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

Area of Science:

• Solid-state image sensor technology
• Complementary Metal-Oxide-Semiconductor (CMOS) processes

Background:

• Optimizing fabrication processes for solid-state image sensors is crucial.
• High-quality photosensors are needed in standard CMOS processes.
• Photosensor quality depends on pixel architecture and photosensitive structure.

Purpose of the Study:

• Compare three photodiode structures (n(+)/p-sub, n-well/p-sub, p(+)/n-well/p-sub) for standard CMOS fabrication.
• Evaluate structures based on spectral sensitivity, noise, and dark current.
• Assess performance in different pixel architectures: standard active pixel sensor and in-pixel capacitive transimpedance amplifier.

Main Methods:

• Fabricated three photodiode structures in a 0.5 µm 3-metal, 2-poly, n-well CMOS process.
• Utilized identical pixel and readout architectures for all structures.
• Compared spectral sensitivity, noise, and dark current across the fabricated photodiodes.

Main Results:

• The n-well/p-sub diode exhibited the highest sensitivity (3.5x and 1.6x improvement over n(+)/p-sub and p(+)/n-well/p-sub).
• The n-well/p-sub diode showed the best signal-to-noise ratio (1.5x and 1.2x improvement over n(+)/p-sub and p(+)/n-well/p-sub).
• The p(+)/n-well/p-sub diode demonstrated the lowest dark current (33% reduction) for a given sensitivity.

Conclusions:

• The n-well/p-sub photodiode structure is optimal for standard CMOS image sensor fabrication due to superior sensitivity and signal-to-noise ratio.
• The p(+)/n-well/p-sub structure offers a viable alternative with reduced dark current, beneficial for specific low-light applications.
• These findings contribute to the development of advanced, high-performance image sensors using established CMOS technology.