In the rapidly developed market of smartphones, drones, robots and automobiles, 3D sensing based on NIR light detection becomes a standard feature. For example, functions like car parking assistance and pre-crash detection in the automobile’s domain; the facial recognition and field of depth detection in smartphone cameras. As a result, the combination for visible and NIR light detection becomes a wise choice for the future applications. Instead of merging two different types of image sensor pixels together, a new image sensor pixel design is developed in this thesis.
In the thesis, a novel CMOS image sensor (CIS) pixel architecture capable of operating across both the visible and near-infrared (NIR) spectral ranges is presented. The proposed design integrates an n-type four-transistor (4T) pinned photodiode (PPD) stacked back-to-back with a p-type 4T PPD structure. Considering the wavelength-dependent photon absorption properties of silicon, the shallower n-type PD is optimized for collecting electrons generated by visible light, whereas the deeper p-type PD is configured to collect holes induced by NIR illumination. The pixel structure was designed and simulated using Sentaurus TCAD and fabricated using a standard 0.18 μm CIS process on an n-type silicon substrate. The resulting pixel forms a multi-junction photodetector (MJPD) featuring a vertically stacked p+-nwell-pwell-nsub configuration—representing a significant difference comparing to the standard 4T PPD architectures. To understand the electrical behavior of the MJPD structure, a three-series-capacitor model is proposed and analyzed in detail. The theoretical analysis demonstrates good agreement with both TCAD simulation and measurement results.