Charge Coupled Devices (CCD)
Charge Coupled Devices (CCD)
The elementary picture element (photosite) for these devices is based on the Metal Oxide Semiconductor (MaS) capacitor on either a p-type or an n-type substrate, as shown in Figure 1 (a) for an n-type substrate device.
The negative potential applied to the photosite electrode creates a depletion region (known as a ‘potential well’) within the substrate. The size of the potential well depends on the electrode shape for width and length and the value of the applied voltage for depth. When the photosite is optically irradiated the incident photons generate hole-electron pairs by virtue of the internal photoeffect. The mobile carriers (electrons in this case) are repelled by the applied voltage and leave behind fixed charges whose number is proportional to the incident radiation Ein, this is therefore equivalent to generating a photocurrent Ip.
Any hole-electron pairs thermally generated within the potential well are also separated by the applied voltage and provide additional charges which, being indistinguishable from the photon generated ones, are equiv alent to a ‘dark’ current Idc.
After a chosen time interval, which is generally referred to as the light ‘integration time’, the charge thus accumulated in the potential well is transferred via gates to a charge coupled shift register (hence the device name) which transports it to a charge sensitive video amplifier, as shown in Figure 3.17(b}. By repeating the process for each photosite (i.e. by scanning the photosite array) the output of the video amplifier will be an electrical signal proportional to the scene focussed on the array, as required.
CCD solid state cameras can be based on either a 2-D photosite array (area scan cameras) or a 1-0 array (linescan cameras). They are geometri cally very stable and accurate devices with a good input dynamic range and are therefore becoming increasingly popular in the fields of robotics and metrology. Their relatively high unit cost, however, has tended to restrict their use to automated inspection applications, such as the GIOTTO European space probe or the British Central Electricity Generating Board nuclear plant maintenance robot. Industrial applications of the CCD camera are, however, growing rapidly as advancements in microelectronics technol ogy increase the CCD optical resolution and help to reduce its unit cost.
