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'Field emission', also known as 'Fowler-Nordheim tunneling', is a form of quantum tunneling in which electrons pass through a barrier in the presence of a high electric field. This phenomenon is highly dependent on both the properties of the material and the shape of the particular cathode, so that higher aspect ratios produce higher field emission currents. The current density produced by a given electric field is governed by the Fowler-Nordheim equation.
The phenomenon of field emission was first noticed by R. W. Wood in 1897, while doing experiments with his discharge tube. Late in 1922, Lilienfeld made observations on this effect in his high vacuum X-ray tube with pointed cathode. A very good account of early work is found in review article by R. O. Jenkins. Classical theories failed to explain the experimental results that field emission was independent of temperature in the region well below onset of thermionic emission. The basic process was then explained by Fowler and Nordheim in 1928 using wave-mechanical formulation.
Applications of field emission include its use as an electron source in flash memory, electron microscopy, MEMS systems, and field emission displays.In the field of vacuum electronics, field emission is seen as an alternative to thermionic emission, with advantages such as dramatically higher efficiency, less scatter of emitted electrons, faster turn-on times, compactness, and, in many cases, redundancy. Some disadvantages include lower current per emission source and, often, lower overall current density. Field emission limits the maximum operating voltage for high voltage vacuum devices such as vacuum capacitors and vacuum switches.
Vacuum tubes based on thermionic emission require several minutes to warm up before they can be used; by contrast, the function of field emission devices is effectively instantaneous, allowing switching times of many megahertz. The ability to modulate the electron source, rather than modifying a stream of electrons from a constant source (i.e., by velocity modulation), has allowed many vacuum devices to be greatly simplified. For instance, the Klystrode functions much like the two-chamber klystron, without the need for a first chamber.
The field emission process is based on the Fowler-Nordheim Model which assumes that,
★ The temperature of metal is zero kelvin.
★ Inside the metal free electron model is valid
★ The surface is a smooth plane &
★ The potential barrier close to the surface in the vacuum region
consists of an image force potential and a potential due to the
applied electric field
★ Cold cathode
★ Field emission microscope
★ Field Emitter Array
★ ''Field emission - Fowler-Nordheim tunneling'', Principles of Semiconductor Devices, Bart Van Zeghbroeck, 1997
| Contents |
| History |
| Applications |
| Field Electron Emission Process |
| See also |
| External links |
History
The phenomenon of field emission was first noticed by R. W. Wood in 1897, while doing experiments with his discharge tube. Late in 1922, Lilienfeld made observations on this effect in his high vacuum X-ray tube with pointed cathode. A very good account of early work is found in review article by R. O. Jenkins. Classical theories failed to explain the experimental results that field emission was independent of temperature in the region well below onset of thermionic emission. The basic process was then explained by Fowler and Nordheim in 1928 using wave-mechanical formulation.
Applications
Applications of field emission include its use as an electron source in flash memory, electron microscopy, MEMS systems, and field emission displays.In the field of vacuum electronics, field emission is seen as an alternative to thermionic emission, with advantages such as dramatically higher efficiency, less scatter of emitted electrons, faster turn-on times, compactness, and, in many cases, redundancy. Some disadvantages include lower current per emission source and, often, lower overall current density. Field emission limits the maximum operating voltage for high voltage vacuum devices such as vacuum capacitors and vacuum switches.
Vacuum tubes based on thermionic emission require several minutes to warm up before they can be used; by contrast, the function of field emission devices is effectively instantaneous, allowing switching times of many megahertz. The ability to modulate the electron source, rather than modifying a stream of electrons from a constant source (i.e., by velocity modulation), has allowed many vacuum devices to be greatly simplified. For instance, the Klystrode functions much like the two-chamber klystron, without the need for a first chamber.
Field Electron Emission Process
The field emission process is based on the Fowler-Nordheim Model which assumes that,
★ The temperature of metal is zero kelvin.
★ Inside the metal free electron model is valid
★ The surface is a smooth plane &
★ The potential barrier close to the surface in the vacuum region
consists of an image force potential and a potential due to the
applied electric field
See also
★ Cold cathode
★ Field emission microscope
★ Field Emitter Array
External links
★ ''Field emission - Fowler-Nordheim tunneling'', Principles of Semiconductor Devices, Bart Van Zeghbroeck, 1997
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