What is Impurity and Its Types , Advantages , Disadvantages , Application

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In semiconductors, an impurity is a foreign atom added to a pure semiconductor (like Silicon or Germanium) in a very small amount to modify its electrical properties.

The process of adding impurity is called doping.

Need for Impurity (Why Doping is Required)

Pure (intrinsic) semiconductors have very low conductivity at room temperature, making them unsuitable for practical use.

Therefore, impurities are added to:

1. Increase electrical conductivity
2. Control charge carriers
3. Improve the performance of devices
4. Make the semiconductor useful in circuits

Doping Process

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

Important Points:

• Impurity concentration is very small (1 atom in millions)
• Does not disturb the crystal structure
• Increases free charge carriers
• Converts an intrinsic semiconductor into an extrinsic semiconductor

Types of Impurities

Impurities are mainly of two types:

(A) Donor Impurity (Pentavalent Impurity)

Donor impurities are those atoms that have 5 valence electrons and donate extra electrons to the semiconductor.

Examples:

Phosphorus (P), Arsenic (As), Antimony (Sb)

Working

• 4 electrons form covalent bonds
• 1 extra electron remains free

This free electron increases conductivity

Result

• Forms Nan -type semiconductor
• Majority carriers → Electrons
• Minority carriers → Holes

Properties of Donor Impurity

1. Provides extra electrons
2. Increases conductivity
3. Creates donor energy level
4. Easily gives electrons to the conduction band
5. Produces N-type material

(B) Acceptor Impurity (Trivalent Impurity)

Acceptor impurities are those atoms that have 3 valence electrons and accept electrons from the semiconductor.

Examples:

Boron (B), Gallium (Ga), Indium (In)

Working

• 3 bonds are formed
• 1 electron is missing
• Creates a hole

This hole helps in conduction

Result

• Forms Pa -type semiconductor
• Majority carriers → Holes
• Minority carriers → Electrons

Properties of Acceptor Impurity

1. Creates holes
2. Increases conductivity
3. Forms acceptor level
4. Accepts electrons from the valence band
5. Produces P-type material

Energy Level Due to Impurities

When impurities are added:

(a) Donor Level

• Close to the conduction band
• Electrons easily move to the conduction band

(b) Acceptor Level

• Close to the valence band
• Electrons easily move, creating holes

These levels reduce the energy required for conduction

Effect of Impurity on Conductivity

• More impurity → more charge carriers
• Conductivity increases
• Resistance decreases

Controlled doping → controlled conductivity

Types of Doping (Based on Level)

1. Light Doping → Low conductivity
2. Moderate Doping → Medium conductivity
3. Heavy Doping → High conductivity

Advantages of Impurity Doping

1. Increases conductivity
2. Controls electrical properties
3. Essential for device fabrication
4. Improves efficiency
5. Enables semiconductor devices

 Applications of Impurity in Semiconductors

1. Diodes
2. Transistors
3. Integrated Circuits
4. Amplifiers
5. Communication systems