Question -
Answer -
Semiconductors aresubstances having conductance in the intermediate range of 10-6 to 104 ohm−1m−1.
The two main types ofsemiconductors are:
(i) n-type semiconductor
(ii) p-type semiconductor
n-type semiconductor: The semiconductor whose increased conductivityis a result of negatively-charged electrons is called an n-typesemiconductor. When the crystal of a group 14 element such as Si or Ge is dopedwith a group 15 element such as P or As, an n-type semiconductor isgenerated.
Si and Ge have fourvalence electrons each. In their crystals, each atom forms four covalent bonds.On the other hand, P and As contain five valence electrons each. When Si or Geis 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 bondswith four neighbouring Si or Ge atoms. The remaining fifth electron becomesdelocalised and increases the conductivity of the doped Si or Ge. 
p-type semiconductor: The semiconductor whose increased inconductivity is a result of electron hole is called a p-typesemiconductor. When a crystal of group 14 elements such as Si or Ge is dopedwith a group 13 element such as B, Al, or Ga (which contains only three valenceelectrons), a p-type of semiconductor is generated.
When a crystal of Si isdoped with B, the three electrons of B are used in the formation of threecovalent bonds and an electron hole is created. An electron from theneighbouring atom can come and fill this electron hole, but in doing so, itwould leave an electron hole at its original position. The process appears asif the electron hole has moved in the direction opposite to that of theelectron that filled it. Therefore, when an electric field is applied,electrons will move toward the positively-charged plate through electron holes.However, it will appear as if the electron holes are positively-charged and aremoving toward the negatively- charged plate.
