Abstract: A pixel unit includes: a base, the base including a substrate, a photosensitive element, and memory nodes; overflow doping regions in the second surface of the substrate; and overflow interconnection structures electrically connected to the overflow doping regions. Vertical-overflow-drain structures are formed between the overflow doping regions and the memory nodes, allowing the charge stored in the memory nodes to be discharged at a certain rate through the overflow interconnection structures. This configuration reduces the oversaturation of memory nodes effectively without altering the exposure time or affecting circuit power consumption, thereby addressing background noise issue. Furthermore, the memory nodes and the overflow doping regions are in the first and second surfaces of the substrate, respectively. Therefore, the placement of the overflow doping regions does not affect the area of the pixel unit, and the resolution of the photosensor is preserved.