The metal-semiconductor (MS) junction is more commonly known as the Schottky-barrier diode. It is sometimes called the surface-barrier diode. Due to the energy-band discontinuity at the interface, injected carriers possess excess energy and the structure is also referred to as a hot-carrier diode or a hot-electron diode. An MS junction is also a useful building block for many other devices. A special type of MS junction is the ohmic contact where the semiconductor is heavily doped. Obviously ohmic contacts are required for every semiconductor device because the final conductor at the chip level is always a metal.
The metal-semiconductor system is among the oldest semiconductor devices. Application of the device can be traced to before 1900. The realization of a potential barrier resulting from space charge in the semiconductor surface was initiated in 1938 by Schottky, and by Mottindependently. The formulation of the thermionic-emission theory was established by Bethe in 1942. This theory was later refined by Crowell and Sze in 1966. The theory of surface states developed by Bardeen in 1947 was instrumental for better understanding of experimental results. The use of silicide in place of metal on silicon substrates was pioneered by Lepselter and coworkers in 1968. The epitaxial silicide process developed by Tung in 1984 provides new insight into intrinsic metal-semiconductor properties.
The early version of the Schottky diode was in the form of a point contact where a metal wire, called a cat’s whisker, is pressed against a clean semiconductor surface. (A point contact has the characteristics of either a Schottky barrier or a p-n junction, depending on the forming process.) Such a structure was unreliable and not reproducible, and was subsequently replaced by vacuum-deposited metal. A diffused guard ring is often used to avoid leakage and breakdown effects caused by the high electric field at the perimeter of the diode. For silicon substrates, metallic silicides can also be used in place of the metals.
A critical step in fabricating a Schottky-barrier diode is to prepare a clean surface for an intimate contact of the metal. In manufacturing, the surface is cleaned chemically. Experimentalists have also explored cleaved surfaces, as well as cleaning by back-sputtering in vacuum. The metal is usually deposited in vacuum, either by evaporation or sputtering. Chemical deposition is gaining popularity, especially for refractory metals. Plating can also be used but contamination from the solution is not controllable. Silicides on silicon substrates are usually made by metal deposition, followed by heat treatment to form the silicides. Such a system can be potentially more ideal because the reaction consumes silicon and the silicide-semiconductor interface propagates below the original surface. One advantage of a Schottky structure is the low temperature processing. The need for high temperature steps in impurity diffusion or impurity activation after ion implantation can be avoided.
After fabrication of a Schottky-barrier diode, it is often required to measure its barrier height. There are altogether five methods to do so and they are listed below:
1. I-V characteristics: A current level is measured when Vj is extrapolated to zero. With a known effective Richardson constant is used to deduce the barrier height. Due to the exponential dependence of current, the accuracy is not critical.
2. Temperature dependence: The dependence of forward current on temperature can yield the barrier height.
3. C-K characteristics are used to obtain the built-in potential and doping concentration. Barrier height is then the sum.
4. Photoresponse: The quantum efficiency for carriers excited from the metal over the barrier is known to be a function of photon energy. If the square root of the photoresponse is plotted against the photon energy, the barrier height can be obtained.
5. Photovoltaic effect: When a Schottky barrier is exposed to light, a short-circuit current, or an open-circuit voltage can be obtained.
