From a functional perspective, the tasks of an image sensor are to convert the incident light (photon) energy into current or charge, collect it, transfer it to a measurement point, and convert it to a readable signal. The operations to accomplish the first two tasks are similar in the two groups of commercial sensors, CMOS and CCD, but the methods of collection and transfer of the photon signal are different. Image sensor sensitivity to incident light is generally given in terms of illuminance in lux (see Sec. 2.3, Table 2.3); 1 lux contains around 4,000 photons/s/μm2 surface area at λ = 550 nm (see Sec. 5.9.7).
In a CMOS image sensor, an individual light-sensing element (photosite) is a photodiode with an adjacent charge-to-voltage converter, buffer, and other preprocessing circuits, depending on the complexity of the device, such as a local amplifier at each pixel location in active pixel sensors (APSs). These diode photosites are connected in a 2D array, and their voltages are collected and transferred to the built-in output node by using the row-column addressing mechanism of random access memory (RAM). In contrast, the photosite in a CCD image sensor is a metal-oxide gate that converts incident photon energy into charge. An individual photosite charge is transported to remote readout registers through a series of photosite-to-photosite transfers; these readout registers are masked CCD gates. The serial outputs of the readout registers are then converted into voltages by the output amplifier and sent to the output node for collection.
Since CMOS image sensors are fabricated by conventional very-large-scale integration (VLSI) memory technology, they are less expensive than CCD devices. However, if a large amount of signal processing hardware is placed next to the photodiodes, the total optical sensing area of a CMOS image sensor is significantly reduced. This reduction in fill factor (the ratio of the sensing area to the total chip area) and the comparatively lower cost of a CMOS image sensor must be compared with the superior image quality of a CCD sensor in the context of specific application requirements. The major limitation of a CCD image sensor is its charge transport mechanism.