CCDs have originally been developed as electronic imagers for optical purposes [1,16]. Until now this is still the main commercial application. This use implies that only a few micrometers of surface depletion are necessary to cover the bandwidth of light from 350 to 650 nm. The depletion is achieved through a conductor-oxide-semiconductor structure (MOS) in nonequilibrium condition. The devices can be modified to make them more sensitive to X-rays by increasing the depleted thickness up to almost 50 |im. A substantial effort was made to manufacture back-illuminated devices, some of which are operating on Chandra and XMM-Newton in the dispersive spectrometers with a thickness of about 30 |m. The operating temperature of the MOS CCDs in orbit is about -120° C and the frame rate a few seconds. The energy resolution of the MOS devices is Fano-limited above 1 keV. The susceptibility to radiation is consistent with previous tests of MOS-type devices. As the pnCCDs were tailored to the requirements of X-ray astronomy, we will concentrate on this concept in the following in more detail.
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