Electrical Conductors, Semi-Conductors and Insulators

Chapter 01

Electrical Conductors, Semiconductors and Insulators

Materials can be divided into three categories depending on their ability to conduct electricity. That is as,

1.    Conductors

2.    Semiconductors

3.    Insulators


Materials that conduct electricity are known as electrical conductors.

All the metals such as copper, aluminium, and iron and the metal alloys such as brass, nichrome, and manganin are electrical conductors.

The reason for electricity to flow easily through metals is the existence of free electrons.

The valence electrons (electrons in the outermost shell of an atom) of different materials possess different energies. The higher the energy of a valence electron, the lesser it is bound to the nucleus. 

In certain materials, particularly metals, the valence electrons possess very high energy that they are loosely attached to the nucleus. Therefore, the valence electrons of metallic atoms can be easily detached from the atom by applying a small amount of external energy. This makes them good conductors of electricity.

A large number of such detached free electrons from the outermost shell of metal atoms are in random motion in the regions between metal atoms as shown below.

These loosely attached valence electrons that move randomly within the material are called free electrons.


Materials that do not conduct electricity are known as electrical insulators.

Ebonite, polythene, plastic, dry wood, asbestos, rubber, glass air etc. are examples of electrical insulators.

The reason for no flow of electricity exists in these materials is because virtually there are no effective free electrons present in them. That is, in insulators valence electrons are bound very tightly to their parent atoms, thus requiring very large energy to remove them from the attraction of their nuclei.


Materials that conduct a smaller amount of electricity are known as semiconductors.

Semiconductors are neither very good conductors nor very good insulators. Its electrical properties lie between insulators and good conductors.

Materials such as silicon (Si) and germanium (Ge) in their crystalline form show such properties. These elements belong to the fourth group in the periodic table and have four electrons in their outermost shell. Such elements form crystal lattice structures by sharing the four electrons in their outermost shell to make covalent bonds with four nearby atoms, thereby acquiring a stable electronic configuration having eight electrons in the outermost shell.

However, these bonds are rather weak and can be broken from the thermal energy available even at room temperature, releasing electrons.

The following figure shows the covalent bonds of the silicon lattice at 0 K. All the bonds are complete at this temperature.

The following figure shows that some bonds have been broken releasing some free electrons at a temperature higher than 0 K. 

An electron deficiency can be observed at the positions that the free electrons occupied previously. Such positions with an electron deficiency are known as holes.

Due to the positively charged protons in the nucleus, a hole gives rise to a positive charge that has not been neutralized (In a neutral atom, the number of protons in the nucleus is equal to the number of electrons). Therefore, a hole is equivalent to a positive charge.

In semiconductors, not only free electrons contribute to the conduction of electricity. When an electron in an adjacent atom jumps to an atom with a hole having a positive charge, the position of the hole can change. By changing the position of a hole from one atom to another in this manner, holes can move around in the lattice and contribute to conducting a current. Free electrons act as negative charge carriers while holes act as positive charge carriers.

Therefore, in semiconductors, the negatively charged electrons as well as the positively charged holes act as the charge carriers that contribute to the conduction of electricity.