A charge coupled device (CCD) is an analog shift register, enabling analog signals to be transported through successive stages (capacitors) controlled by a clock signal. Initially F. Sangster and K. Teer of the Philips Research Labs invented the Bucket-Brigade Device or BBD in 1969. The main function of this device basically transfers charge packets from one transistor to another. One year later, W. Boyle and G. Smith of the Bell Laboratories modified this concept by inventing a transport mechanism from one capacitor to another one. This new device called Charge Coupled Device or simply CCD.
A Charge Coupled Device (CCD) is a two dimensional electronic image detector. Charge coupled devices can be used as a form of memory or for delaying analog, sampled signals. The CCD chip is an array of Metal-Oxide-Semiconductor capacitors (MOS capacitors), each capacitor represents a pixel. Charges (electrons (e-) or holes (h+)) can be stored in the resulting potential well by applying an external voltage to the top plates of the MOS structure. When we applying digital pulses to the top plates (gates) then these charges can be shifted from one pixel to another pixel. As a result the charges can be transferred row by row to a serial register output. The picture is the display of the electron distribution.
Since electrons can be optical generated or more precisely excited from the valence band in the conduction band, the CCD may be used for cameras as a light sensor. Cameras where the light penetrates through the gate structure to reach the region where electrons are collected, are called front-illuminated. More sophisticated in the production, but with a higher sensitivity are cameras where the CCD chip is exposed from the opposite side. These cameras are known as back-illuminated. To insure charge transport from the back to the front side where the electrons are collected, the silicon bulk is thinned.
Charge Coupled Device (CCDs) offer a considerably smaller area than photographic plates, but the improved performance generally outweighs this disadvantage. CCDs suffer from readout noise, which makes them less suitable than an IPCS at very low count rates. CCDs integrate signal during the exposure, and then exhibit a finite read-out time afterwards- this can be many 10s of seconds for the large arrays; in contrast the IPCS has a real-time data collection. Charge Coupled Device (CCDs) are used on La Palma for both spectroscopic and imaging observations.
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