A photosensitive receptor and electronics embedded into a substrate material in a silicon chip make up a CCD. Incident light from a scintillator strikes the detector and electron/hole pairs are produced in the silicon. The number of electron/hole pairs is related to the amount of light that is absorbed. The electrons are held by electrostatic forces in the array until the charge is read out to form the image. The chip is made up of a polysilicon layer, a silicon dioxide layer, and a silicon substrate (Figure 6-2). The polysilicon layer is coated with a photosensitive material and contains the electronic gates. The silicon dioxide layer is an insulator and the silicon substrate contains the charge storage area.

Each pixel or detector element (del) contains three electrodes that hold the electrons in a electrical potential well. The dels are formed by voltage gates that, at readout, are opened and closed like gates to allow the flow of electrons. To collect the charge on the silicon chips, the voltage sign is changed on the electrodes within each del, moving the electrons by rows down the columns until the readout row is reached. This process is commonly known as the “bucket brigade scheme” (Figure 6-3). Once the readout row is reached, the data are sent sequentially to the sense amplifier and are then digitized. The readout process requires that the movement of the electrons is timed, and multiple voltage changes occur; however, the process is very quick. There can be issues with overfill in the dels, which causes excess electrons to spill out of the del into an adjacent del. This is handled by building overflow drains into the array to reduce or prevent this “blooming” effect.