Editing Field electron emission (section)

== Practical applications: past and present ==

=== Field electron microscopy and related basics ===
As already indicated, the early experimental work on field electron emission (1910–1920)<ref name="Lilienfeld1922"/> was driven by [[Julius Edgar Lilienfeld|Lilienfeld's]] desire to develop miniaturized [[X-ray]] tubes for medical applications. However, it was too early for this technology to succeed.{{Why|date=March 2025}}

After Fowler–Nordheim theoretical work in 1928, a major advance came with the development in 1937 by [[Erwin Wilhelm Mueller|Erwin W. Mueller]] of the spherical-geometry [[field emission microscope|field electron microscope]] (FEM)<ref name=Mueller1937>{{cite journal|year = 1937|author = Mueller, E.W.|journal = Z. Phys.|pages = 541–550|volume = 106|doi = 10.1007/BF01339895|title = Elektronenmikroskopische Beobachtungen von Feldkathoden|bibcode = 1937ZPhy..106..541M|issue = 9–10 |s2cid = 120836411}}</ref> (also called the "field emission microscope"). In this instrument, the electron emitter is a sharply pointed wire, of apex radius ''r''. This is placed, in a vacuum enclosure, opposite an image detector (originally a phosphor screen), at a distance ''R'' from it. The microscope screen shows a projection image of the distribution of current-density ''J'' across the emitter apex, with magnification approximately (''R''/''r''), typically 10<sup>5</sup> to 10<sup>6</sup>. In FEM studies the apex radius is typically 100&nbsp;nm to 1 μm. The tip of the pointed wire, when referred to as a physical object, has been called a "field emitter", a "tip", or (recently) a "Mueller emitter".

When the emitter surface is clean, this FEM image is characteristic of: (a) the material from which the emitter is made: (b) the orientation of the material relative to the needle/wire axis; and (c) to some extent, the shape of the emitter endform. In the FEM image, dark areas correspond to regions where the local work function ''φ'' is relatively high and/or the local barrier field ''F'' is relatively low, so ''J'' is relatively low; the light areas correspond to regions where ''φ'' is relatively low and/or ''F'' is relatively high, so ''J'' is relatively high. This is as predicted by the exponent of Fowler–Nordheim-type equations [see eq. (30) below].

The [[adsorption]] of layers of gas atoms (such as oxygen) onto the emitter surface, or part of it, can create surface [[electric dipole]]s that change the local work function of this part of the surface. This affects the FEM image; also, the change of work-function can be measured using a Fowler–Nordheim plot (see below). Thus, the FEM became an early observational tool of [[surface science]].<ref name=Gomer1961>{{Cite book|title = Field emission and field ionization|year = 1961|author = Gomer, R.|publisher = Harvard Univ. Press|location = Cambridge, Massachusetts|isbn = 1-56396-124-5}}</ref><ref>{{cite journal|title = Recent advances in field electron microscopy of metals|year = 1975|journal =Advances in Electronics and Electron Physics|pages = 193–309|volume = 32|last1 = Swanson|first1 = L.W.|last2 = Bell|first2 = A.E.|doi = 10.1016/S0065-2539(08)60236-X|isbn = 9780120145324}}</ref> For example, in the 1960s, FEM results contributed significantly to discussions on [[heterogeneous catalysis]].<ref>"The role of the adsorbed state in heterogeneous catalysis", Discuss. Faraday Soc., Vol. 41 (1966)</ref> FEM has also been used for studies of [[surface diffusion|surface-atom diffusion]]. However, FEM has now been almost completely superseded by newer surface-science techniques.

