The two main types of semiconductors are as follows:
i. n-type semiconductor (Electron rich or donor impurities):
The electron rich (or donor) impurities provide electrons in the following way:
a. Silicon and germanium belonging to group 14 of the periodic table have four valence electrons. In their crystals, each atom forms four covalent bonds with its neighbours.
b. If some atoms with five valence electrons (of group 15) such as arsenic (As) or phosphorus (P) are added to the germanium crystal, a minute proportion of ‘Ge’ atoms are randomly replaced by ‘As’ atoms.
c. As a ‘Ge’ atom is substituted by an atom of ‘As’, four of the electrons in arsenic form covalent bonds with surrounding ‘Ge’ atoms and the fifth electron remains free as shown in figure (a).
d. Hence, an extra electron, (more than the number required for forming the four covalent bonds), gets introduced in the crystal. This extra electron which is not needed for bonding, becomes delocalised and thus helps in conducting electricity. Therefore, germanium containing traces of arsenic (or arsenic doped germanium) exhibits high electrical conductivity.
e. This type of conduction is known as n-type semiconduction, where ‘n’ stands for negative because electrons are responsible for semiconducting behaviour. This is shown in figure (b). Doping of germanium or silicon with other group-15 elements such as P, Sb or Bi also give n-type semiconductors.
ii. p-type semiconductor (Electron deficient or acceptor impurity):
An electron deficient or acceptor impurity helps in conduction in the following way:
a. Some atoms of germanium are doped with acceptor atoms having only three outer shell electrons such as Indium (of Group 13).
b. Each indium atom uses its three electrons to form three bonds in the lattice and is unable to form fourth bond to complete the network structure of Ge. As a result, some sites normally occupied by electrons are left empty and gives rise to electrons deficiencies. The electron deficient sites are called electron vacancies or positive holes because the net charge at these sites is positive.
c. When electric field is applied, a valence electron on adjacent Ge atom may gain sufficient energy to move into the hole. This forms a new positive hole on the adjacent Ge atom.
d. The migration of positive hole continues and current is carried throughout the crystal. This is equivalent to moving an electron in the opposite direction, and therefore, current is carried. Thus, doping of germanium with traces of indium increases the electrical conductivity of the germanium crystal. This type of conduction is called p-type semiconduction because holes (positive in charge) appears to be responsible for the semiconducting properties. This is shown in figure (b).
e. Doping of silicon or germanium with other group-13 elements such as B, Al or Ga also give p-type semiconductors.
