Extrinsic semiconductor

An extrinsic semiconductor that has been doped with electron donor atoms is called an n-type semiconductor, because the majority of charge carriers in the crystal are negative electrons.

An extrinsic semiconductor which has been doped with electron acceptor atoms is called a p-type semiconductor, because the majority of charge carriers in the crystal are positive holes.

A solid substance can conduct electric current only if it contains charged particles, electrons, which are free to move about and not attached to atoms.

Unlike in metals, the atoms that make up the bulk semiconductor crystal do not provide the electrons which are responsible for conduction.

In semiconductors, electrical conduction is due to the mobile charge carriers, electrons or holes which are provided by impurities or dopant atoms in the crystal.

In an extrinsic semiconductor, the concentration of doping atoms in the crystal largely determines the density of charge carriers, which determines its electrical conductivity, as well as a great many other electrical properties.

This is the key to semiconductors' versatility; their conductivity can be manipulated over many orders of magnitude by doping.

Semiconductors and dopant atoms are defined by the column of the periodic table in which they fall.

Group IV atoms can act as both donors and acceptors; therefore, they are known as amphoteric impurities.

Extrinsic semiconductors are components of many common electrical devices.

Transistors (devices that enable current switching) also make use of extrinsic semiconductors.

As opposed to BJTs, they are called unipolar because they involve single carrier type operation – either N-channel or P-channel.

Band structure of an n-type semiconductor. Dark circles in the conduction band are electrons and light circles in the valence band are holes. The image shows that the electrons are the majority charge carrier.
Band structure of a p-type semiconductor. Dark circles in the conduction band are electrons and light circles in the valence band are holes. The image shows that the holes are the majority charge carrier.