A consequence of FEM development, and subsequent experimentation, was that it became possible to identify (from FEM image inspection) when an emitter was "clean", and hence exhibiting its clean-surface work-function as established by other techniques. This was important in experiments designed to test the validity of the standard Fowler–Nordheim-type equation.<ref name=Dyke1953>{{Cite journal|title = Field emission: Large current densities, space charge, and the vacuum arc
|year = 1953|journal = Physical Review|pages = 799–808
|volume = 89|issue = 4|last1 = Dyke |first1 = W.P.
|last2 = Trolan|first2 = J.K.|doi=10.1103/PhysRev.89.799|bibcode = 1953PhRv...89..799D }}</ref><ref>{{cite journal|title = Field emission
|year = 1956|journal = Advances in Electronics and Electron Physics
|pages = 89–185|volume = 8|last1 = Dyke|first1 = W.P.
|last2 = Dolan|first2 = W.W.|doi=10.1016/S0065-2539(08)61226-3| bibcode=1956AEEP....8...89D |isbn = 9780120145089}}</ref> These experiments deduced a value of voltage-to-barrier-field conversion factor ''β'' from a Fowler–Nordheim plot (see below), assuming the clean-surface ''φ''–value for tungsten, and compared this with values derived from [[transmission electron microscope|electron-microscope]] observations of emitter shape and electrostatic modeling. Agreement to within about 10% was achieved. Only very recently<ref>{{cite journal|title = Field emission from crystalline niobium|year = 2009|journal = Physical Review Special Topics - Accelerators and Beams
|page = 023501|volume = 12|issue = 2|doi=10.1103/PhysRevSTAB.12.023501|last1 = Pandey|first1 = A D|last2 = Muller|first2 = Gunter|last3 = Reschke|first3 = Detlef|last4 = Singer|first4 = Xenia|bibcode = 2009PhRvS..12b3501D |doi-access = free}}</ref> has it been possible to do the comparison the other way round, by bringing a well-prepared probe so close to a well-prepared surface that approximate parallel-plate geometry can be assumed and the conversion factor can be taken as 1/''W'', where ''W'' is the measured probe-to emitter separation. Analysis of the resulting Fowler–Nordheim plot yields a work-function value close to the independently known work-function of the emitter.

=== Field electron spectroscopy (electron energy analysis) ===
Energy distribution measurements of field-emitted electrons were first reported in 1939.<ref name=AH39/> In 1959 it was realized theoretically by Young,<ref name=Y59>{{cite journal|doi=10.1103/PhysRev.113.110|title=Theoretical Total-Energy Distribution of Field-Emitted Electrons|year=1959|last1=Young|first1=Russell D.|journal=Physical Review|volume=113|issue=1|pages=110–114|bibcode = 1959PhRv..113..110Y }}</ref> and confirmed experimentally by Young and Mueller<ref name=YM59>{{cite journal|doi=10.1103/PhysRev.113.115|title=Experimental Measurement of the Total-Energy Distribution of Field-Emitted Electrons|year=1959|last1=Young|first1=Russell D.|last2=Müller|first2=Erwin W.|journal=Physical Review|volume=113|issue=1|pages=115–120|bibcode = 1959PhRv..113..115Y }}</ref> that the quantity measured in spherical geometry was the distribution of the total energy of the emitted electron (its "total energy distribution"). This is because, in spherical geometry, the electrons move in such a fashion that [[angular momentum]] about a point in the emitter is very nearly conserved. Hence any [[kinetic energy]] that, at emission, is in a direction parallel to the emitter surface gets converted into energy associated with the radial direction of motion. So what gets measured in an energy analyzer is the [[total energy]] at emission.

With the development of sensitive electron energy analyzers in the 1960s, it became possible to measure fine details of the total energy distribution. These reflect fine details of the [[surface physics]], and the technique of Field Electron Spectroscopy flourished for a while, before being superseded by newer surface-science techniques.<ref name=mo84>{{cite book|author=A. Modinos|title=Field, Thermionic and Secondary Electron Emission Spectroscopy|publisher=Plenum, New York|year=1984|isbn=0-306-41321-3}}</ref><ref name=GP73>{{cite journal|doi=10.1103/RevModPhys.45.487|title=Field Emission Energy Distribution (FEED)|year=1973|last1=Gadzuk|first1=J. W.|last2=Plummer|first2=E. W.|journal=Reviews of Modern Physics|volume=45|pages=487–548|bibcode=1973RvMP...45..487G|issue=3}}</ref>

