Polarization Processes in Partially Illuminated Photoconducting Insulators

A quantitative analysis of polarization phenomena in photoconducting insulators is presented for the case in which a strip of the sample is illuminated far from the electrodes. It is shown that if the drift length of the photo-excited carriers is small compared with the dimensions of the dark and illuminated regions, the photocurrent decays exponentially during both the buildup and release of polarization. The decay constant is essentially independent of the characteristics of the contacts and the applied voltage, and can be expressed in terms of geometrical factors and the photoconductance of the illuminated strip. The conclusions of the analysis are checked in colored single crystals of NaCl. The experimental results are found to be in satisfactory agreement with the theory presented.