Editing Single-photon avalanche diode (section)

==== Pixel pitch ====
The above fill-factor enhancement methods, mostly concentrating on SPAD geometry along with other advancements, have led SPAD arrays to recently push the 1 mega pixel barrier.<ref>{{Cite journal|last=Kazuhiro Morimoto, Andrei Ardelean, Ming-Lo Wu, Arin Can Ulku, Ivan Michel Antolovic, Claudio Bruschini, and Edoardo Charbon|date=2020|title=Megapixel time-gated SPAD image sensor for 2D and 3D imaging applications|url=https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-4-346|journal=Optica|volume=7|issue=4|pages=346–354|doi=10.1364/OPTICA.386574|arxiv=1912.12910|bibcode=2020Optic...7..346M|s2cid=209515304|via=OSA}}</ref> While this lags CMOS image sensors (with pitches now below 0.8&nbsp;um), this is a product of both the youth of the research field (with CMOS SPADs introduced in 2003) and the complications of high voltages, avalanche multiplication within the silicon and the required spacing rules.