Photons incident on a semiconductor generate electron-hole pairs through a process known as the photoelectric absorption effect or simply the photoelectric effect (refer to Fig. 2.1). Once created, the carriers are free to move in the semiconductor lattice, where they are quickly collected by pixel elements (pixels). The majority of silicon CCD and CMOS imagers are designed and fabricated to generate electrons and discard holes through a device ground return. Particles other than photons can also produce signal carriers by ionizing silicon atoms as they travel through the lattice, including high-energy electrons, protons, ions, etc. Any high-energy particle that is in a charged state can free primary valence electrons that collide with other silicon atoms to generate secondary electrons (e.g., refer to Fig. 4.5, which shows proton ionizing events). For PT discussions in this text, it will be assumed that photons, electrons, and silicon are responsible for generating electronic pictures.
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