# What is a semiconductor?

While studying electrical engineering, most topics revolve around these three types of materials conductor, insulator, and semiconductor.

Conductors are materials that possess high conductivity and low resistivity which allows them to conduct electricity. Gold, copper, silver, etc. are few conducting materials.

On the other hand, insulators are materials that possess low conductivity and slightly high resistivity due to which they aren’t able to conduct electricity. Examples of insulating materials are glass, rubber, quartz, etc.

Whereas semi-conductor is the median type of material. Because neither it is a good conductor nor a good insulator. Whereat absolute temperature it acts as an insulator but at room temperature, its conductivity is lower than the conductor and higher than the insulator. Which makes it electrically conducting at room temperature.

When the temperature increases it increases the resistance of the conductor, so it is said that the conductor has a +ve temperature coefficient of resistance. On the other side, the semiconductor has -ve temp. coefficient of resistance i.e. when the temp. increases the resistance of it decreases.

## Energy band of the semiconductor

In the energy bands of the solid article, we discussed energy bands and energy bands of different materials.

As the conductivity of the semi-conductor lies between the conductor and the insulator i.e at some temp. it conducts and at some, it did not. It is seen during experiments that at absolute temperature (-273°C approx) the valence band is full of electrons and the conduction band is empty. So, it doesn’t conduct at absolute temperature.

But when external energy is imparted on it, then electrons can move from the valence band to the conduction band. This external energy must be greater than the Fermi level energy gap, which in the case of semiconductors is around 1eV (for silicon it is 1.2 eV & for germanium, it is 0.7 eV). If the energy imparted is lower than the energy gap then electrons are not able to jump from the valence band to the empty conduction band.

At room temperature semiconductors have a sufficient amount of energy to move electrons from the valence band to the conduction band. Therefore the semiconductors are capable of conduction at ambient temperature

## Types of semiconductor

A semiconductor is classified into two classes:

### Intrinsic

A very pure material is used to make an intrinsic semiconductor. Also, it is made up of only a single element. Silicon, germanium, tellurium, boron, etc are various elements but silicon (Si) and germanium (Ge) are the most widely used elements.

A hole-electron pair is formed when the temp. rises and the electron moves to the conduction band leaving a hole in its place. These hole and electron pair is responsible for the conduction of electricity in the intrinsic semi-conductor.

### Extrinsic

Extrinsic are those in which some foreign impurities are added into a pure semiconductor.

It can be further divided into two types one is N-type or donor semiconductor and the other is P-type or acceptor semiconductor.

• N-type or donor

In N-type, a pentavalent impurity such as arsenic, phosphorus, or antimony is added to the pure semiconductor.

When the pentavalent impurity is added to silicon, the atom of pentavalent impurity forms a covalent bond with the silicon atom. As silicon has 4 electrons in the valence band, so each impurity added produces one free electron which moves to the conduction band.

In this electrons are majority carriers are electrons and holes are minority carriers.

• P-type or acceptor

Similarly in the P-type, a trivalent element is added to the pure semiconductor. Trivalent impurities are boron, gallium, or aluminium.

When the trivalent impurity is added to silicon it forms the covalent bond with the surrounding atoms of silicon. As silicon has 4 atoms in the valence band and trivalent impurity has 3 atoms this gives rise to a hole. Now, the hole can accept an electron that is why this is also known as acceptor semi-conductor.

Here majority carriers are holes and minority carriers are holes.

## Properties of semiconductors

1. The resistance of a semiconductor depends on various factors so it can be controlled.
2. In this the temperature coefficient of resistance is negative.
3. The capacity of conduction changes when impurities are added to it.

The semi-conductor used in several devices such as a diode, transistor, etc. has many advantages.

• The devices have a compact size and lightweight
• They are very cheap.
• They are resistive to shock and vibrations.
• Their operating frequency is high
• They work for a long period if used within the permissible limit of temperature and frequency

## Application of semiconductor

In today’s digital world where mostly thing electrical and electronic items, their semiconductors are the backbone of it. Diodes, transistors, IC, and many more are made from semiconductors.

### In everyday use

• In self-driving cars
• Computers, laptops, and many more electronic devices
• Used in solar plates