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

Ans: Semiconductors are substances having conductance in the intermediate range 10^{-6} to 10^{4} ohm ^{-1} \mathrm{~m}^{-1}.
The two main types of semiconductors are:
(i) n-type semiconductor
(ii) p-type semiconductor
n-type semiconductor: The semiconductor whose increased conductivity is a result of negatively-charged electrons is called an n-type semiconductor. When the crystal of a group 14 element such as Si or Ge is doped with a group 15 element such as P or As, an n-type semiconductor is generated.
Si and Ge have four valence electrons each. In their crystals, each atom forms four covalent bonds. On the other hand, \mathrm{P} and As contain five valence electrons each. When Si or Ge is doped with P or As, the latter occupies some of the lattice sites in the crystal. Four out of five electrons are used in the formation of four covalent bonds