Ridley Watkins Hilsum theory
Encyclopedia
The Ridley–Watkins–Hilsum
Theory (RWH) explains the mechanism by which differential negative resistance
is developed in a bulk solid state device when a voltage is applied to the terminals of the sample.
Such effects had been observed in the laboratory by J. B. Gunn in 1962, and were thus named the "Gunn Effect", but it was Kroemer
who suggested that J. B. Gunn's observations were in complete agreement with the RWH in 1964.
In essence, RWH mechanism is the transfer of conduction electrons in a semiconductor from a high mobility
valley to lower-mobility, higher-energy satellite valleys. This phenomenon can only be observed in materials that have such energy band structures.
Normally, in a conductor, increasing electric field
causes higher charge carrier
(usually electron) speeds and results in higher current consistent with Ohm's Law
. 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 (NDR). Normal voltage-current relation resumes once the bulk of the carriers are kicked into the higher energy-mass valley.
Of the type of semiconducting materials satisfying these conditions, gallium arsenide (GaAs) is the most widely understood and used. However RWH mechanisms can also be observed in indium phosphide (InP), cadmium telluride
(CdTe), zinc selenide
(ZnSe) and indium arsenide (InAs) under hydrostatic or uniaxial pressure.
Cyril Hilsum
Cyril Hilsum CBE FRS FREng HonFInstP is a British physicist and academic.-Life:He entered Raine's Foundation School in 1936 as the middle of three brothers, leaving in 1943 after being accepted into University College London, where he did his Bsc. In 1945 he joined the Royal Naval Scientific...
Theory (RWH) explains the mechanism by which differential negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
is developed in a bulk solid state device when a voltage is applied to the terminals of the sample.
Such effects had been observed in the laboratory by J. B. Gunn in 1962, and were thus named the "Gunn Effect", but it was Kroemer
Herbert Kroemer
Herbert Kroemer , a professor of electrical and computer engineering at the University of California, Santa Barbara, received his Ph.D. in theoretical physics in 1952 from the University of Göttingen, Germany, with a dissertation on hot electron effects in the then-new transistor, setting the stage...
who suggested that J. B. Gunn's observations were in complete agreement with the RWH in 1964.
In essence, RWH mechanism is the transfer of conduction electrons in a semiconductor from a high mobility
Electron mobility
In solid-state physics, the electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility...
valley to lower-mobility, higher-energy satellite valleys. This phenomenon can only be observed in materials that have such energy band structures.
Normally, in a conductor, increasing 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...
causes higher charge carrier
Charge carrier
In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...
(usually electron) speeds and results in higher current consistent with Ohm's Law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
. 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 (NDR). Normal voltage-current relation resumes once the bulk of the carriers are kicked into the higher energy-mass valley.
Of the type of semiconducting materials satisfying these conditions, gallium arsenide (GaAs) is the most widely understood and used. However RWH mechanisms can also be observed in indium phosphide (InP), cadmium telluride
Cadmium telluride
Cadmium telluride is a crystalline compound formed from cadmium and tellurium. It is used as an infrared optical window and a solar cell material. It is usually sandwiched with cadmium sulfide to form a p-n junction photovoltaic solar cell...
(CdTe), zinc selenide
Zinc selenide
Zinc selenide , is a light yellow binary solid compound. It is an intrinsic semiconductor with a band gap of about 2.70 eV at 25 °C. ZnSe rarely occurs in nature...
(ZnSe) and indium arsenide (InAs) under hydrostatic or uniaxial pressure.