15.1 CCD Sensors
A CCD sensor is a charge-coupled device. Potential wells for electrons are created by extrinsic doping of a semiconductor. The metal-oxide gate of a field-effect transistor determines the depth of a well. An external voltage may raise or lower a gate. A specific gate sequence transfers electrons between wells. The charge-coupling process converts photoelectrons into a video signal. Holst and Lomheim, provide a broad review of CCD and complementary metal-oxide semiconductor (CMOS) sensors. Janesick provides a more rigorous review of CCD technology.
Figure 15.1 displays the structure of a potential well of a CCD sensor. An n-type material donates negatively charged electrons. A p-type material donates positively charged holes. The bulk material is p-type silicon. The Fermi level defines an equal probability for a hole and an electron. n-implantation raises the Fermi level by donation of electrons. Consequently, the conduction-band edge is lower in the n-doped region. This defines the base of the well.