Extrinsic Semiconductor

Intrinsic semiconductors have very low conductivity at room temperature. Therefore, they are not useful for constructing electronic devices. Their electrical conductivity can be increased by adding a suitable impurity. This process is called doping.

A doped semiconductor is called an extrinsic semiconductor or impurity semiconductor.

The impurity added is called a dopant.

The semiconductor to which the dopant is added is called the host.

The addition of a dopant should not disturb the original crystal lattice. Therefore, the sizes of the dopant and host atoms should be nearly the same.

Silicon and germanium belong to the 14th group of the periodic table and are tetravalent. Their dopants are chosen from the neighbouring 15th and 13th groups of the periodic table.

Pentavalent Dopants

Examples of pentavalent dopants are:

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

Trivalent Dopants

Examples of trivalent dopants are:

• Indium (In)
• Boron (B)
• Aluminium (Al)

Different impurities are added to get the desired properties and conduction mechanisms. The number of electrons or holes changes according to the type of impurity added.

Thus, extrinsic semiconductors are of two types:

• n-type semiconductor
• p-type semiconductor

• Intrinsic semiconductors have very low conductivity at room temperature.
• Doping increases conductivity.
• A doped semiconductor is called an extrinsic semiconductor.
• The impurity added is called a dopant.
• The semiconductor receiving the impurity is called the host.
• The dopant size should be nearly the same as that of the host atom.
• Pentavalent and trivalent impurities are used as dopants.
• Extrinsic semiconductors are of two types: n-type and p-type.