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Single-photon avalanche diodes (SPADs) measure individual photons' time of arrival. Low detector jitter is required in many SPAD applications. This letter describes a method for significantly reducing SPAD jitter using an area-efficient shallow-trench-isolation guard ring. The structure prevents lateral drift and diffusion of charge carriers, resulting in improved timing resolution. Experimental results of the device, fabricated in a 0.18-mum CMOS technology, are presented. The timing resolution of the SPAD is 27-ps full-width at half-maximum. Importantly, the diffusion tail exhibits only 96-ps full-width at hundredth-maximum, a three times improvement over previously published SPAD results. The reduced jitter can be translated to improved bit error rates in quantum key distribution systems, faster bit rates in pulse position modulation optical links, and greater contrast in high-resolution fluorescence lifetime imaging microscopy.