A monolithic optical receiver containing four single-photon avalanche diodes (SPADs) fabricated in 0.35-μm high-voltage (HV) CMOS is introduced and compared with two 4-SPAD receivers realized in pin-photodiode CMOS belonging to the same process family. This HV-CMOS SPAD receiver achieves sensitivities of −55.1  dBm at 50  Mbit  /  s and −52.0  dBm at 100  Mbit  /  s, both with digital processing, a bit error rate (BER) of 2  ×  ^{10  −  3}, and return-to-zero coding using a wavelength of 642 nm. Also at 143  Mbit  /  s, this BER is achievable. This receiver is especially interesting for applications in which low light intensities can be expected, such as quantum key distribution, optical communications from deep space, and visible light communication for short-range consumer applications.