230. What is a semiconductor? Describe two main types of semiconductors, giving examples and their distinctive features.

Semiconductors : These are the solids with conductivities in the intermediate range from 10–6 to 104 ohm^–1m^–1.

In case of semiconductors, the gap between the valence band and conduction band is small Therefore, some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semiconductors increases with rise in temperature, since
more electrons can jump to the conduction band. Substances like silicon and germanium show this type of behaviour and are called intrinsic semiconductors.
The conductivity of these intrinsic semiconductors is too low to be of practical use. Their conductivity is increased by adding an appropriate amount of suitable impurity. This process is called doping.

There are two types of semiconductor
i) n- type semiconductor
ii) p- type semiconductor

Element of group14 of the periodic table, when doped with a group 15 element like P or As, Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron is extra and becomes delocalised.

These delocalised electrons increase the conductivity of doped silicon (or germanium). Here the increase in conductivity is due to the negatively charged electron, hence silicon doped with electron-rich impurity is called n-type semiconductor.

p-type : 

Silicon or germanium can also be doped with a group 13 element like B, Al or Ga which contains only three valence electrons. The place where the fourth valence electron is missing is called electron hole or electron vacancy. This hole can be act as charge carrier in the lattice. This known as p - type semiconductor.