Because deposited metals can themselves oxidise in ambient conditions, to the detriment of the contacts' electrical properties, it is common to form ohmic contacts with layered structures. The bottom layer, in contact with the semiconductor, is chosen for its ability to induce ohmic behaviour. The top layer is chosen for its low reactivity. Optionally, a three-layer structure may be used. Where this is the case, the middle layer serves as a diffusion barrier, preventing the metals from mixing during any annealing process.