Quantum efficiency (QE) is the measure of the
effectiveness of an imager to produce electronic
charge from incident photons. This is an especially
important property when doing low-light-level imaging.
Because most CCD imagers are made from silicon,
it is useful to examine the properties of this element
and the way that it interacts with light.
In the high-purity crystalline form, each atom
of silicon is covalently bonded to its neighbor.
Energy greater than the band gap energy, about 1.1
eV, is required to break a bond and create an electron/hole
pair. The wavelength of incoming light and photon
absorption depth are directly related; the shorter
the wavelength, the shorter the penetration depth
into the silicon.
Light normally enters the CCD through gates of
the parallel register (front-illuminated CCD). These
gates are made of very thin polysilicon, which is
reasonably transparent at long wavelengths, but
becomes opaque at wavelengths shorter than 400 nm.
Thus, at short wavelengths, gate structure attenuates
It is possible, using acid-etching techniques,
to uniformly thin a CCD to a thickness of approximately
10 µm and focus an image on the backside of the
CCD register where there is no gate structure (back-illuminated
CCD). Thinned CCDs exhibit high sensitivity to light
from the soft x-ray to the near-infrared regions
of the spectrum.
To improve the sensitivity of CCDs in the blue-visible
and ultraviolet wavelengths (200 nm to 400 nm),
it is also possible to coat a CCD with Metachrome