Expalin why does conductivity of germainum crystals increase on doping with galium ?

Explanation

On doping germanium with gallium, some of the positions of lattice of germanium are occupied by gallum. Gallium atom has only three valence ecletrons. Therefore, fourth valency of nearby germanium atom is not satishfied and this site remains vacant. This place is deficient of electrons and is called electron hole or electron vacancy. Electron from neighbouring atoms moves to fill the gap, thereby creating a hole in its original position. Under the influence of electric field, electrons move towards postively charged plates through these holes and conduct electricity. The holes appear to move towards negatively charged plates. The movement of electrons (or electron holes) results in increase in conductivity of germanium.