Quantum Efficiency

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Published on Thursday, 15 December 2011 19:40

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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 incoming light.

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 II.