Question

Why does conductivity of germanium crystals increase on adding gallium atoms?

Answer 1

as we know gallium is trivalent element & germanium is tetravalent so on adding germanium with gallium, some of the crystal sites are occupied by gallium atoms..so
3 electrons of gallium are used for making bonds with germanium while the 4th electron of germanium remains unbounded and thus is free for movement and increases the conductivity of crystal....