The Saturation Characteristics Control of Photo-diodes with Overflow Drain

Abstract

Photo-diodes with vertical overflow drain has been used to suppress blooming. The γ characteristics at the saturation region of the diodes is important factor to minimize smear and to expand dynamic range for incident light. However, the characteristics have not been analyzed yet. In this paper, we have clarified the dependence of γ characteristics on the substrate impurity profile that is most important design parameter in the diode from view point of γ control. The γ characteristics were calculated by one dimensional analysis to understand how the characteristics are decided and by three dimensional numerical analysis to obtain more exact results. The analyses indicate that γ characteristics became lager for the lower substrate density. However, lower substrate density brings about high reset voltage. To reduce reset voltage while keeping large γ characteristics, we proposed photo-diodes with three substrate layers, and succeeded, in simulation, to realize both characteristics at a same time.