Photoconductor materials are well known in the art and are used in a familiar manner in electronic image sensors. In practice, an image sensor includes a housing which has a window of electrically-conducting material through which radiation enters the housing. A photoconductor layer, typical of such a sensor, is electrically insulating and is exposed to incident radiation through the window.
A vacuum is created within the housing so that the opposite surface of the photoconductor is exposed to a vacuum. In operation, a positive voltage is applied to the conducting layer and the vacuum-side face of the photoconductor, in response, is charged with electrons to a cathode potential which establishes a bias field across the photoconductor.
Once charged, the photoconductor, when exposed to a pattern of radiation, exhibits electron-hole pairs which are swept by the bias field moving electrons to the conducting layer and moving holes to the insulating surface of the photoconductor. When holes reach the insulating surface, they recombine with electrons at that surface in a charge pattern representative of the input radiation. The operation is characteristic of the photoconductive action of the standard vidicon-type image tube.
The charge image, so stored, may be read out, for example, by an electron beam which scans the charge surface as in a vidicon as exemplified by U.S. Pat. No. 5,195,118. As the electron beam replaces the charge, removed from the vacuum-side face of the photoconductor by the radiation exposure, a capacitively-coupled signal is sensed by a preamplifier connected to the electrically conducting layer. Although a scanning beam method works well in such sensors, the inherent drawback to such a system is the physical size necessary for the large vacuum bottle which supports the electron gun and the associated electrodes necessary for the operation of scanning beam devices.