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Field emission gun
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<p>A field emission gun (FEG) is a type of <a href="/facts/Electron_gun/l6nahAeW">electron gun</a> in which a sharply pointed Müller-type emitter: 87–128 is held at several <a href="/facts/Kilovolts/KF18MXP2">kilovolts</a> negative <a href="/facts/Potential/8Fcbtnt9">potential</a> relative to a nearby <a href="/facts/Electrode/251PaKX8">electrode</a>, so that there is sufficient potential <a href="/facts/Gradient/TsR7uukj">gradient</a> at the emitter surface to cause <a href="/facts/Field_electron_emission/ReNmucpE">field electron emission</a>. Emitters are either of <a href="/facts/Cold-cathode/da9mlNne">cold-cathode</a> type, usually made of single crystal <a href="/facts/Tungsten/WYwbNDYX">tungsten</a> sharpened to a tip radius of about 100 nm, or of the <a href="/facts/Walter_H._Schottky/2rYjmcAC">Schottky</a> type,: 1–28 in which <a href="/facts/Thermionic_emission/3oujvv1U">thermionic emission</a> is enhanced by barrier lowering in the presence of a high <a href="/facts/Electric_field/tt4MooBs">electric field</a>. Schottky emitters are made by coating a tungsten tip with a layer of <a href="/facts/Zirconium_oxide/d2RN7wGQ">zirconium oxide</a> (ZrO2) decreasing the <a href="/facts/Work_function/VHaHTyXg">work function</a> of the tip by approximately 2.7 <a href="/facts/Electronvolt/YMT1fqX9">eV</a>. </p><p>In <a href="/facts/Electron_microscope/XtaBktwF">electron microscopes</a>, a field emission gun is used to produce an electron beam that is smaller in diameter, more <a href="/facts/Coherence_(physics)/NF1Rgdoa">coherent</a> and with up to three orders of magnitude greater <a href="/facts/Current_density/IlUKSe2B">current density</a> or <a href="/facts/Brightness/e93R5I4b">brightness</a> than can be achieved with conventional <a href="/facts/Thermionic_emission/3oujvv1U">thermionic emitters</a> such as tungsten or <a href="/facts/Lanthanum_hexaboride/60pwIKbE">lanthanum hexaboride</a> (LaB6)-tipped <a href="/facts/Electrical_filament/31frjg0A">filaments</a>. The result in both <a href="/facts/Scanning_electron_microscope/yzK0hsSx">scanning</a> and <a href="/facts/Transmission_electron_microscope/Rpr8jpCn">transmission electron microscopy</a> is significantly improved <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a> and <a href="/facts/Spatial_resolution/ll8LNYxN">spatial resolution</a>, and greatly increased emitter life and reliability compared with thermionic devices. </p>