Charge-coupled devices (CCDs), the standard imagers at all observatories today, consist of integrated circuits made through the same process as computer memory or the chips in cell phones. Complementary metal oxide semiconductors (CMOS) are an alternative image-sensor technology with high noise immunity and low static power consumption; however, CCDs are the dominant imagers today, so we will concentrate our discussion on their use.
Silicon crystals are sensitive to light through the process by which incident photons of sufficient energy can excite electrons into the valence levels of the atom. Photons with energies less than the valence levels fail to create photoelectrons and are therefore not detected, while the higher-energy photons are absorbed near the surface of the silicon layer before creating usable photoelectrons. If one applies a voltage to the silicon in a controlled manner, these photoelectrons can be either held in place (during the integration) or moved through the silicon lattice (during readout) and collected.
What is a charge-coupled device?
When a CCD is constructed, a square grid of microscopic electrodes called gates is fabricated on the surface of a silicon wafer (see Fig. 14.1). The orthogonal axes of the grid are called columns and rows and the grid elements are the pixels, which have typical sizes of 10–20 üm. When exposed to optical light, the silicon substrate reacts to each absorbed photon by creating one photoelectron–hole pair. The gate voltages control the movement and position of these photoelectrons.