Schottky barrier

A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifying characteristics, suitable for use as a diode. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by ΦB (see figure). The value of ΦB depends on the combination of metal and semiconductor. Not all metal–semiconductor junctions form a rectifying Schottky barrier; a metal–semiconductor junction that conducts current in both directions without rectification, perhaps due to its Schottky barrier being too low, is called an ohmic contact. To a first approximation, the barrier between a metal and a semiconductor is predicted by the Schottky-Mott rule to be proportional to the difference of the metal-vacuum work function and the semiconductor-vacuum electron affinity. In practice, however, 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 strong contrast to the Schottky-Mott rule. Different semiconductors exhibit this Fermi level pinning to different degrees, but a technological consequence is that ohmic contacts are usually difficult to form in important semiconductors such as silicon and gallium arsenide. Non-ohmic contacts present a parasitic resistance to current flow that consumes energy and lowers device performance. In a rectifying Schottky barrier, the barrier is high enough that there is a depletion region in the semiconductor, near the interface.This gives the barrier a high resistance when small voltage biases are applied to it.Under large voltage bias, the electric current flowing through the barrier is essentially governed by the laws of thermionic emission, combined with the fact that the Schottky barrier is fixed relative to the metal's Fermi level. Note: the discussion above is for a Schottky barrier to an n-type semiconductor; similar considerations apply for a p-type semiconductor. The current-voltage relationship is qualitatively the same as with a p-n junction, however the physical process is somewhat different. For very high Schottky barriers where ΦB is a significant fraction of the band gap of the semiconductor, the forward bias current may instead be carried 'underneath' the Schottky barrier, as minority carriers in the semiconductor. An example of this is seen in the Point-contact transistor.