Tunneling is studied in two main single-photon avalanche diode (SPAD) topologies, which are n-tub guard ring (NTGR) and p-tub guard ring (PTGR). Device simulation, I-V measurements, and dark count calculations and measurements demonstrate that tunneling is the main source of noise in NTGR, but it is less dominant in PTGR SPADs. All structures are characterized with respect to dark noise, photon detection probability, timing jitter, afterpulsing probability, and breakdown voltage. Noise performance is disturbed because of tunneling, whereas jitter performance is disturbed because of the short diffusion time of photo-generated minority carriers in NTGR SPADs. The maximum photon detection probability is enhanced because of an improvement in absorption thickness.