Silicon exhibits an energy gap of 1.14 eV.  Incoming photons with
energy greater than this can excite valence electrons into the
conduction band thus creating electron-hole pairs.  These pairs
diffuse thorugh the silicon lattice structure.  The average lifetime
for these carriers is 100 microsecs.  After this time the e-h pair
will recombine.

Photons with energy of 1.1 to 5 eV generate single e-h pairs.  Photons
with energy > 5 eV produce multiple pairs.

Example:

a 10 eV photon (Lyman alpha) will produce 3 e-h pairs, on average,
for every incident photon.  Soft X-ray photons can generate thousands
of signal electrons make it possible for a CCD to detect single 
photons.

Limits in Silicon Detectors:

Photons with energy less than 1.1 eV (about 1.2 microns) pass
through silicon unimpeded.

Photons with energy greater than 10 kev have such small wavelength
that the probability of interaction is very small.

To use as an infrared imager, CCDs must be made of other material
like germanium (band gap 0.55 eV).