Detector Pixel Geometry?
Usually the separation exists, but is negligible compared to the size of the pixels.  Hence, the loss due to photons "falling through the cracks" is usually very small, and can be well accounted for using either Monte Carlo-based methods or experimentally using a focused source of known brightness. For example, attached is the user manual for the WIYN One-Degree Imager (ODI), a CCD detector that I have done some work on. There is a section on detector geometry that will probably help. 

http://www.wiyn.org/ODI/ODIUserManual.pdf