solid state physics Solid-state physics is the study of rigid matter, or solids, through methods such as solid-state chemistry, quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state p ...

the Ridley–Watkins–Hilsum theory (RWH) explains the mechanism by which differential

negative resistance In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it. This is in contrast to an ordina ...

is developed in a bulk solid state

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

material when a voltage is applied to the terminals of the sample. It is the theory behind the operation of the

Gunn diode A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative differential resistance, used in high-frequency electronics. It is based on the "Gunn effect" d ...

as well as several other microwave semiconductor devices, which are used practically in

electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillating or alternating current (AC) signal, usually a sine wave, square wave or a triangle wave, powered by a direct current (DC) source. Oscillators are found ...

s to produce

microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequency, frequencies between 300&n ...

power. It is named for British physicists

Brian Ridley Brian Kidd Ridley (2 March 1931 – 22 May 2024) was a British solid-state physicist specialising in semiconductor theory. He was an emeritus professor at the University of Essex. Early life and education Ridley was born on 2 March 1931. He ...

, Tom Watkins and

Cyril Hilsum Cyril Hilsum (born 17 May 1925) is a British physicist and academic. Hilsum was elected a member of the National Academy of Engineering in 1983 for the inventiveness and leadership in introducing III- V semiconductors into electronic technol ...

who wrote theoretical papers on the effect in 1961. Negative resistance oscillations in bulk semiconductors had been observed in the laboratory by J. B. Gunn in 1962, and were thus named the "Gunn effect", but physicist

Herbert Kroemer Herbert Kroemer (; August 25, 1928 – March 8, 2024) was a German-American physicist who, along with Zhores Alferov, received the Nobel Prize in Physics in 2000 for "developing semiconductor heterostructures used in high-speed- and opto-electro ...

pointed out in 1964 that Gunn's observations could be explained by the RWH theory. In essence, RWH mechanism is the transfer of conduction electrons in a semiconductor from a high

mobility Mobility may refer to: Social sciences and humanities * Economic mobility, ability of individuals or families to improve their economic status * Geographic mobility, the measure of how populations and goods move over time * Mobilities, a conte ...

valley to lower-mobility, higher-energy satellite valleys. This phenomenon can only be observed in materials that have such

energy band In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...

structures. Normally, in a conductor, increasing

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

causes higher charge carrier (usually electron) speeds and results in higher current consistent with

Ohm's law Ohm's law states that the electric current through a Electrical conductor, conductor between two Node (circuits), points is directly Proportionality (mathematics), proportional to the voltage across the two points. Introducing the constant of ...

. In a multi-valley semiconductor, though, higher energy may push the carriers into a higher energy state where they actually have higher effective mass and thus slow down. In effect, carrier velocities and current drop as the voltage is increased. While this transfer occurs, the material exhibits a decrease in current – that is, a negative differential resistance. At higher voltages, the normal increase of current with voltage relation resumes once the bulk of the carriers are kicked into the higher energy-mass valley. Therefore the negative resistance only occurs over a limited range of voltages. Of the type of semiconducting materials satisfying these conditions,

gallium arsenide Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...

(GaAs) is the most widely understood and used. However RWH mechanisms can also be observed in

indium phosphide Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic ("zincblende (crystal structure), zincblende") crystal structure, identical to that of gallium arsenide, GaAs and most of the List of ...

(InP),

cadmium telluride Cadmium telluride (CdTe) is a stable crystalline compound formed from cadmium and tellurium. It is mainly used as the semiconducting material in cadmium telluride photovoltaics and an infrared optical window. It is usually sandwiched with ...

(CdTe),

zinc selenide Zinc selenide is the inorganic compound with the formula ZnSe. It is a lemon-yellow solid although most samples have a duller color due to the effects of oxidation. It is an intrinsic semiconductor with a band gap of about 2.70 eV at , equi ...

(ZnSe) and

indium arsenide Indium arsenide, InAs, or indium monoarsenide, is a narrow-bandgap semiconductor composed of indium and arsenic. It has the appearance of grey cubic crystals with a melting point of 942 °C. Indium arsenide is similar in properties to gallium ars ...

(InAs) under hydrostatic or uniaxial pressure.

References

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* * * * * Electronic engineering {{CMP-stub de:Gunn-Effekt

See also

References

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