=== Field electron emitters as electron-gun sources ===
[[File:Schottky-Emitter 01.jpg|thumb|Schottky-emitter electron source of an [[Electron microscope]]]]
To achieve high-resolution in [[transmission electron microscope|electron microscopes]] and other electron beam instruments (such as those used for [[electron beam lithography]]), it is helpful to start with an electron source that is small, optically bright and stable. Sources based on the geometry of a Mueller emitter qualify well on the first two criteria. The first electron microscope (EM) observation of an individual atom was made by [[Albert Crewe]], J. Wall and J. Langmore in 1970,<ref name=CWL70>{{cite journal|doi=10.1126/science.168.3937.1338|title=Visibility of Single Atoms|year=1970|last1=Crewe|first1=A. V.|last2=Wall|first2=J.|last3=Langmore|first3=J.|journal=Science|volume=168|pages=1338–40|pmid=17731040|issue=3937|bibcode = 1970Sci...168.1338C |s2cid=31952480}}</ref> using a [[Scanning transmission electron microscopy|scanning transmission electron microscope]] equipped with an early field emission gun.

From the 1950s onwards, extensive effort has been devoted to the development of field emission sources for use in [[electron gun]]s.<ref name=Ch95>{{cite journal|doi=10.1016/0169-4332(95)00517-X|title=Developing and using the field emitter as a high intensity electron source|year=1996|last1=Charbonnier|first1=F|journal=Applied Surface Science|volume=94-95|pages=26–43|bibcode = 1996ApSS...94...26C }}</ref><ref name=OR08>{{cite book|editor=J.Orloff|title=Handbook of Charged Particle Optics|edition=2|publisher=CRC Press|year=2008}}</ref><ref>L.W. Swanson and A.E. Bell, Adv. Electron. Electron Phys. 32 (1973) 193</ref> [e.g., DD53] Methods have been developed for generating on-axis beams, either by field-induced emitter build-up, or by selective deposition of a low-work-function [[adsorption|adsorbate]] (usually [[Zirconium oxide]] – ZrO) into the flat apex of a [[Miller index|(100) oriented]] [[Tungsten]] emitter.<ref name=Sw75>{{cite journal|doi=10.1116/1.568503|title=Comparative study of the zirconiated and built-up W thermal-field cathode|year=1975|last1=Swanson|first1=L. W.|journal=Journal of Vacuum Science and Technology|volume=12|page=1228|bibcode = 1975JVST...12.1228S|issue=6 }}</ref>

Sources that operate at room temperature have the disadvantage that they can become covered with adsorbate [[molecule]]s that arrive from the [[vacuum]] system walls, and the emitter has to be cleaned from time to time by "flashing" to high temperature. Nowadays, it is common to use Mueller-emitter-based sources that are operated at elevated temperatures, either in the [[Thermionic emission|Schottky emission]] regime or in the so-called temperature-field intermediate regime. Most modern high-resolution electron microscopes and electron beam instruments use some form of field emission electron source. Currently, attempts are being made to develop [[carbon nanotubes]] (CNTs) as electron-gun field emission sources.<ref name=Milne/><ref name=JB04>{{cite journal|doi=10.1098/rsta.2004.1438|title=Carbon nanotube electron sources and applications|year=2004|last1=De Jonge|first1=Niels|last2=Bonard|first2=Jean-Marc|journal=Philosophical Transactions of the Royal Society A|volume=362|pages=2239–66|pmid=15370480|issue=1823|bibcode = 2004RSPTA.362.2239D |s2cid=14497829}}</ref>

