Metal-semiconductor junction
Encyclopedia
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.
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 transistor
s. 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
.
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.
Solid-state physics
Solid-state physics is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from...
, a metal–semiconductor junction is a type of junction in which a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
comes in close contact with a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
material. Similar to a p-n junction
P-n junction
A p–n junction is formed at the boundary between a P-type and N-type semiconductor created in a single crystal of semiconductor by doping, for example by ion implantation, diffusion of dopants, or by epitaxy .If two separate pieces of material were used, this would...
, it has rectifying properties.
History
The rectification property of metal–semiconductor contacts was discovered by Ferdinand Braun in 1874 using mercuryMercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
metal contacted with copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
semiconductors.
G.W. Pickard
Greenleaf Whittier Pickard
Greenleaf Whittier Pickard was a United States radio pioneer. Pickard was a researcher in the early days of wireless. He experimented with crystal detectors, antennas, wave propagation, and noise suppression...
received a patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
in 1906 on a point-contact rectifier using silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
. In 1907, Pierce published a paper in Physical Review
Physical Review
Physical Review is an American scientific journal founded in 1893 by Edward Nichols. It publishes original research and scientific and literature reviews on all aspects of physics. It is published by the American Physical Society. The journal is in its third series, and is split in several...
showing rectification properties of diodes made by sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
many metals on many semiconductors. The use of the metal-semiconductor (M/S) diode rectifier was proposed by Lilienfeld
Julius Edgar Lilienfeld
Julius Edgar Lilienfeld was an Austro-Hungarian physicist. He was born in Lemberg in Austria-Hungary , moved to the United States in the early 1920s, and became American citizen in 1934...
in 1926 in the first of his three transistor patents as the gate of the field-effect transistor
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
s. The correct theory of the field-effect transistor using a metal/semiconductor gate was advanced by Shockley
William Shockley
William Bradford Shockley Jr. was an American physicist and inventor. Along with John Bardeen and Walter Houser Brattain, Shockley co-invented the transistor, for which all three were awarded the 1956 Nobel Prize in Physics.Shockley's attempts to commercialize a new transistor design in the 1950s...
in 1939.
The earliest M/S diodes in electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
application occurred in the early 1920's when the cat's whisker rectifiers were used in broadcast
Broadcast
Broadcast or Broadcasting may refer to:* Broadcasting, the transmission of audio and video signals* Broadcast, an individual television program or radio program* Broadcast , an English electronic music band...
receivers
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. They consisted of pointed tungsten wire (in the shape of a cat's whisker) whose tip
Tip
A tip is an extra payment made to certain service sector workers in addition to the advertised price of the transaction. Such payments and their size are a matter of social custom. Tipping varies among cultures and by service industry...
or point is pressed against the surface of a lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
sulfide crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
. 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
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
, 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
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
metal wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
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
Frederick Seitz
Frederick Seitz was an American physicist and a pioneer of solid state physics. Seitz was president of Rockefeller University, and president of the United States National Academy of Sciences 1962–1969. He was the recipient of the National Medal of Science, NASA's Distinguished Public Service...
in 1942 and successfully undertaken by the Experimental Station of the E. I du Pont de Nemours Company
E. I du Pont de Nemours Company
E. I du Pont de Nemours Company was the holding company formed in the early 1900s by T. Coleman du Pont, Alfred I. du Pont and Pierre S. du Pont to save the family business from being bought out by a rival....
.
Theory of metal-semiconductor junction
The first theory that predicted the correct direction of rectification of the metal–semiconductor junction was given by MottNevill Francis Mott
Sir Nevill Francis Mott, CH, FRS was an English physicist. He won the Nobel Prize for Physics in 1977 for his work on the electronic structure of magnetic and disordered systems, especially amorphous semiconductors. The award was shared with Philip W. Anderson and J. H...
in 1939. He found the solution for both the diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
and drift
Drift
- Film and literature :* Drift , a 2002 Doctor Who novel* Drift , a series of Japanese films written and directed by Futoshi Jinno* Drift, 2007 experimental short film by Max Hattler* Drift , a fictional character...
currents of the majority carriers through the semiconductor surface space charge layer which has been known since about 1948 as the Mott barrier. Schottky
Walter H. Schottky
Walter Hermann Schottky was a German physicist who played a major early role in developing the theory of electron and ion emission phenomena, invented the screen-grid vacuum tube in 1915 and the pentode in 1919 while working at Siemens, and later made many significant contributions in the areas of...
and Spenke extended Mott's theory by including a donor ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
whose density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
is spatially constant through the semiconductor surface layer. This changed the constant electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
assumed by Mott to a linearly decaying electric field. This semiconductor space-charge layer under the metal is known as the Schottky barrier
Schottky barrier
A Schottky barrier, named after Walter H. Schottky, is a potential barrier formed at a metal–semiconductor junction which has rectifying characteristics, suitable for use as a diode...
. A similar theory was also proposed by Davydov
Davydov
Davydov , or Davydova , is a surname common in Russia and Ukraine.*Alexander Davydov, Soviet and Ukrainian physicist*Avgust Davydov, Russian mathematician and mechanic*Boris Davydov, Russian hydrographer and geodesist...
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
Hans Bethe
Hans Albrecht Bethe was a German-American nuclear physicist, and Nobel laureate in physics for his work on the theory of stellar nucleosynthesis. A versatile theoretical physicist, Bethe also made important contributions to quantum electrodynamics, nuclear physics, solid-state physics and...
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
Thermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
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
Schottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...
, since the Schottky theory does not predict the modern M/S diode characteristics correctly.