|Class 12 Chemistry The Solid State||Electron rich impurities|
Electron – rich impurities
In a periodic table Silicon and germanium belongs to group 14 with four valence electrons each. In their crystals each atom forms four covalent bonds with surrounding atom.
When doped with a group 15 element like P or As, which contains five valence electrons,
They occupy some of the lattice sites in silicon or germanium crystal.
Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron in P is extra and becomes delocalized.
These delocalized electrons increase the conductivity of doped silicon (or germanium).
This type of semiconductor is known as n-type semiconductor because the increase in conductivity is due to the negatively charged electron.