The use of field emission sources in electron optical instruments has involved the development of appropriate theories of charged particle optics,<ref name=OR08/><ref name=HK96>{{cite book|author1=P.W. Hawkes |author2=E. Kaspar |title=Principles of Electron Optics|publisher=Academic Press, London|year= 1996|chapter=44,45|volume=2}}</ref> and the development of related modeling. Various shape models have been tried for Mueller emitters; the best seems to be the "Sphere on Orthogonal Cone" (SOC) model introduced by Dyke, Trolan. Dolan and Barnes in 1953.<ref name=DTDB53>{{cite journal|doi=10.1063/1.1721330|title=The Field Emitter: Fabrication, Electron Microscopy, and Electric Field Calculations|year=1953|last1=Dyke|first1=W. P.|last2=Trolan|first2=J. K.|last3=Dolan|first3=W. W.|last4=Barnes|first4=George|journal=Journal of Applied Physics|volume=24|page=570|bibcode = 1953JAP....24..570D|issue=5 }}</ref> Important simulations, involving trajectory tracing using the SOC emitter model, were made by Wiesener and Everhart.<ref name=Ev67>{{cite journal|doi=10.1063/1.1709260|title=Simplified Analysis of Point-Cathode Electron Sources|year=1967|last1=Everhart|first1=T. E.|journal=Journal of Applied Physics|volume=38|page=4944|bibcode = 1967JAP....38.4944E|issue=13 }}</ref><ref>{{cite journal|doi=10.1063/1.1662526|title=Point-cathode electron sources-electron optics of the initial diode region|year=1973|last1=Wiesner|first1=J. C.|journal=Journal of Applied Physics|volume=44|page=2140|bibcode = 1973JAP....44.2140W|issue=5 |doi-access=free}}</ref><ref>{{cite journal|doi=10.1063/1.1663676|title=Point-cathode electron sources-Electron optics of the initial diode region: Errata and addendum|year=1974|last1=Wiesner|first1=J. C.|journal=Journal of Applied Physics|volume=45|page=2797|bibcode = 1974JAP....45.2797W|issue=6 |doi-access=free}}</ref> Nowadays, the facility to simulate field emission from Mueller emitters is often incorporated into the commercial electron-optics programmes used to design electron beam instruments. The design of efficient modern field-emission electron guns requires highly specialized expertise.

=== Atomically sharp emitters ===
Nowadays it is possible to prepare very sharp emitters, including emitters that end in a single atom. In this case, electron emission comes from an area about twice the crystallographic size of a single atom. This was demonstrated by comparing FEM and [[field ion microscope]] (FIM) images of the emitter.<ref name=Fi88>{{cite journal|doi=10.1088/0031-8949/38/2/029|title=Point source for ions and electrons|year=1988|last1=Fink|first1=Hans-Werner|journal=Physica Scripta|volume=38|pages=260–263|bibcode = 1988PhyS...38..260F|issue=2 |s2cid=250806259 }}</ref> Single-atom-apex Mueller emitters also have relevance to the [[scanning probe microscopy]] and [[scanning Helium Ion Microscope|helium scanning ion microscopy]] (He SIM).<ref name=WNB06>{{cite journal|doi=10.1116/1.2357967|title=Helium ion microscope: A new tool for nanoscale microscopy and metrology|year=2006|last1=Ward|first1=B. W.|last2=Notte|first2=John A.|last3=Economou|first3=N. P.|s2cid=55043024|journal=Journal of Vacuum Science and Technology B|volume=24|page=2871|bibcode = 2006JVSTB..24.2871W|issue=6 }}</ref> Techniques for preparing them have been under investigation for many years.<ref name=Fi88/><ref name=BGP96>{{cite journal|doi=10.1016/S1076-5670(08)70156-3|title=Electron Field Emission from Atom-Sources: Fabrication, Properties, and Applications of Nanotips|year=1996|last1=Binh|first1=Vu Thien|last2=Garcia|first2=N.|last3=Purcell|first3=S.T.|volume=95|pages=63–153|journal=Advances in Imaging and Electron Physics|bibcode=1996AdIEP..9582b83B |isbn=9780120147373}}</ref> A related important recent advance has been the development (for use in the He SIM) of an automated technique for restoring a three-atom ("trimer") apex to its original state, if the trimer breaks up.<ref name=WNB06/>

=== Large-area field emission sources: vacuum nanoelectronics ===

==== Materials aspects ====
Large-area field emission sources have been of interest since the 1970s. In these devices, a high density of individual field emission sites is created on a substrate (originally silicon). This research area became known, first as "vacuum microelectronics", now as "vacuum nanoelectronics".

