Silicon SPADs are typically used in applications at wavelengths between 400 nm and 900 nm and have been manufactured with two different types of processes. Non-planar fabrication methods yields devices with a thick absorption region, with high sensitivity in the 600 nm – 800 nm wavelength range and moderately good timing resolution up to 150 ps full width at half maximum (FWHM). Thin SPADs, based on a planar process, are more sensitive to blue-green light, have a timing resolution typically below 50 ps FWHM and are easy to integrate in monolithic arrays for direct imaging applications.
Recently the authors of this article introduced a third technology called red-enhanced SPAD (RE-SPAD) that combines the advantages of silicon thin and thick SPADs with detection efficiency extended over the entire visible spectrum and beyond, exceeding 40% at 800 nm, timing resolution better than 100 ps FWHM and the possibility to integrate an array of devices on the same chip.
In this Optics Express article Giulia Acconcia et al. are leveraging a new-generation 0.18 µm high-voltage CMOS process for the design of an active quenching circuit (AQC) specifically tailored to be used for RE SPAD arrays. The AQC can handle supply voltages up to 50 V, while allowing for extremely high frequency operation thanks to the internal logic running at 1.8 V. Count rates up to 80 million counts per second are achievable, a record performance for a fully integrated AQC. Remarkably, temporal resolutions better than 120 ps can be obtained without additional dedicated timing electronics. The AQC footprint has a 1:1 aspect ratio and occupies an extremely low area, only 150 µm x 150 µm, making it suitable for the design of read-out electronics for large and densely packed SPAD arrays that will improve single photon imaging capabilities and enable advanced multichannel single photon counting and timing.
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