Field emission
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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.
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.