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Blooming, the lateral spreading of charge from an intensely illuminated area of an image sensor, can be present in an especially objectionable form in charge-coupled devices. Bright lines are formed in the display, owing to a charge propagation along the transfer channels. The introduction of overflow drains, i.e., stripes of a reverse-biased p-n junction placed between the transfer channels, provides a well-defined leakage path for excess carriers from saturated potential wells. An implanted threshold barrier between these drains and the integrating potential wells determines the saturation potential of the integration sites and prevents the total collapse of the associated depletion region. Therefore, the lateral diffusion of minority carriers is also greatly reduced. An existing design of a charge-coupled area image sensor with 64 × 106 resolution elements and frame transfer organization has been modified to study these methods of blooming suppression. The lateral distance of charge spreading has been reduced by a factor of seven at overload conditions of 20 dB. For stronger overloads the booming suppression becomes even more striking, but the performance of the device iS limited by frame transfer smearing. Furthermore, lens flare limits most optical systems at overloads above 30 dB. However, by using an optical fiber, well-localized overloads of 50 dB could be generated. Although, in our particular test device, the addition of blooming suppression reduced the light sensitivity by a factor of four, other geometries for overflow drain structures that should result in only a negligible loss in sensitivity are discussed.
Date of Publication: Jun 1974