Types of semiconductors – intrinsic (pure) and extrinsic (impure)

Semiconductors may be classified into 2 types: (1) Intrinsic (pure) semiconductors, and (2) Extrinsic (impure) semiconductors.

In this post, we will learn more about intrinsic (pure) semiconductors, the Movement of Holes in Intrinsic or pure Semiconductors, Extrinsic (impure) semiconductors, and the purpose of adding impurity.

Intrinsic (pure) semiconductors

Semiconductor in its extremely pure form is called an intrinsic semiconductor. At 0 Kelvin it behaves as an insulator because there are no free electrons.

At room temperature, a few electrons get sufficient energy to break the covalent bond. These electrons can move freely and act as charge carriers.

A vacancy is created in the covalent bond when a free electron is formed and this vacancy is called a hole. Holes in the valence band can be filled by other electrons in the valence band. This results in the movement of holes in the valence band. These holes can be viewed as positive charge carriers.

In an intrinsic semiconductor, the number of holes in the valence band is equal to the number of free electrons. The free electrons exist in the conduction band and holes exist in the valence band.

Movement of Holes in Intrinsic or pure Semiconductors

If an electron in the valence band gets sufficient energy, it can jump across the forbidden energy gap and enter the conduction band.

Suppose a covalent bond ‘A’ breaks due to the external energy received by the atom and an electron is released. A hole is left behind in the valence band. Under favorable conditions, the electron from another covalent bond ‘B’ jumps into the hole at ‘A’.

This fills the original hole but creates a new hole in ’B’. Next, an electron from another bond ‘C’ jumps into the hole in ‘B’ and so on,

In this way a succession of electron movement takes place.

So we can say that the hole is moved from ‘A‘ to ‘C’. Even if the actual movement is by electrons, for convenience we will consider it as hole movement.

It should be noted that holes are filled by electrons that move from adjacent atoms without passing through the forbidden energy gap. It means that hole movement takes place in the valence band only.

facts about Intrinsic or pure Semiconductors:

• Holes exist in the valence band and they flow in the valence band.
• Conduction electrons are found in the conduction band and they flow in the conduction band, as well.
• The Hole current is in the valence band and the electronic current is in the conduction band.
• Conduction electrons move twice as fast as the holes.

Extrinsic (impure) semiconductors

A pure form of semiconductor is a bad conductor of electricity because of the absence of charge carriers that are free to move about.

Therefore in order to have conduction, more free charge carriers have to be created in the crystal structure. This can be achieved by adding suitable materials (impurity) to pure semiconductors.

Impurity-added semiconductors are called extrinsic semiconductors. The process of adding impurities to a semiconductor is known as doping.

Now let us discuss how these free charge carriers are formed when impurity atoms are added.

The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor. If a pentavalent impurity (having 5 valence electrons) is added to the semiconductor, free electrons are produced in the semiconductor. On the other hand, the addition of trivalent impurity (having 3 valence electrons) creates holes in the crystal. Depending on the type of impurity added, extrinsic semiconductors are classified into:
(i) N-type semiconductor
(ii) P-type semiconductor