Semiconductors are essential components of modern electronic devices, including computers, smartphones, solar cells, and transistors. Pure semiconductors such as silicon have limited electrical conductivity. Their conductivity can be increased by adding small amounts of impurities through a process called doping.
Depending on the type of impurity added, semiconductors are classified as P-type and N-type semiconductors. Both are important in electronic circuits and are used to create semiconductor devices such as diodes and transistors.
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What Is a Semiconductor?
A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator.
Silicon and germanium are common examples. Their conductivity can be controlled by temperature, light, and the addition of impurities.
A pure semiconductor is called an intrinsic semiconductor. When a small amount of impurity is added to improve conductivity, it becomes an extrinsic semiconductor.
There are two main types of extrinsic semiconductors: P-type and N-type.
What Is a P-Type Semiconductor?
A P-type semiconductor is formed by adding a trivalent impurity to a pure semiconductor such as silicon.
A trivalent impurity has three valence electrons. Examples include boron, aluminium, gallium, and indium.
When boron is added to silicon, three of its electrons form covalent bonds with neighbouring silicon atoms. One bond remains incomplete, creating a hole.
A hole behaves like a positive charge carrier because an electron can move into the vacant position, leaving another hole behind.
Characteristics of P-Type Semiconductor
- Doped with trivalent or acceptor impurities.
- Holes are the majority charge carriers.
- Electrons are the minority charge carriers.
- The impurity atoms are called acceptors.
- Electrical conduction mainly occurs through holes.
The semiconductor remains electrically neutral overall, even though holes act as positive charge carriers.
What Is an N-Type Semiconductor?
An N-type semiconductor is formed by adding a pentavalent impurity to a pure semiconductor.
A pentavalent impurity has five valence electrons. Common examples include phosphorus, arsenic, and antimony.
When phosphorus is added to silicon, four of its electrons form covalent bonds with neighbouring silicon atoms. The fifth electron is loosely bound and can become a free electron.
These free electrons increase the electrical conductivity of the semiconductor.
Characteristics of N-Type Semiconductor
- Doped with pentavalent or donor impurities.
- Electrons are the majority charge carriers.
- Holes are the minority charge carriers.
- The impurity atoms are called donors.
- Electrical conduction mainly occurs through electrons.
N-type semiconductors are also electrically neutral overall.
Energy Band Diagram of P-Type and N-Type Semiconductors
The energy band diagram explains the energy levels of electrons in a semiconductor.
There are three important terms:
- Conduction band: The energy band where electrons can move freely and conduct electricity.
- Valence band: The energy band containing electrons involved in bonding.
- Forbidden energy gap: The energy gap between the valence band and conduction band.
<text weight=”medium”>N-Type Semiconductor Band</text>
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In an N-type semiconductor, the donor energy level lies just below the conduction band. Electrons require only a small amount of energy to move from the donor level into the conduction band.
The Fermi level lies closer to the conduction band.
<text weight=”medium”>P-Type Semiconductor Band</text>
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In a P-type semiconductor, the acceptor energy level lies just above the valence band. Electrons can move from the valence band into the acceptor level, leaving holes behind.
The Fermi level lies closer to the valence band.
Difference Between P-Type and N-Type Semiconductors
| Feature | P-Type | N-Type |
|---|---|---|
| Impurity | Trivalent | Pentavalent |
| Examples | Boron, aluminium | Phosphorus, arsenic |
| Majority carriers | Holes | Electrons |
| Minority carriers | Electrons | Holes |
| Impurity type | Acceptor | Donor |
| Fermi level | Near valence band | Near conduction band |
| Main conduction | By holes | By electrons |
Applications of P-Type and N-Type Semiconductors
Both types are used together in many electronic components.
- PN junction diodes: A junction between P-type and N-type materials allows current to flow preferentially in one direction.
- Transistors: Semiconductor regions with different doping types are used to control current and amplify signals.
- Solar cells: P-type and N-type semiconductor layers help separate charge carriers generated by sunlight.
- Integrated circuits: Doped semiconductor regions are essential for constructing transistors and other electronic components.
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Conclusion
P-type and N-type semiconductors are formed by doping pure semiconductors with different impurities. P-type materials have holes as majority carriers, while N-type materials have electrons as majority carriers.
Their different energy levels and charge carrier properties make them fundamental to modern electronics.
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