When extraction voltage of a few kV is put through a sharpened metal single crystal with a tip of about 0.1 microns, the electrons inside the metal are drawn out into the vacuum. These electrons are known as field-emission electrons, and their characteristics are that their directivity is united, and the dispersion in energy is small at about 0.3 to 0.4 eV.
The part where field-emission electrons are accelerated at a certain speed and sent into the electron microscope is the field-emission electron gun. Compared to general thermionic electron guns (heats metals, etc., to draw out electrons), it generates an electron beam of about one-thousand-times brighter.
