Which Photodiode to Use: A Comparison of CMOS-Compatible Structures

While great advances have been made in optimizing fabrication process technologies for solid state image sensors, the need remains to be able to fabricate high quality photosensors in standard CMOS processes. The quality metrics depend on both the pixel architecture and the photosensitive structure. This paper presents a comparison of three photodiode structures in terms of spectral sensitivity, noise and dark current. The three structures are n⁺/p-sub, n-well/p-sub and p⁺/n-well/p-sub. All structures were fabricated in a 0.5 mum 3-metal, 2-poly, n-well process and shared the same pixel and readout architectures. Two pixel structures were fabricated-the standard three transistor active pixel sensor, where the output depends on the photodiode capacitance, and one incorporating an in-pixel capacitive transimpedance amplifier where the output is dependent only on a designed feedback capacitor. The n-well /p-sub diode performed best in terms of sensitivity (an improvement of 3.5 times and 1.6 times over the n⁺/p-sub and p⁺/n-well/p-sub diodes, respectively) and signal-to-noise ratio (1. 5times and 1.2 times improvement over the n⁺/p-sub and p⁺/n-well/p-sub diodes, respectively) while the p⁺/n-well/p-sub diode had the minimum (33% compared to other two structures) dark current for a given sensitivity.