The tunnel cathode is a promising development in the search for a cold emitter. Ideally, it should operate at high current densities, low cathode power, low noise, and at low temperatures. High-frequency modulation of emission is also possible A research effort has been under way in various countries in recent years, with regard to the development of electron emission from cold, solid-state devices into vacuum, an insulator, or a semiconductor. The term “cold” in this connection is used to describe devices that emit electrons at operating temperatures of approximately 300°K and below. This is in contrast to the emission of electrons from a heated material in which the electrons receive the excess energy necessary for their emission exclusively from their equilibrium with the thermal vibrations of the heated host body.