crystal Structure
Each silicon atom has four valence electrons.
These electrons form four covalent bonds with neighbouring atoms.
At absolute zero (0 K):
- All valence electrons are bound.
- No free electrons exist.
- The semiconductor behaves like an insulator.
At room temperature:
Some covalent bonds break due to thermal energy.
As a result:
- Free electrons are generated.
- Holes are created.
- Both electrons and holes contribute to current.
Charge Carriers
Intrinsic semiconductors contain two charge carriers:
- Electrons (Negative charge)
- Holes (Positive charge)
Number of electrons = Number of holes
Therefore,
n = p
where
n = Electron concentration
p = Hole concentration
Characteristics
- Pure semiconductor.
- Equal number of electrons and holes.
- Low conductivity.
- Conductivity increases with temperature.
- No impurity atoms.
Examples
- Pure Silicon
- Pure Germanium
Need for Doping
Intrinsic semiconductors have low electrical conductivity.
For practical electronic devices, higher conductivity is required.
This is achieved by adding a very small quantity of impurity atoms.
This process is called Doping.
Doping Process
Definition
Doping is the process of adding a very small amount of impurity atoms to a pure semiconductor to increase its electrical conductivity.
Typically, one impurity atom is added for every several million semiconductor atoms.
Doping greatly increases conductivity without significantly changing the crystal structure.
Types of Impurity Atoms
There are two types of impurities:
- Pentavalent impurities
- Trivalent impurities
Extrinsic Semiconductor
Definition
An Extrinsic Semiconductor is a semiconductor whose conductivity has been increased by adding suitable impurity atoms.
There are two types:
- N-type Semiconductor
- P-type Semiconductor
N-Type Semiconductor
Formation
An N-type semiconductor is formed by adding a Pentavalent impurity to pure silicon or germanium.
Pentavalent atoms have five valence electrons.
Examples:
- Phosphorus (P)
- Arsenic (As)
- Antimony (Sb)
Working
Four electrons form covalent bonds with neighbouring silicon atoms.
The fifth electron becomes free.
This free electron moves easily and contributes to electrical conduction.
Thus,
Electrons become the majority charge carriers.
Charge Carriers
Majority carriers:
Electrons
Minority carriers:
Holes
Characteristics
- High conductivity.
- Electrons are majority carriers.
- Holes are minority carriers.
- Neutral overall.
P-Type Semiconductor
Formation
A P-type semiconductor is formed by adding a Trivalent impurity.
Trivalent atoms have three valence electrons.
Examples:
- Boron (B)
- Aluminium (Al)
- Gallium (Ga)
- Indium (In)
Working
The impurity atom forms only three covalent bonds.
One bond remains incomplete.
This incomplete bond behaves like a hole.
Neighbouring electrons move to fill this hole, creating new holes.
Hence, current is mainly carried by holes.
Charge Carriers
Majority carriers:
Holes
Minority carriers:
Electrons
Characteristics
- Higher conductivity than intrinsic semiconductor.
- Holes are majority carriers.
- Electrons are minority carriers.
- Electrically neutral.
Comparison of Intrinsic and Extrinsic Semiconductors
| Property | Intrinsic Semiconductor | Extrinsic Semiconductor |
|---|---|---|
| Purity | Pure | Doped |
| Conductivity | Low | High |
| Impurities | None | Present |
| Majority Carriers | Equal electrons and holes | Electrons (N-type) or Holes (P-type) |
| Applications | Research and study | Electronic devices |
Comparison of N-Type and P-Type Semiconductors
| Property | N-Type | P-Type |
|---|---|---|
| Dopant | Pentavalent | Trivalent |
| Majority Carrier | Electrons | Holes |
| Minority Carrier | Holes | Electrons |
| Examples | Phosphorus, Arsenic | Boron, Gallium |
Applications
Intrinsic semiconductors:
- Semiconductor research
- Laboratory experiments
Extrinsic semiconductors:
- PN Junction Diodes
- Transistors
- LEDs
- Solar Cells
- Integrated Circuits
- Rectifiers
- Voltage Regulators
- Microprocessors
- Mobile Phones
- Computers
Advantages of Doping
- Increases electrical conductivity.
- Enables the manufacture of electronic devices.
- Improves current carrying capability.
- Controls semiconductor properties.
- Essential for integrated circuits.
Key Points to Remember
- Intrinsic semiconductor is pure.
- Extrinsic semiconductor is doped.
- Doping increases conductivity.
- Pentavalent impurities produce N-type semiconductors.
- Trivalent impurities produce P-type semiconductors.
- Electrons are majority carriers in N-type semiconductors.
- Holes are majority carriers in P-type semiconductors.
Frequently Asked Questions (FAQs)
1. What is an intrinsic semiconductor?
An intrinsic semiconductor is a pure semiconductor without any added impurities.
2. What is an extrinsic semiconductor?
An extrinsic semiconductor is a doped semiconductor whose conductivity has been increased by adding impurity atoms.
3. What is doping?
Doping is the process of adding a small amount of impurity atoms to a pure semiconductor to increase its conductivity.
4. Name two pentavalent impurities.
- Phosphorus
- Arsenic
5. Name two trivalent impurities.
- Boron
- Gallium
6. Which charge carrier is the majority carrier in an N-type semiconductor?
Electrons.
7. Which charge carrier is the majority carrier in a P-type semiconductor?
Holes.
Conclusion
Semiconductors are the building blocks of modern electronics. A pure semiconductor is called an intrinsic semiconductor, while a semiconductor whose conductivity is improved by adding impurity atoms is known as an extrinsic semiconductor. The process of adding these impurities is called doping, which produces N-type and P-type semiconductors. Understanding intrinsic and extrinsic semiconductors is essential for learning electronic devices such as diodes, transistors, LEDs, integrated circuits, and solar cells.
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