Development of high electric field domains under varying drift field conditions in photoconducting CdS samples

High resolution measurements of the electric field distribution in photoconduction CdS samples is reported under two specific cases of drift field variations. In the first case, by using short bias voltage pulses, the high electric field domain was found to develop, at first, into a saturated moving domain; then, for a decrease of the bias voltage pulse, the domain collapsed rapidly from a crest to a dip shape, through the formation of a small dip (frontaldip) in the front part of the domain. In the second case, where the drift field was larger in the upstream half than in the down stream half of the sample, the domain has nucleated in the upstream-half of the sample. The domain has also formed a frontal-dip for the time decrease of the drift field when it has moved into the low field half of the sample. It was found that the appearance of the frontal-dip corresponds to the condition that the drift field, outside of the domain, has dropped below the value of the sonic drift field. Thus the latter condition does not correspond to the time of voltage-current crossover as it was suggested by other authors. We have also found that the drift field, outside of a saturated moving domain, is indeed larger than the sonic drift field as it was suggested by other authors on the basis of theoretical studies. Two methods are presented here for the calculation of the value of the sonic drift field.