One of the original two device types, the "[[Spindt tip|Spindt array]]",<ref name=SBHW76>{{cite journal|doi=10.1063/1.322600|title=Physical properties of thin-film field emission cathodes with molybdenum cones|year=1976|last1=Spindt|first1=C. A.|journal=Journal of Applied Physics|volume=47|pages=5248–5263|bibcode = 1976JAP....47.5248S|issue=12 }}</ref> used [[integrated circuit|silicon-integrated-circuit (IC)]] fabrication techniques to make regular arrays in which [[molybdenum]] cones were deposited in small cylindrical voids in an oxide film, with the void covered by a counterelectrode with a central circular aperture. This overall geometry has also been used with [[carbon nanotubes]] grown in the void.

The other original device type was the "Latham emitter".<ref name=la95>{{cite book|editor=R.V. Latham|title=High-Voltage Vacuum Insulation: Basic Concepts and Technological Practice|publisher=Academic, London|year=1995}}</ref><ref name=F01>{{cite journal|doi=10.1016/S0038-1101(00)00208-2|title=Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism|year=2001|last1=Forbes|first1=R|journal=Solid-State Electronics|volume=45|pages=779–808|bibcode=2001SSEle..45..779F|issue=6}}</ref> These were MIMIV (metal-insulator-metal-insulator-vacuum) – or, more generally, CDCDV (conductor-dielectric-conductor-dielectric-vacuum) – devices that contained conducting particulates in a dielectric film. The device field-emits because its microstructure/nanostructure has field-enhancing properties. This material had a potential production advantage, in that it could be deposited as an "ink", so IC fabrication techniques were not needed. However, in practice, uniformly reliable devices proved difficult to fabricate.

Research advanced to look for other materials that could be deposited/grown as thin films with suitable field-enhancing properties. In a parallel-plate arrangement, the "macroscopic" field ''F''<sub>M</sub> between the plates is given by {{nowrap|1=''F''<sub>M</sub> = ''V''/''W''}}, where ''W'' is the plate separation and ''V'' is the applied voltage. If a sharp object is created on one plate, then the local field ''F'' at its apex is greater than ''F''<sub>M</sub> and can be related to ''F''<sub>M</sub> by
: <math> F = \gamma F_{\mathrm{M}}.</math>

The parameter ''γ'' is called the "field enhancement factor" and is basically determined by the object's shape. Since field emission characteristics are determined by the local field ''F'', then the higher the ''γ''-value of the object, then the lower the value of ''F''<sub>M</sub> at which significant emission occurs. Hence, for a given value of ''W'', the lower the applied voltage ''V'' at which significant emission occurs.

For a roughly ten year-period from the mid-1990s, there was great interest in field emission from plasma-deposited films of [[diamond-like carbon|amorphous and "diamond-like" carbon]].<ref name=Ro02>{{cite journal|doi=10.1016/S0927-796X(02)00005-0|title=Diamond-like amorphous carbon|year=2002|last1=Robertson|first1=J|journal=Materials Science and Engineering: R: Reports|volume=37|pages=129–281|issue=4–6|s2cid=135487365 }}</ref><ref>{{cite book|author1=S.R.P. Silva |author2=J.D. Carey |author3=R.U.A. Khan |author4=E.G. Gerstner |author5=J.V. Anguita |chapter=9|title=Handbook of Thin Film Materials|editor=H.S. Nalwa|publisher=Academic, London|year=2002}}</ref> However, interest subsequently lessened, partly due to the arrival of [[carbon nanotube|CNT]] emitters, and partly because evidence emerged that the emission sites might be associated with particulate carbon objects created in an unknown way during the [[chemical vapor deposition|deposition process]]: this suggested that [[quality control]] of an industrial-scale production process might be problematic.

