Explain how a p-type & n-type semiconductors are formed ????
Doping is a process of deliberate addition of a desirable impurity atoms to a pure semiconductor to modify its properties in a controlled manner.
Doped semiconductors are also called extrinsic semiconductors. They are of 2 types:
n type semiconductor
we know that Si has valency of 4. Now when a pure semiconductor like Si in which each atom has 4 valence electrons, is doped with a controlled amount of pentavalent atoms like Antimony, which have 5 valence electrons, the impurity atom replaces a Si atom and 4 out of the 5 electrons form covalent bonds by sharing the electrons with the adjoining 4 toms of Si while the fifth electron is free to move hence constituting the current in the semiconductor. Since this type of semiconductor has free electrons to donate, it is also called donor type semiconductor.
p- type semiconductor
when a pure semiconductor such as Ge is doped with controlled amount of trivalent atoms say Indium, which have three valence electrons , the impurity atom replaces the Ge atom. Three valence electrons of the impurity atoms form covalent bonds by sharing the electrons of the adjoining 3 atoms of Ge , while there will be 1 incomplete covalent bond with the neighbouring Ge atom, due to deficiency of electron. This deficiency is compensated by taking an electron from another Ge- Ge bond . This electron leaves a hole or positive charge behind. This positive charge constitutes the hole current, in p – type semiconductor.
Such a semi conductor is also called acceptor type semi conductor.
This involves chemical elements silicon and germanium. The addition of a small amount of foreign atoms in the regular crystal lattice of silicon or germanium produces the changes in their electrical properties, producing the n-type and p-type semiconductors.
The elements silicon and germanium are not actually semiconductors, they are "doped" to create the desired changes. The doping process is a way of implanting dopant atoms. Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing extra electrons. Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a hole or electron deficiency.