We report on a new single photon avalanche diode (SPAD) fabricated in a 130nm CMOS imaging process. A novel circular structure combining shallow trench isolation (STI) and a passivation implant creates an effective guard ring against premature edge breakdown. Thanks to this guard ring, unprecedented levels of miniaturization may be reached at no cost of added noise, decreased sensitivity or timing resolution. The detector integrated along with quenching and read out electronics was fully characterized. Optical measurements show the effectiveness of the guard ring and the high degree of electric field planarity across the sensitive region of the detector. With a photon detection probability of up to 30% and a timing jitter of 125 ps at full-width-half-maximum this SPAD is well suited for applications such as 3-D imaging, fluorescence lifetime imaging and biophotonics.