Enhanced Sensitivity of CMOS Image Sensors by Stacked Diodes

We have investigated and compared the performance of photodiodes built with stacked p/n junctions operating in parallel versus conventional ones made with single p/n junctions. We propose a method to characterize and compare photodiodes sensitivity. For this purpose, a dedicated chip in the standard AMS 180-nm HV technology has been fabricated. Four different sensor structures were implemented and compared. Experimental results are provided. Measurements show sensitivity enhancement ranging from 55% to 70% within the 500-1100 nm spectral region. The larger increment is happening in the near infrared band (up to 62%). Such results make stacked photodiodes suitable candidates for the implementation of photosensors in vision chips designed for standard CMOS technologies.