The role of work functions in electron emissive materials is very important. A high emission current density can be obtained from thermionic emission materials (thermionic cathodes) having low work functions and kept at high temperature. The figure of merit is, thus, the work function divided by the melting temperature of the meterial. In the case of field emission (cold) cathodes, the magnitude of the work function is not as important as its time variation, which creates field emission current fluctuation (noise). The reduction of the current fluctuation is, hence, the major issue of the development of field emission cathode materials.