What is a semiconductor ? Describe two main types of semiconductors and explain mechanism for their conduction.

Explanation

Solids which have intermediate conductivities between those of conductors and insulators are called semiconductors. The range of conductivity for such solids is `10^(-6)` to `10^(4) ("ohm m")^(-1)`. The atomic orbitals of the semiconductor atoms form molecular orbitals which are too ciose to each other so as to form band. In the case of semiconductors, the gap between the valence band and conduction band is small. Some of the elections jump from valence band to conduction band and some conductivity is observed.
n-type semiconductor : When a silicon crystal is doped with group 15 element such as P or As, the dopant atom with 5 valence electrons occupies the site normally occupied by silicon atom. Four electrons of dopant form four covalent bonds with Si and the fifth electron is free to take part in conduction of electricity. This type of semiconductor is called n-type semiconductor. n stands for negative. Thus n-type semiconductor is one in which the conductance takes place through movement of electrons.
p-type semiconductor : When a silicon is doped with group 13 element like B, Al or Ga (having three valence electrons), one of the positions held by Si is taken up by the dopant (B, Al or Ga). The dopant in this case is short of one electron to form 4 bonds with Si. Thus, a positive charge is created. In an electric field, this positive charge (hole) moves through the solid and conducts electricity. This is called p-type semiconductor. p-represents positive charge. The semiconductors in which the conduction take place through positive holes are called p-type semiconductors.