Adding impurities to pure semiconductors is called doping. ... it is done to increase conductivity. ... by creating holes or moblie electrons

Introduction:
Adding impurities to a semiconductor is a common practice in order to modify its electrical properties and make it suitable for specific applications. These impurities, also known as dopants, can either increase or decrease the electrical conductivity of the semiconductor.

Explanation:
The correct answer for this question is option 'C', which states that impurities are added to a semiconductor to increase its electrical conductivity. This is achieved by introducing dopant atoms that either donate or accept extra electrons, thereby increasing the number of charge carriers in the semiconductor material.

Types of Impurities:
There are two types of impurities commonly used in semiconductors: donor impurities and acceptor impurities.

1. Donor impurities:
- Donor impurities are atoms that have more valence electrons than the atoms in the host semiconductor material.
- When a donor impurity is added, it provides extra electrons to the semiconductor material, creating excess negative charge carriers called electrons.
- This increases the electrical conductivity of the semiconductor as the number of charge carriers available for conduction is increased.

2. Acceptor impurities:
- Acceptor impurities are atoms that have fewer valence electrons than the atoms in the host semiconductor material.
- When an acceptor impurity is added, it creates holes in the valence band of the semiconductor material.
- These holes act as positively charged carriers and increase the electrical conductivity of the semiconductor.

Application of Impurities:
The type of impurity added to a semiconductor depends on its intended use. Here are a few examples:

1. N-type semiconductors:
- N-type semiconductors have donor impurities added to them, such as phosphorus or arsenic.
- These impurities provide extra electrons, making the semiconductor more conductive.
- N-type semiconductors are commonly used in devices such as diodes, transistors, and solar cells.

2. P-type semiconductors:
- P-type semiconductors have acceptor impurities added to them, such as boron or gallium.
- These impurities create holes in the valence band, increasing the conductivity.
- P-type semiconductors are used in devices such as diodes, transistors, and integrated circuits.

Conclusion:
Adding impurities to semiconductors is essential for tailoring their electrical properties. The choice of impurity depends on the desired electrical conductivity and the type of semiconductor device being fabricated. Donor impurities increase the number of electrons, while acceptor impurities create holes in the valence band. By carefully selecting and controlling impurities, semiconductors can be optimized for specific applications in electronics and optoelectronics.