Novel charge-storage-diode structure for use with light-activated displays

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Abstract
A theoretical model is described for the photosensor‐substrate structure presently used with the ac liquid‐crystal light valve. This structure consists of a thin film (sputter deposited) of n‐type CdS on which is evaporated a thin film of p‐type CdTe. The model describes the photosensitivity of this structure in terms of the depletion‐width photocapacity of the charge‐storage diode formed by these two coatings. The model developed based on this photocapacity effect allows a consistent explanation of the experimentally observed fact that good device photosensitivity is observed at high frequencies (e.g., 10 kHz), contrary to the prediction that would result from a quantum‐photodiode model. By assuming an electronic‐defect‐center structure that consists of fast and slow electron trap levels and a recombination level, as in the case of a photoconductor, the charge stored in the diode is related to image‐activating light intensity. This provides descriptive information on device response time, gray scale, and sensitivity. Additionally, the model allows for the identification of those material properties that presently limit photosensor performance.

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