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A field emission gun (FEG) is a type of electron gun in which a sharply pointed Müller-type[ clarification needed] emitter [1]: 87–128 is held at several kilovolts negative potential relative to a nearby electrode, so that there is sufficient potential gradient at the emitter surface to cause field electron emission. Emitters are either of cold-cathode type, usually made of single crystal tungsten sharpened to a tip radius of about 100 nm, or of the Schottky type, [1]: 1–28 in which thermionic emission is enhanced by barrier lowering in the presence of a high electric field. Schottky emitters are made by coating a tungsten tip with a layer of zirconium oxide (ZrO2) decreasing the work function of the tip by approximately 2.7 eV. [2]
In
electron microscopes, a field emission gun is used to produce an electron beam that is smaller in diameter, more
coherent and with up to three orders of magnitude greater
current density or
brightness than can be achieved with conventional
thermionic emitters such as tungsten or
lanthanum hexaboride (LaB
6)-tipped
filaments. The result in both
scanning and
transmission electron microscopy is significantly improved
signal-to-noise ratio and
spatial resolution, and greatly increased emitter life and reliability compared with thermionic devices.