Silicon oxynitrides can be doped with metal atoms. Amplification is achieved by stimulated emission of photons from dopant ions in the doped fiber. Thus semiconductors should be highly doped. To be proton conductive, PBI usually is doped with acid. Silica glass can be doped with various materials. Crystals of calcium sulfate or calcium fluoride are doped with dysprosium. Hence, highly doped semiconductors behave metallically. For photorefractive applications, barium titanate can be doped by various other elements, e.g. iron. To allow conduction, semiconductors are doped with impurities which donate either mobile electrons or holes. Semiconductors are doped with donors and/or acceptors, which are typically ionized, and are thus charged. Finally, the hole is filled with conducting (doped) polysilicon. Graphene doped with potassium in ultra-high vacuum at low temperature can reduce mobility 20-fold. It is simply a wide band gap, highly doped diode. These aerogels are used as catalysts, especially when "doped" with a metal other than aluminium. Having a lightly doped base ensures recombination rates are low.