I have read the textbook but it does not explain it in a very intuitive manner. This is the explanation (the bolded bits are the unintuitive bits): Rectangular electrodes and an insulating layer are thin enough to allow light photons to pass through and liberate an individual electron in the light sensitive material underneath. When collecting charge, the central electrode in each pixel is held at +10 V and the two outer electrodes at +2 V, which ensures that the liberated electrons accumulate under the central electrode. After the pixels have collected charge for a certain time, the charge of each pixel is shifted towards the output electrode via the adjacent pixels. This is achieved by altering the voltage level of each electrode in a sequence of three step cycles.