In practice, most metal-semiconductor interfaces do not follow this rule to the predicted degree. Instead, the chemical termination of the semiconductor crystal against a metal creates electron states within its band gap. The nature of these metal-induced gap states and their occupation by electrons tends to pin the center of the band gap to the Fermi level, an effect known as Fermi level pinning. Thus the heights of the Schottky barriers in metal-semiconductor contacts often show little dependence on the value of the semiconductor or metal work functions, in stark contrast to the Schottky-Mott rule. Different semiconductors exhibit this Fermi level pinning to different degrees, but a technological consequence is that high quality (low resistance) ohmic contacts are usually difficult to form in important semiconductors such as silicon and gallium arsenide.