Influence of bulk and surface properties on image sensing silicon diode arrays

Silicon diode arrays for use as the electron-beam accessed target in camera tubes for the Picturephone® visual telephone set have been fabricated and their properties evaluated. These targets offer significant advantages over the antimony trisulfide target commonly used in vidicon-type tubes. But there are certain potential limitations which must be dealt with in developing a silicon target. Three of its critical requirements are adequate sensitivity to visible light, low dark current, and junction uniformity and freedom from defects across at least 300,000 diodes per square centimeter. Sensitivity to visible light is expressed here by the efficiency for conversion of incident photons to electrons in the read-out circuit. Conversion efficiencies exceeding 50 percent in the visible region have been achieved by oxidizing or by diffusing phosphorus into the light-receiving surface to reduce the surface-recombination velocity. Diode leakage currents of ≦1 × 10−13 A per diode are required, and are obtained for target voltages up to about 5 to 7 V. Surface generated current dominates in the 8-μ diameter diodes of the array, but this component of current can be reduced substantially by use of (100) surfaces or by hydrogen annealing. Visible defects in a picture can result from leaky diodes or oxide pinholes which cause bright spots, and diodes covered by oxide which cause dark spots. Our best targets show a video display with only a few defects; processing must be improved to eliminate defects completely.

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