The introduction of CNT field emitters,<ref name=JB04/> both in "mat" form and in "grown array" forms, was a significant step forward. Extensive research has been undertaken into both their physical characteristics and possible technological applications.<ref name=Milne/> For field emission, an advantage of CNTs is that, due to their shape, with its high [[aspect ratio]], they are "natural field-enhancing objects".

In recent years there has also been massive growth in interest in the development of other forms of thin-film emitter, both those based on other carbon forms (such as "carbon nanowalls"<ref>{{cite journal | last1 = Hojati-Talemi | first1 = P. | last2 = Simon | first2 = G. | year = 2011| title = Field emission study of graphene nanowalls prepared by microwave-plasma method | journal = Carbon | volume = 49 | issue = 8| pages = 2875–2877 | doi = 10.1016/j.carbon.2011.03.004 | bibcode = 2011Carbo..49.2875H }}</ref>) and on various forms of wide-band-gap semiconductor.<ref name=XH05>{{cite journal|doi=10.1016/j.mser.2004.12.001|title=Novel cold cathode materials and applications|year=2005|last1=Xu|first1=N|last2=Huq|first2=S|journal=Materials Science and Engineering: R: Reports|volume=48|pages=47–189|issue=2–5}}</ref> A particular aim is to develop "high-''γ''" nanostructures with a sufficiently high density of individual emission sites. Thin films of nanotubes in form of nanotube webs are also used for development of field emission electrodes.<ref name=understand >{{cite journal | year = 2013| title = Understanding parameters affecting field emission properties of directly spinnable carbon nanotube webs | journal = Carbon | volume = 57 | pages = 388–394 | doi = 10.1016/j.carbon.2013.01.088 | last1 = Hojati-Talemi | first1 = Pejman | last2 = Hawkins | first2 = Stephen | last3 = Huynh | first3 = Chi | last4 = Simon | first4 = George P. | bibcode = 2013Carbo..57..388H }}</ref><ref name=high >{{cite journal | year = 2013| title = Highly efficient low voltage electron emission from directly spinnable carbon nanotube webs | journal = Carbon | volume = 57 | pages = 169–173 | doi = 10.1016/j.carbon.2013.01.060 | last1 = Hojati-Talemi | first1 = Pejman | last2 = Hawkins | first2 = Stephen C. | last3 = Huynh | first3 = Chi P. | last4 = Simon | first4 = George P. | bibcode = 2013Carbo..57..169H }}</ref><ref>{{cite journal | year = 2010| title = Electron field emission from transparent multiwalled carbon nanotube sheets for inverted field emission displays | journal = Carbon | volume = 48 | pages = 41–46 | doi = 10.1016/j.carbon.2009.08.009 | last1 = Kuznetzov | first1 = Alexander A. | last2 = Lee | first2 = Sergey B. | last3 = Zhang | first3 = Mei | last4 = Baughman | first4 = Ray H. | last5 = Zakhidov | first5 = Anvar A. | issue = 1 | bibcode = 2010Carbo..48...41K }}</ref> It is shown that by fine-tuning the fabrication parameters, these webs can achieve an optimum density of individual emission sites.<ref name=understand/> Double-layered electrodes made by deposition of two layers of these webs with perpendicular alignment towards each other are shown to be able to lower the turn-on electric field (electric field required for achieving an emission current of 10&nbsp;μA/cm<sup>2</sup>) down to 0.3&nbsp;V/μm and provide a stable field emission performance.<ref name=high/>

Common problems with all field-emission devices, particularly those that operate in "industrial vacuum conditions" is that the emission performance can be degraded by the adsorption of gas atoms arriving from elsewhere in the system, and the emitter shape can be in principle be modified deleteriously by a variety of unwanted subsidiary processes, such as bombardment by ions created by the impact of emitted electrons onto gas-phase atoms and/or onto the surface of counter-electrodes. Thus, an important industrial requirement is "robustness in poor vacuum conditions"; this needs to be taken into account in research on new emitter materials.

