Abstract
The use of a physical guard ring in CMOS single-photon avalanche diodes (SPADs) based on n + /(deep)p-well and p + /(deep)n-well structures is a common solution to control the electric field of the SPADs periphery and prevent the premature lateral breakdown. However, this leads to a decrease of the detection efficiency, i.e., the fill-factor, especially when the SPADs size is reduced. Our paper presents an experimental and simulation study on replacing the physical guard ring by a virtual guard ring to improve the fill-factor and the scalability of a n + / p-well SPAD implemented in 0.35-μm pin-photodiode CMOS technology. Accordingly, the optimization of the virtual guard ring and its superiority at downscaling are discussed, and the SPAD scalability in size with respect to the fill-factor is quantified in this technology.
Highlights
Detecting weak optical signals is critical to a variety of opto-electronic applications, including time-of-flight sensors, quantum cryptography, optical wireless communication, and optical tomography in medical diagnostics
The difference between the radii of the effective active areas in the two structures is larger than the ∼2 to 3 μm predicted by Fig. 3(d) and is around 7 μm. This is due to the lateral electric field from the lateral diode (p-well/n-well) in SPAD1, which is stronger than that of the p-well/p-epi transition in SPAD2, and it has a stronger effect on the trajectory of the carriers toward the cathode and results in a narrower region with a high avalanche triggering probability (ATP)
This proves an improvement of around 45% and for the same diameter of 116 μm the fill-factors of 23% and 34% are obtained for SPAD1 and SPAD2 leading to the photon detection efficiencies (PDEs, PDE 1⁄4 photon detection probability (PDP) times fill-factor) of 7.75% and 11.46%, respectively
Summary
Detecting weak optical signals is critical to a variety of opto-electronic applications, including time-of-flight sensors, quantum cryptography, optical wireless communication, and optical tomography in medical diagnostics This makes the single-photon avalanche diode (SPAD) an attractive candidate as it has a sensitivity level of detecting single photons.[1,2,3,4] The SPAD operation can be thought as a simple diode with a reverse bias above its breakdown voltage and an absorbed photon generates an electron–hole pair, which might gain enough energy to create a self-sustaining avalanche due to a strong electric field formed in a multiplication zone. The effect of the virtual guard ring on the parasitic noises, including the DCR and afterpulsing probability (APP), and the breakdown voltage (Vbr) as the key performance factors of the SPAD are studied.
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