Field emission
Also known as Fowler-Nordheim tunneling,
field emission 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.
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.
*
Cold cathode*
Field emission - Fowler-Nordheim tunneling, Principles of Semiconductor Devices, Bart Van Zeghbroeck, 1997 
