Metal–semiconductor junction

In solid-state physics, a metal–semiconductor junction is a type of junction in which a metal comes in close contact with a semiconductor material. Similar to a p-n junction, it has rectifying properties.

History

The rectification property of metal–semiconductor contacts was discovered by Ferdinand Braun in 1874 using mercury metal contacted with copper and iron sulfide semiconductors.

G.W. Pickard received a patent in 1906 on a point-contact rectifier using silicon. In 1907, Pierce published a paper in Physical Review showing rectification properties of diodes made by sputtering many metals on many semiconductors. The use of the metal-semiconductor (M/S) diode rectifier was proposed by Lilienfeld in 1926 in the first of his three transistor patents as the gate of the field-effect transistors. The correct theory of the field-effect transistor using a metal/semiconductor gate was advanced by Shockley in 1939.

The earliest M/S diodes in electronics application occurred in the early 1920's when the cat's whisker rectifiers were used in broadcast receivers. They consisted of pointed tungsten wire (in the shape of a cat's whisker) whose tip or point is pressed against the surface of a lead sulfide crystal. The first large area rectifier appeares around 1926 which consisted of a copper-oxide semiconductor thermally grown on a copper substrate. Subsequently, semiconductor film, such as selenium, were evaporated onto large metal substrates to form the rectifying M/S diodes. These large-area diodes were used to convert a.c. current to d.c. current in electrical power applications. During 1925-1940, M/S diodes consisting of a pointed tungsten metal wire in contact with a silicon crystal base, were fabricated in laboratories to detect microwaves in the UHF range. A World War II program to manufacture high-purity silicon as the crystal base for the point-contact rectifier was suggested by Frederick Seitz in 1942 and successfully undertaken by the Experimental Station of the E. I du Pont de Nemours Company.

Theory of metal-semiconductor junction

The first theory that predicted the correct direction of rectification of the metal–semiconductor junction was given by Mott in 1939. He found the solution for both the diffusion and drift currents of the majority carriers through the semiconductor surface space charge layer which has been known since about 1948 as the Mott barrier. Schottky and Spenke extended Mott's theory by including a donor ion whose density is spatially constant through the semiconductor surface layer. This changed the constant electric field assumed by Mott to a linearly decaying electric field. This semiconductor space-charge layer under the metal is known as the Schottky barrier. A similar theory was also proposed by Davydov in 1939. Although it gives the correct direction of rectification, it has also been proven that the Mott theory and its Schottky-Davydov extension gives the wrong current limiting mechanism and wrong current-voltage formulae in silicon metal/semiconductor diode rectifiers. The correct theory was developed by Hans Bethe and reported by him in a M.I.T. Radiation Laboratory Report dated November 23, 1942. In Bethe's theory, the current is limited by thermionic emission of electrons over the metal–semiconductor potential barrier. Thus, the appropriate name for the metal/semiconductor diode should be the Bethe diode, instead of the Schottky diode, since the Schottky theory does not predict the modern M/S diode characteristics correctly.^[1]

References

1. ^ Fundamentals of Solid-State Electronics, Chih-Tang Sah. World Scientific, first published 1991, reprinted 1992, 1993 (pbk), 1994, 1995, 2001, 2002, 2006, ISBN 9810206372. -- ISBN 9810206380 (pbk).