A back-illuminated single-photon avalanche diode, SPAD, image sensor, comprising: a sensor wafer, comprising: a SPAD region, comprising: an anode gradient layer comprising a first dopant type; a cathode region positioned within the anode gradient layer and adjacent to a front surface of the sensor wafer and comprising a second dopant type; and an anode avalanche layer positioned within the anode gradient layer and adjacent to the cathode region opposite to the front surface and comprising the first dopant type; wherein: the cathode region has a first area and the anode avalanche layer has a second area that is less than the first area; and a circuit wafer attached to the front surface of the sensor wafer; and characterised in that a dopant concentration of the first dopant type in the anode gradient layer increases from a surface of the anode avalanche layer that faces away from the front surface toward a back surface of the sensor wafer that is opposite to the front surface, providing a dopant concentration gradient in the anode gradient layer that guides a photon-generated charge carrier, induced by light entering the back surface, through the anode gradient layer to the anode avalanche layer. a sensor wafer, comprising: a SPAD region, comprising: an anode gradient layer comprising a first dopant type; a cathode region positioned within the anode gradient layer and adjacent to a front surface of the sensor wafer and comprising a second dopant type; and an anode avalanche layer positioned within the anode gradient layer and adjacent to the cathode region opposite to the front surface and comprising the first dopant type; wherein: the cathode region has a first area and the anode avalanche layer has a second area that is less than the first area; and a circuit wafer attached to the front surface of the sensor wafer; a SPAD region, comprising: an anode gradient layer comprising a first dopant type; a cathode region positioned within the anode gradient layer and adjacent to a front surface of the sensor wafer and comprising a second dopant type; and an anode avalanche layer positioned within the anode gradient layer and adjacent to the cathode region opposite to the front surface and comprising the first dopant type; an anode gradient layer comprising a first dopant type; a cathode region positioned within the anode gradient layer and adjacent to a front surface of the sensor wafer and comprising a second dopant type; and an anode avalanche layer positioned within the anode gradient layer and adjacent to the cathode region opposite to the front surface and comprising the first dopant type; wherein: the cathode region has a first area and the anode avalanche layer has a second area that is less than the first area; and characterised in that a dopant concentration of the first dopant type in the anode gradient layer increases from a surface of the anode avalanche layer that faces away from the front surface toward a back surface of the sensor wafer that is opposite to the front surface, providing a dopant concentration gradient in the anode gradient layer that guides a photon-generated charge carrier, induced by light entering the back surface, through the anode gradient layer to the anode avalanche layer.