Analysis of n-p-n photodiode in p-well CPD image sensors

A CPD image sensor with the p-well structure having two kinds of impurity profiles has been developed. The potential along the depth of the n-p-n photodiode in the p-well is calculated for a linearly graded junction. Based on this calculation, the capability of the blooming suppression and the dynamic range of the photodiode are discussed. The blooming suppression efficiency is found to increase with Vsub(the voltage applied to the n-substrate with respect to the p-well) up to a certain value, beyond which the dynamic range of the photodiode decreases. It is shown that the lightly doped p-well is strikingly effective for the blooming suppression.