What is Extrinsic Semiconductor and Its Types , Advantage , Disadvantage , Application

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An extrinsic semiconductor is a semiconductor in which a small amount of impurity is added intentionally to improve its electrical conductivity. This process of adding impurity is called doping.

Extrinsic semiconductors are more useful than intrinsic semiconductors because their conductivity can be controlled as required.

Example: Silicon doped with Phosphorus or Boron

Need for Extrinsic Semiconductor

Intrinsic semiconductors have very low conductivity at room temperature, making them unsuitable for practical applications.

Therefore, extrinsic semiconductors are used because:

1. They have higher conductivity
2. Electrical properties can be controlled
3. Suitable for electronic devices
4. Better performance in circuits

Doping Process

Doping is the process of adding a very small amount of impurity to a pure semiconductor.

Types of impurities:

• Pentavalent impurities (5 valence electrons) → Donor atoms
• Trivalent impurities (3 valence electrons) → Acceptor atoms

Important Points:

• Doping does not change the crystal structure
• Only increases free charge carriers
• Improves conductivity

Types of Extrinsic Semiconductor

Extrinsic semiconductors are classified into two types:

(A) N-Type Semiconductor

An N-type semiconductor is formed by doping a pure semiconductor with pentavalent impurity atoms.

Example: Phosphorus, Arsenic, Antimony

Working Principle

• A pentavalent atom has 5 valence electrons
• 4 electrons form covalent bonds
• 1 extra electron becomes free

This extra electron increases conductivity

Charge Carriers

• Majority carriers → Electrons
• Minority carriers → Holes

Properties of N-type Semiconductor

1. High conductivity
2. Free electrons are in large numbers
3. Negative charge carriers dominate
4. Donor impurity provides extra electrons. The he 
5. Fermi level shifts towards the conduction band

Energy Band Diagram

• The donor level is close to the conduction band
• Electrons easily move to the conduction band
• Conduction increases significantly

(B) P-Type Semiconductor

A P-type semiconductor is formed by doping a pure semiconductor with trivalent impurity atoms.

Example: Boron, Gallium, Indium

Working Principle

• A trivalent atom has 3 valence electrons
• Only 3 bonds are formed
• 1 electron is missing → creates a hole

This hole helps in conduction

Charge Carriers

• Majority carriers → Holes
• Minority carriers → Electrons

Properties of P-type Semiconductor

1. Conductivity due to holes
2. Positive charge carriers dominate
3. Acceptor impurities create holes 
4. Fermi level shifts towards the valence band
5. Slightly lower conductivity than N-type

Energy Band Diagram

• The acceptor level is close to the valence band
• Electrons easily move from the VB to the acceptor level
• Holes are created in the valence band

Majority and Minority Carriers

In extrinsic semiconductors:

• Majority carriers: Charge carriers in large numbers
• Minority carriers: Charge carriers in small numbers

Example:

• N-type → Electrons (majority), Holes (minority)
• P-type → Holes (majority), Electrons (minority)

Conductivity in an Extrinsic Semiconductor

Conductivity increases due to the presence of extra charge carriers.

Factors affecting conductivity:

1. Type of impurity
2. Amount of doping
3. Temperature

More doping → more carriers → higher conductivity

Temperature Effect

• Increase in temperature → increase in conductivity
• More electron-hole pairs generated
• Extrinsic behaviour may become intrinsic at high temperature

Advantages of Extrinsic Semiconductor

1. High conductivity
2. Controlled electrical properties
3. Suitable for electronic devices
4. Efficient performance
5. Widely used in circuits

Applications

Extrinsic semiconductors are used in:

1. Diodes
2. Transistors
3. Integrated Circuits (ICs)
4. Amplifiers
5. Rectifiers
6. Communication devices