The method of the invention for extracting electrons in a vacuum consists in: making a cathode presenting at least one junction (9) between a metal (7) acting as an electron reservoir and an n-type semiconductor (8) possessing a surface potential barrier with a height of a few tenths of an electron volt, and presenting thickness lying in the range 1 nm to 20 nm; injecting electrons through the metal/semiconductor junction (9) to create a space charge in the semiconductor (8) sufficient to lower the surface potential barrier of the semiconductor to a value that is less than or equal to 1 eV relative to the Fermi level of the metal (7); and using the bias source creating an electric field in the vacuum to control the height of the surface potential barrier (V.sub.p) of the n-type semiconductor in order to control the emission of the electron flux towards the anode.