Blooming suppression mechanism for an interline CCD image sensor with a vertical overflow drain

The blooming suppression mechanism for a vertical overflow drain (VOD) structure for an interline CCD image sensor(1) was quantitatively analyzed. An analytical formula, describing photodiode potential VPDchange as a function of integration time and light intensity, was obtained using VOD structure punch-through characteristics. Calculated results at strong light intensity, for which blooming suppression is required, indicate that (1) at a fixedV_{SUB}, V_{PD}after integration time decreases linearly with an exponential increase in the light intensity. This fact implies that a slight VVDdecrease remarkably increases punch-through current, which overflows excess signal charge into the drain. (2) VVDafter integration time markedly increases with VSUB. By combining this fact with (1), it can be deduced that VSUB, necessary for blooming suppression, increases only slightly even for several orders of incident light increase. Experimental results for both VOD image sensor cell and an actual interline sensor verified the analytical results. Therefore, the VOD image sensor has an excellent blooming suppression capability.