Electron-rich impurities

 Electron-rich impurities

Silicon and germanium belong to group 14 of the periodic table and have four valence electrons each. 

In their crystals each atom forms four covalent bonds with its neighbors (Fig. )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 (Fig.).

 Four out of five electrons are used In the formation of four covalent bonds with the four neighboring silicon atoms. 

The fifth electron is extra and becomes delocalized. These delocalized electrons increase the conductivity of doped silicon (or germanium). 

Here the increase in conductivity is due to the negatively charged electron. hence silicon doped with electron-rich impurity is called n-type semiconductor.