Different kinds of CCD sensors
It is important to prevent light from falling onto the CCD pixels during the readout process to avoid artifacts on the image. The most simple method is to use a shutter in front of the CCD which is actually raelized by the ICCD image intensifiers shutter function. Though, the most simple kind of CCD chip which can be used is a so called Full-Frame CCD, as shown in the according picture.
Frame-transfer CCD chips first transfer the collected charges very fast into a light insensitive area, before the actual readout process starts. The covered area is located below the CCD's sensitive surface.
Interline-transfer CCD chips only shift the pixel charges into adjacent covered pixels, which needs only one single shifting, and thus is even faster than the frame-transfer.
Frame-interline-transfer CCD chips combine the two latter structres to minimize additional noise and optimize readout performance.
The higher the resolution of a CCD sensor with a fixed surface area, the smaller are the pixels. The smaller the pixels are, the lower is the signal to noise ratio. This is due to the fact that some noise sources do not scale with the pixels surface area, as the signal does. Thus, the higher the resolution, the lower is the signal to noise ratio.
Read more about CCD sensors here.