A back-illuminated single-photon avalanche diode (SPAD) image sensor includes a sensor wafer stacked vertically over a circuit wafer. The sensor wafer includes one or more SPAD regions, with each SPAD region including an anode gradient layer, a cathode region positioned adjacent to a front surface of the SPAD region, and an anode avalanche layer positioned over the cathode region. Each SPAD region is connected to a voltage supply and an output circuit in the circuit wafer through inter-wafer connectors. Deep trench isolation elements are used to provide electrical and optical isolation between SPAD regions.