1. A method for operating a photoreceiver cell for generating electrical current responsive to light incident on a surface area of a semiconductor substrate from above the semiconductor substrate, the photoreceiver cell comprising: a plurality of first regions of a first conductivity type in the semiconductor substrate, each first region being present underneath said surface area to receive the incident light and generate minority charge carriers in response to the incident light, wherein underneath said surface area, the first regions are spaced from each other and overlie one another; one or more second regions of a second conductivity type opposite to the first conductivity type, each second region connected to the photoreceiver cell's output, wherein each first region forms a p-n junction with the one or more second regions; wherein underneath said surface area, in at least one vertical cross section passing through said surface area, for at least two first regions R 1 , R 2 in said plurality of the first regions, the semiconductor substrate's entire region which underlies the first region R 1 and overlies the first region R 2 has the first conductivity type but is heavier doped to the first conductivity type than the first regions R 1 , R 2 underneath said surface area, said entire region providing a potential barrier to minority charge carriers formed in the first region R 2 when the p-n junctions formed by the first regions R 1 , R 2 with the one or more second regions are reverse-biased; the method comprising: connecting the one or more second regions to a first potential through one or more readout circuits; and connecting the semiconductor substrate to a second potential to reverse-bias the p-n junctions.