5 Solid State Detectors
169
diode structure, which is also used in the semiconductor drift chamber. Their first
use was in two space-based X-ray telescopes: XMM [27] and ABRIXAS [28].
CCDs are non-equilibrium detectors. Signal charge is stored in potential pockets
within a space-charge region, the content of which is then transferred to a collecting readout electrode. In order to retain the thermal non-equilibrium condition,
thermally generated charge that also assembles in the potential pockets has to be
removed from time to time. Usually this is done during the readout cycle of the
device.
While in conventional MOS CCDs minority carriers (electrons in a p-type
bulk) are collected, the p-n CCDs are majority carrier (electrons in an n-type
bulk) devices. The conventional MOS CCDs to be described in the following
for didactic purposes store and transfer the charge directly at the semiconductorinsulator interface. These devices are in practice not used anymore and have been
replaced by buried-channel CCDs, in which the store-and-transfer region is moved
a small distance away from the surface. As a result they are less sensitive to surface
radiation damage. In p-n CCDs, this region is moved a considerable distance into
the bulk.
5.9.1 MOS CCDs
The CCD transfer mechanism is explained in Fig. 5.29 that shows a cut along the
transfer channel. The top part of the p-type bulk is depleted of charge carriers and
the potential along the Si-SiO 2 interface is modulated in a periodic fashion with the
help of the metal electrodes on top of the SiO 2 . Electrons created in the sensitive
bulk region assemble in the potential maxima (minima for electrons) at the Si-SiO 2
interface.
The charge can now be moved towards the readout electrode by a periodic change
of the voltages φ 1 , φ 2 , and φ 3 , as shown in the figure. First φ 2 is increased to the
same level as φ 1 and the signal charge will spread between φ 1 and φ 2 . If now φ 1 is
lowered, the signal charge will transfer below the electrodes φ 2 . If this procedure is
followed for φ 2 and φ 3 and then again for φ 3 and φ 1 , the signal charge is transferred
by a complete cell. After several cycles the charge will finally arrive at the anode,
where it can be measured.
Placing many of these channels next to each other and separating them by
so called channel stops one arrives at a matrix CCD. Channel stops prevent the
spreading of signal charge to neighbour channels. They can be realized by doping
variations as for example an increased p-doping between channels. Usually charge
is transferred into one additional charge transfer channel oriented perpendicular to
the matrix channel (Fig. 5.30) so that the pixel charge can be shifted towards a single
output node.
169
diode structure, which is also used in the semiconductor drift chamber. Their first
use was in two space-based X-ray telescopes: XMM [27] and ABRIXAS [28].
CCDs are non-equilibrium detectors. Signal charge is stored in potential pockets
within a space-charge region, the content of which is then transferred to a collecting readout electrode. In order to retain the thermal non-equilibrium condition,
thermally generated charge that also assembles in the potential pockets has to be
removed from time to time. Usually this is done during the readout cycle of the
device.
While in conventional MOS CCDs minority carriers (electrons in a p-type
bulk) are collected, the p-n CCDs are majority carrier (electrons in an n-type
bulk) devices. The conventional MOS CCDs to be described in the following
for didactic purposes store and transfer the charge directly at the semiconductorinsulator interface. These devices are in practice not used anymore and have been
replaced by buried-channel CCDs, in which the store-and-transfer region is moved
a small distance away from the surface. As a result they are less sensitive to surface
radiation damage. In p-n CCDs, this region is moved a considerable distance into
the bulk.
5.9.1 MOS CCDs
The CCD transfer mechanism is explained in Fig. 5.29 that shows a cut along the
transfer channel. The top part of the p-type bulk is depleted of charge carriers and
the potential along the Si-SiO 2 interface is modulated in a periodic fashion with the
help of the metal electrodes on top of the SiO 2 . Electrons created in the sensitive
bulk region assemble in the potential maxima (minima for electrons) at the Si-SiO 2
interface.
The charge can now be moved towards the readout electrode by a periodic change
of the voltages φ 1 , φ 2 , and φ 3 , as shown in the figure. First φ 2 is increased to the
same level as φ 1 and the signal charge will spread between φ 1 and φ 2 . If now φ 1 is
lowered, the signal charge will transfer below the electrodes φ 2 . If this procedure is
followed for φ 2 and φ 3 and then again for φ 3 and φ 1 , the signal charge is transferred
by a complete cell. After several cycles the charge will finally arrive at the anode,
where it can be measured.
Placing many of these channels next to each other and separating them by
so called channel stops one arrives at a matrix CCD. Channel stops prevent the
spreading of signal charge to neighbour channels. They can be realized by doping
variations as for example an increased p-doping between channels. Usually charge
is transferred into one additional charge transfer channel oriented perpendicular to
the matrix channel (Fig. 5.30) so that the pixel charge can be shifted towards a single
output node.