Charge-coupled devices (CCDs) have been the dominant form of conventional imaging circuits for detecting and converting a packet of light photons into an electrical signal that represents the intensity of the light in a particular pixel region of the image. Most commonly, CCDs use a photogate to detect and store the light energy as electrical charge, and a series of electrodes to transfer the collected charge serially to an output.
CCDs have many advantages for obtaining very high quality images. These include high sensitivity, high well capacity, near unity fill factors, low leakage currents and mature processes optimized for imaging. However, CCDs also suffer from some system shortcomings, such as limited readout rates, high power dissipation that increases linearly with read rates, limited linearity and difficulty in integrating signal processing electronics onto the imager focal plane.
To overcome these limitations, recent imaging arrays use active pixel sensor (APS) cells to convert the light photons into electrical charge. With APS, a conventional photodiode is typically combined with MOS Field Effect Transistors (MOSFETs), which provide amplification, readout and timing control.