Impurity level in a crystal

A spatially localized electronic level produced by a defect or foreign atom in a crystalline potential. Such levels can lie in a host band gap, where no extended host state has the same energy, while impurity resonances can also occur inside an energy band. Dilute localized levels are nearly independent; increasing overlap can broaden them into an impurity band. The location and existence of a bound level depend on the impurity potential.