3.2.6.4 CCD detectors

A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be further processed. This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between p-doped MOS capacitors arranged as pixels in a plane. These detecting capacitors are biased above the threshold for inversion when acquisition begins, allowing the conversion of incoming photons into electron charges at the semiconductor-oxide interface. Each pixel has the photoactive region (an epitaxial layer of silicon), and a transmission region made out of a shift register (the CCD, properly speaking). The charge packets are collected in potential wells created by applying positive voltage at the gate electrodes (GE). Applying positive voltage to the gate electrode in the correct sequence transfers the charge packets, which are further processed.

CCDs are widely used in professional, medical, and scientific applications where high-quality image data is required. The photoactive region is also sensitive to x-rays, thus allowing their use for x-ray detection, providing that the sensor is not exposed to the direct light. If the detector is cooled to reduce the electronic noise, energy resolution down to that achieved by SSD detectors can be reached. As each pixel allows collecting a whole spectrum, CCD detectors have been recently used for x-ray imaging (see 5.2.3).