The spectrum of microwave radiation from InSb

The frequency spectrum of the microwave radiation emitted from InSb in the presence of applied electric and magnetic fields at 77°K was observed using a spectrum analyzer. In the frequency region from 7 to 26 Gc/s the spectrum showed oscillatory behavior with a period of 2.2 Gc/s, with the average value decreasing with increasing frequency. Such a structure could be due to a series of harmonics of a fundamental mode with frequency of 2.2 Gc/s. The frequencies of the peaks did not shift on changing the magnetic field and current. The spectrum near the frequency of 2.2 Gc/s was measured in detail in a coaxial-type cavity. The intensity of the radiation in this frequency region was much higher than that in the higher frequency region. The spectrum did not show clearly the expected peak at 2.2 Gc/s, but showed a further oscillatory behavior with a period of about 150 Mc/s. The physical mechanism which gives such a structure to the spectrum is not yet understood. It was also observed that the threshold conditions for the observation of radiation depended on the sample geometry, the thresholds being lower for thin samples, with the magnetic field perpendicular to both the current and the thin direction.