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

Abstract

A photodiode (PD) structure based on ${N} \times {N}$ 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} \times {5}$ subsections is designed and fabricated in a standard AMS 0.35- $\mu \text{m}$ 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 $100~\mu \text{m} \,\, \times 100~\mu \text{m}$ . A responsivity improvement of around 10%–15% is measured between 400- and 550-nm wavelengths. The normalized ac responsivity shows a $1.7\times $ 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.