3 Microelectronic solutions for digital photgraphy
3.1 Charge-coupled device (CCD) detectors
The first digital imagers to be developed were called charge-coupled devices, because of the way in which accumulated charges are passed along rows in order to read the contents of each element of an array.
A CCD array has at each pixel an MOS photocapacitor. Potential on a gate electrode holds the accumulating, photo-generated charges in place during the exposure interval. The first thing to appreciate is that the photons that generate the electron–hole pairs have to penetrate the polysilicon gate and its underlying silicon dioxide. For visible wavelengths, the absorption coefficient of silicon changes from a little over 105 m−1 for red light to a little under 107 m−1 for blue light. That means for red light passing through 1 µm of silicon the intensity would fall to about 90% [= exp(−105 × 10−6)] of its initial value. The same path would extinguish blue light altogether. The specification for gate thickness is therefore critical and is considerably less than 1 µm; some designs also have small holes etched in the gates of the blue sensors. Across the entire array, the thickness of gate material and the distribution of holes in the gates of blue sensors must be highly uniform, otherwise different parts of the array will see similar things somewhat differently.
The next point to note is that not all the silicon surface of an imaging chip is available for image capture. This is due to two more important strategies. The first is the electronic read-out from the pixels. The second is an electronic shutter that effectively freezes an image before reading out.