Current instabilities in cadmium sulphide

Abstract

During the investigation of current instabilities in semi-insulating single CdS crystals, caused by applying a high voltage pulse and using various illumination distributions, some completely new phenomena have been observed. The major phenomena are discussed and explanations have been offered for the observed effects with various types of illumination. Phenomena observed have been compared with the results of other workers. One type of instability was initiated by applying high voltage pulses to CdS specimens in complete darkness. It is observed from the field measurements that the period of oscillation is equal to the transit time of the acoustic wave from the position of high field region to the anode end of the specimen and not related to the wave transit time between electrodes. High frequency non-sinusodial electrical oscillations were observed in uniformly illuminated specimens. These were attributed to the effect of longitudinal potential gradients which were found when the electric field was measured under oscillatory conditions. Continuous acousto-electric current oscillations were observed in a specimen by shading a length near to the anode end. Extremely high fields were detected in the high resistivity region so formed when amplification occurred. Obscuring a narrow strip perpendicular to the length of the sample and applying a high voltage pulse resulted in current oscillations and two step saturation of the current. A number of oscillatory modes were often found in a specimen, depending on the position of the shielded strip on the specimen. Extremely high electricfields were observed near the position of the obscured strip of the specimen. In two-step current saturation phenomena the changes in current pulses were recorded. Low frequency current oscillations were observed after the current had fallen to a final steady value. The conductivity of the material at different times after the application of a high voltage pulse was recorded and it remained constant below threshold voltages and thereafter differed from the ohmic low voltage value. The electric field reached maximum values 0.5mm from the cathode and 1mm from the anode ends of the specimen. High frequency non-sinusodial current oscillations were recorded by localised illumination of the specimens with an He-Ne laser beam. With low voltage pulses, the I-V characteristics of these specimens fell below the linear ohmic relation, but at high voltages the current became proportional to the square of the applied voltage. Qualitative explanations of these oscillations have been given and it has been shown that they are consistent with existing theory.