Electron beam prebunching by direct modulation of the electron current density emitted into vacuum is one of the potential advantages of cold cathodes. We develop a small alternating current signal equivalent circuit of planar cold cathode with negative electron affinity and current carrying thin films at the surface. For a specific InP/CdS/LaS cold cathode, device parameters and biasing conditions are found leading to large (60%) cathode current modulation at a frequency well within the K band. (C) 2004 American Vacuum Society.