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Indium Antimonide emits microwave noise when it is subjected simultaneously to parallel dc electric and magnetic fields whose values exceed certain thresholds. Comparison of the emission characteristics at 77°K with those at 4.2°K shows two major differences. 1) The threshold magnetic field at 4.2°K is approximately half of that at 77°K. 2) With increasing magnetic field the emission at 4.2°K is comprised of a background continuum upon which are superimposed equally-spaced resonant spikes. At 77°K, the background continuum only is observed. A theoretical model for the threshold characteristics is presented based on the assumption that the observed instability arises from phonon excitation by the drifting electrons.