An interline CCD image sensor with reduced image lag

An undesirable image lag with a long time constant was found in the interline CCD image sensor having an n⁺-p^-junction photodiode (PD). This paper clarifies the image lag mechanism and proposes a new photodiode structure having very little image lag. The image lag measurement method and the experimental results are also given. The experimental results quantitatively agree 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⁺-n-p^-structure PD with low donor concentration is proposed, in which all the signal electrons can be quickly transferred before the subthreshold condition begins. As a result, decay lag values for the first and the second fields were reduced to half and no decay lags were observed after the third field.