Abstract: An electron gun used in a particle beam device, for example in an electron microscope, has a relatively good brightness and may be operated under vacuum conditions which can be easily achieved (i.e., for example, at a residual pressure of about 10?6 or 10?7 mbar). The electron gun comprises an electron source having an electron emission surface. Furthermore, the electron gun comprises a first electrode configured to control a path of electrons emitted from the electron emission surface, a second electrode which is configured to suppress emissions of electrons from a side surface of the electron source and a third electrode configured to accelerate electrons emitted from the electron source to a final energy. A first voltage, a second voltage and a third voltage are adjusted to avoid any crossover of electrons emitted from the electron emission surface.