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This paper deals with an interesting type of voltage-controlled negative resistance in heavily doped semiconductor junction diodes. The effect, discovered and explained by Esaki, is due to quantum-mechanical tunneling of carriers through the junction. In the present article, experimental and theoretical results are given which show the diode has great promise for frequencies in the kilomegacycle region. Diodes with a negative conductance of a mho or more have been made; they oscillate above 1 kmc, generate harmonics over 4 kmc, and switch in 2 mÂ¿sec. From a gain-bandwidth analysis based on a proposed equivalent circuit, the limiting time constant is shown to be the product of the negative resistance and the junction transition capacitance. According to quantum theory, this product can be varied over a wide range by nominal changes in the free carrier concentration. Germanium diodes with 4.8 by 1019 carriers/cm2 have a measured gain-bandwidth of 1 kmc. Further material development should increase this factor to 10 kmc. The high negative conductance of the junction coupled with its high shunt susceptance make the device admittance much higher than is normally encountered. As a result, the series impedance of the device mount becomes important. Since the negative resistance is voltage controlled, establishing an operating point requires a voltage supply with internal resistance lower than the magnitude of the negative resistance. Under such conditions, the problem of suppressing parasitic oscillations in the mount and external circuit is serious.