Rf oscillations in optically activated semi-insulating GaAs

It has been previously shown that semi-insulating GaAs can give rise to RF oscillationswhen the sample is optically activated[1]. The typical frequency range for the observed oscillation is several hundred mega hertz. An intriguing feature of these oscillations is that the observed frequency far exceeds the frequency associated with Gunn effects, i.e., the transit time of the sample (typically 1 cm long) was much longer than the observed RF period. Another important characteristic is that the oscillations persist during the lock-on of the current pulse, which can continue for hundreds of nanoseconds or even microseconds. In addition, under certain conditions, the depth of modulation (RF amplitude) occasionally observed was very large, approaching 20% or so. However, the results (although promising) have been inconsistent and not reproducible, particularly in regard to the RF amplitude. One reason for the inconsistency may be connected with the fact that the previous tests were conducted using a lumped circuit, and no attempt was made to couple the semiconductor to an RF cavity. In this paper, preliminary experimental results are reported in which an optically activated GaAs sample is coupled to the fields in an RF quarter wave cavity. The cavity frequency is manually adjustable from 0.2 to 1.0 GHz. Unlike the previous test circuits, the use of an RF cavity technique allows one to decouple the video pulse from the RF and also facilitates the maximization of the RF signal. A description of the cavity and preliminary results are presented. In particular, the dependence of the oscillations on bias voltage, optical wavelength and intensity, sample size, and its relationship to lock-on, are discussed.