View source for Engineering:Ridley–Watkins–Hilsum theory

In solid state physics the '''Ridley–Watkins–Hilsum theory''' ('''RWH''') explains the mechanism by which differential [[Physics:Negative resistance|negative resistance]] is developed in a bulk solid state [[Physics:Semiconductor|semiconductor]] material when a voltage is applied to the terminals of the sample.<ref>
{{cite journal
 | author1 = B.K. Ridley
 | author2 = T.B. Watkins
 | year = 1961
 | title = The Possibility of Negative Resistance Effects in Semiconductors
 | journal = [[Organization:Proceedings of the Physical Society|Proceedings of the Physical Society]]
 | volume = 78 | issue = 2 | page = 293 
 | bibcode = 1961PPS....78..293R
 | doi = 10.1088/0370-1328/78/2/315
}}</ref>  It is the theory behind the operation of the [[Engineering:Gunn diode|Gunn diode]] as well as several other microwave semiconductor devices, which are used practically in [[Engineering:Electronic oscillator|electronic oscillator]]s to produce [[Physics:Microwave|microwave]] power.  It is named for British physicists Brian Ridley,<ref>{{cite web|last1=Ridley|first1=Brian|title=B. K. Ridley|url=http://www.essex.ac.uk/csee/research/omd_lab/ridley.htm|website=www.essex.ac.uk|accessdate=3 March 2015}}</ref> Tom Watkins and [[Biography:Cyril Hilsum|Cyril Hilsum]] who wrote theoretical papers on the effect in 1961.

Negative resistance oscillations in bulk semiconductors had been observed in the laboratory by [[Biography:J. B. Gunn|J. B. Gunn]] in 1962,<ref name=gunn_effect>
{{cite journal
  | author = J. B. Gunn
  | year = 1963
  | title = Microwave Oscillation of Current in III-V Semiconductors
  | journal = [[Physics:Solid State Communications|Solid State Communications]]
  | volume = 1 | issue = 4 | pages = 88–91
  | bibcode = 1963SSCom...1...88G
  | doi = 10.1016/0038-1098(63)90041-3
}}</ref> and were thus named the "Gunn effect", but physicist [[Biography:Herbert Kroemer|Herbert Kroemer]] pointed out in 1964 that Gunn's observations could be explained by the RWH theory.<ref>
{{cite journal
 | author = H. Kroemer
 | year = 1964 
 | title = Theory of the Gunn effect
 | journal = [[Proceedings of the IEEE]]
 | volume = 52 | issue = 12 | page = 1736
 | bibcode =
 | doi =10.1109/proc.1964.3476
}}</ref>

In essence, RWH mechanism is the transfer of conduction electrons in a semiconductor from a high [[Physics:Electron mobility|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 [[Physics:Electric field|electric field]] causes higher charge [[Physics:Charge carrier|carrier]] (usually electron) speeds and results in higher current consistent with [[Physics:Ohm's law|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 [[Physics:Effective mass (solid-state physics)|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, [[Chemistry:Gallium arsenide|gallium arsenide]] (GaAs) is the most widely understood and used. However RWH mechanisms can also be observed in [[Chemistry:Indium phosphide|indium phosphide]] (InP), [[Chemistry:Cadmium telluride|cadmium telluride]] (CdTe), [[Chemistry:Zinc selenide|zinc selenide]] (ZnSe) and [[Chemistry:Indium arsenide|indium arsenide]] (InAs) under hydrostatic or uniaxial pressure.

== See also ==
* [[Engineering:Gunn diode|Gunn diode]]

== References ==
{{Reflist}}

==Other sources==
*{{Cite book
 | last = Liao | first = Samual Y
 | year = 1990
 | title = Microvave Devices and Circuits
 | edition = 3rd
 | publisher = Prentice Hall
 | isbn = 0-13-583204-7
}}
*{{Cite book
 |last1=Averkov |first1=Y.O.
 |year=2001
 |chapter=The role of the Ridley–Watkins–Hilsum effect in stabilization of millimeter and sub-millimeter surface electromagnetic waves excited byan electron beam moving parallel to the surface of GaAs
 |title=The Fourth International Kharkov Symposium on Physics and Engineering of Millimeter and Sub-Millimeter Waves
 |volume=1 |pages=299–301
 |doi=10.1109/MSMW.2001.946832
 |isbn=0-7803-6473-2
|s2cid=122627167
 }}
*{{Cite journal
 |last1=Sterzer |first1= F
 |year=1971
 |title= Transferred electron (Gunn) amplifiers and oscillators for microwave applications
 |journal=[[Proceedings of the IEEE]]
 |volume=59 |issue=8 |pages=1155–1163
 |bibcode=
 |doi=10.1109/PROC.1971.8361
}}
*{{Cite journal
 |last1= Agamalyan |first1= N. R.
 |last2=Vartanyan |first2=E. S.
 |last3=Hovsepyan |first3=R. K.
 |year=1996
 |title= Photoelectric properties of lead molybdate crystals
 |journal=Physica Status Solidi A
 |volume=157 |issue=2 |pages=421–425
 |bibcode = 1996PSSAR.157..421A
 |doi=10.1002/pssa.2211570226
}}
*{{cite web
 |title=Phenomena/Theories – 1961
 |url=http://www.geocities.com/semnews/milestones.html
 |work=Milestones in Semiconductor Science and Technology
 |archiveurl=https://web.archive.org/web/20091022064123/http://geocities.com/semnews/milestones.html
 |url-status=dead
 |archivedate=2009-10-22
}}

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