During the imaging stage, the photodiodes of the CCD absorb the incident photons, resulting in liberation of electrons and the formation of the corresponding electron-deficient sites (holes) within the Si. One electron-hole pair is generated for each absorbed photon, so the amout of charges that accumulate in each photodiode is linearly proportional to the number of incident photons. External voltages applied to each photodiode’s gate electrodes control the storage and movement of charges accumulated following a precise sequence determined by the imaging electronics. Although either negatively charged electrons or positively charged holes can be accumulated (depending on the CCD design), here we assume electrons are the charge carriers.
Readout of CCD Array Photoelectrons. When taking an image, each CCD photodiode is exposed to light for a specified time interval, referred to as the integration time or exposure time. To record the image taken by the CCD sensor, the stored charge from each photodiode in a CCD sensor must be transferred to a readout node while maintaining the integrity of the charge packet. A fast and efficient charge-transfer process, as well as a rapid readout mechanism, are crucial to the function of CCDs as imaging devices. This is achieved by controlling the voltages applied to the capacitor gates in a pattern that causes stored charge to spill from one capacitor to the next, or from one row of capacitors to the next. The charge transfer along the channel is effectively coupled to clocked voltage patterns applied to the gates, thus the term “charge-coupled” device. This process is schematically shown in Figure 6.