No image lag photodiode structure in the interline CCD image sensor

An undesirable image lag with a long time constant was found in the interline CCD image sensor having N⁺P-junction photodiode (PD). This paper clarifies the image lag mechanism and proposes a new photodiode structure having no image lag. The image lag measurement method and the experimental results are also given. The experimental results quantitatively agreed with the analytical model, in which signal electrons are assumed to be transferred from the PD to the vertical CCD as a small subthreshold current. To eliminate the image lag, a P⁺NP^-structure PD with low donor concentration was proposed, where all the signal electrons can be quickly transferred before the subthreshold condition begins. As a result, the decay lag values of the 1st and the 2nd fields were reduced to half and the decay lags after the 3rd field were not observed.