Design and Modeling of Blue-Enhanced and Bandwidth-Extended PN Photodiode in Standard CMOS Technology

A photodiode (PD) structure based on N x N junctions is presented to enhance the responsivity in the blue region and extend the optical bandwidth. The use of subsections or multiple junctions increase the number of generated blue photo-carriers as well as the collection speed of photo-carriers at the edges of the depletion regions. An N-well/Psub PD formed of 5 × 5 subsections is designed and fabricated in a standard AMS 0.35-μm CMOS technology with an OPTO process option. The PD is compared to a solid structure with a single section having the same optical window of 100 μm × 100 μm. A responsivity improvement of around 10%-15% is measured between 400and 550-nm wavelengths. The normalized ac responsivity shows a 1.7x increase in bandwidth compared with the solid PD at 1-V reverse-bias voltage. S-parameter-based voltage-bias-dependent lumped-RLC models are proposed to accurately represent the optoelectrical conversion and the output impedance for both PDs as a function of the reverse-bias voltage and operating frequency range.