At the time of writing, the most promising forms of large-area field emission source (certainly in terms of achieved average emission current density) seem to be Spindt arrays and the various forms of source based on CNTs.

==== Applications ====
The development of large-area field emission sources was originally driven by the wish to create new, more efficient, forms of [[flat panel display|electronic information display]]. These are known as "[[field-emission display]]s" or "nano-emissive displays". Although several prototypes have been demonstrated,<ref name=Milne/> the development of such displays into reliable commercial products has been hindered by a variety of industrial production problems not directly related to the source characteristics [En08].

Other proposed applications of large-area field emission sources<ref name=Milne>{{cite journal |author=Milne WI |title=E nano newsletter |date=Sep 2008 |issue=13 |url=http://www.phantomsnet.net/Foundation/Enano_newsletter13.php|display-authors=etal}}</ref> include [[microwave]] generation, space-vehicle neutralization, [[X-ray generation]], and (for array sources) multiple [[electron beam lithography|e-beam lithography]]. There are also recent attempts to develop large-area emitters on flexible substrates, in line with wider trends towards "[[plastic electronics]]".

The development of such applications is the mission of vacuum nanoelectronics. However, field emitters work best in conditions of good ultrahigh vacuum. Their most successful applications to date (FEM, FES and EM guns) have occurred in these conditions. The sad fact remains that field emitters and industrial vacuum conditions do not go well together, and the related problems of reliably ensuring good "vacuum robustness" of field emission sources used in such conditions still await better solutions (probably cleverer materials solutions) than we currently have.

=== Vacuum breakdown and electrical discharge phenomena ===
As already indicated, it is now thought that the earliest manifestations of field electron emission were the electrical discharges it caused. After Fowler–Nordheim work, it was understood that CFE was one of the possible primary underlying causes of vacuum breakdown and electrical discharge phenomena. (The detailed mechanisms and pathways involved can be very complicated, and there is no single universal cause)<ref name=Mil03>{{cite news|url=http://isdeiv.lbl.gov/MillerBiblio/WBib2000Txt.htm |title=Bibliography: electrical discharges in vacuum: 1877-2000 |date=November 2003 |author=H. Craig Miller |url-status=dead |archive-url=https://web.archive.org/web/20071113184640/http://isdeiv.lbl.gov/MillerBiblio/WBib2000Txt.htm |archive-date=November 13, 2007 }}</ref> Where vacuum breakdown is known to be caused by electron emission from a cathode, then the original thinking was that the mechanism was CFE from small conducting needle-like surface protrusions. Procedures were (and are) used to round and smooth the surfaces of electrodes that might generate unwanted field electron emission currents. However the work of Latham and others<ref name=la95/> showed that emission could also be associated with the presence of semiconducting inclusions in smooth surfaces. The physics of how the emission is generated is still not fully understood, but suspicion exists that so-called "triple-junction effects" may be involved. Further information may be found in Latham's book<ref name=la95/> and in the on-line bibliography.<ref name=Mil03/>

=== Internal electron transfer in electronic devices ===
In some electronic devices, electron transfer from one material to another, or (in the case of sloping bands) from one band to another ("[[Zener diode|Zener tunneling]]"), takes place by a field-induced tunneling process that can be regarded as a form of Fowler–Nordheim tunneling. For example, [[Emlyn Rhoderick|Rhoderick's]] book discusses the theory relevant to [[metal–semiconductor junction|metal–semiconductor contacts]].<ref>{{cite book |last1= Rhoderick|first1= E. H.|author-link1= Emlyn Rhoderick|title= Metal-Semiconductor Contacts|year= 1978|publisher= [[Clarendon Press]]|location= Oxford|isbn= 0-19-859323-6}}</ref>