Blooming characteristics of a solid-state imager overlaid with a photoconductor

The control mechanisms of blooming phenomena in a solid-state imager overlaid with a photoconductor have been analyzed. The device studied is composed of an interline transfer type of scanner and a thin-film heterojunction ZnSe-Zn1-xCdxTe photoconductor. Two types of operation exist depending on the dc voltage or the pulse voltage applied to the ITO electrode on the photoconductor and it has been proved that pulse mode operation is superior to dc mode to sustain normal sensitivity under blooming suppressed condition. From the analysis of the operation it is made clear that one condition for blooming control in this device is based upon the principle of biasing the photoconductor so that in strong light the readout transistor is held in the cut-off condition except during the readout period. In the case that this condition is met, additional blooming can still occur due to the carriers excited in the Si substrate by the light that passed through the photoconductor and through the gap area between the charge collecting electrodes. By special shielding the blooming control capability can be effective up to 1100 times the saturation exposure compared to 120 times without the special shielding. The remaining blooming signal is clearly shown to be due to photo-excited carriers in the photoconductor during the blanking period.