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G. Lutz and R. Klanner
(LHC), where the time difference between consecutive crossings of particle bunches
(25 ns) is much shorter than the time it takes to decide whether or not the data of
a particular event needs to be kept (approximately 2 μs), chips with high-speed
operation and radiation hardness have been developed successfully. In addition
to fast low-noise amplifiers and radiation hardness, it is required to store the
information for approximately hundred bunch crossings.
The task of designing radiation hard electronics has been considerably eased by
the industrial development of submicron integrated circuit technology which, due to
the use of ultra-thin oxide, to a large extend has eliminated the problem of radiation
induced threshold shifts in MOS transistors [17]. Taking some precautions in the
design these technologies can be considered “intrinsically radiation hard”.
5.8 Silicon Drift Detectors
The semiconductor drift detector was invented by E. Gatti and P. Rehak [1]. First
satisfactorily working devices in silicon were realised in a collaborative effort by J.
Kemmer at the Technical University Munich, the Max Planck Institute for Physics
in Munich and the inventors [18].
The working principle may be explained by starting from the diode (Figs. 5.1
and 5.21a) if one realizes that the ohmic n + contact does not have to extend over
the full area of one wafer side but can instead be placed anywhere on the undepleted
conducting bulk (Fig. 5.21b). Then there is space to put diodes on both sides of
the wafer (Fig. 5.21c). At small voltages applied to the n + electrode, there are two
space-charge regions separated by the conducting undepleted bulk region (hatched
in Fig. 5.21). At sufficiently high voltages (Fig. 5.21d) the two space-charge regions
will touch each other and the conductive bulk region will retract towards the vicinity
of the n + electrode. Thus it is possible to obtain a potential valley for electrons in
which thermally or otherwise generated electrons assemble and move by diffusion
only, until they eventually reach the n + electrode (anode), while holes are drifting
rapidly in the electric field towards the p + electrodes.
Based on this double-diode structure the concept of the drift detector is realised
by adding an additional electric field component parallel to the surface of the wafer
in order to provide for a drift of electrons in the valley towards the anode. This can
be accomplished by dividing the diodes into strips and applying a graded potential
to these strips on both sides of the wafer (Fig. 5.5).
Other drift field configurations (e.g. radial drift) can be obtained by suitable
shapes of the electrodes. Drift chambers may be used for position and/or energy
measurement of ionizing radiation. In the first case the position is determined from
the drift time. Furthermore, segmenting the n + -strip anode in Fig. 5.5 into pads, a
two-dimensional position measurement is achieved.
Due to the small capacitive load of the readout electrode to the readout amplifier,
drift detectors are well suited for high precision energy measurement.
G. Lutz and R. Klanner
(LHC), where the time difference between consecutive crossings of particle bunches
(25 ns) is much shorter than the time it takes to decide whether or not the data of
a particular event needs to be kept (approximately 2 μs), chips with high-speed
operation and radiation hardness have been developed successfully. In addition
to fast low-noise amplifiers and radiation hardness, it is required to store the
information for approximately hundred bunch crossings.
The task of designing radiation hard electronics has been considerably eased by
the industrial development of submicron integrated circuit technology which, due to
the use of ultra-thin oxide, to a large extend has eliminated the problem of radiation
induced threshold shifts in MOS transistors [17]. Taking some precautions in the
design these technologies can be considered “intrinsically radiation hard”.
5.8 Silicon Drift Detectors
The semiconductor drift detector was invented by E. Gatti and P. Rehak [1]. First
satisfactorily working devices in silicon were realised in a collaborative effort by J.
Kemmer at the Technical University Munich, the Max Planck Institute for Physics
in Munich and the inventors [18].
The working principle may be explained by starting from the diode (Figs. 5.1
and 5.21a) if one realizes that the ohmic n + contact does not have to extend over
the full area of one wafer side but can instead be placed anywhere on the undepleted
conducting bulk (Fig. 5.21b). Then there is space to put diodes on both sides of
the wafer (Fig. 5.21c). At small voltages applied to the n + electrode, there are two
space-charge regions separated by the conducting undepleted bulk region (hatched
in Fig. 5.21). At sufficiently high voltages (Fig. 5.21d) the two space-charge regions
will touch each other and the conductive bulk region will retract towards the vicinity
of the n + electrode. Thus it is possible to obtain a potential valley for electrons in
which thermally or otherwise generated electrons assemble and move by diffusion
only, until they eventually reach the n + electrode (anode), while holes are drifting
rapidly in the electric field towards the p + electrodes.
Based on this double-diode structure the concept of the drift detector is realised
by adding an additional electric field component parallel to the surface of the wafer
in order to provide for a drift of electrons in the valley towards the anode. This can
be accomplished by dividing the diodes into strips and applying a graded potential
to these strips on both sides of the wafer (Fig. 5.5).
Other drift field configurations (e.g. radial drift) can be obtained by suitable
shapes of the electrodes. Drift chambers may be used for position and/or energy
measurement of ionizing radiation. In the first case the position is determined from
the drift time. Furthermore, segmenting the n + -strip anode in Fig. 5.5 into pads, a
two-dimensional position measurement is achieved.
Due to the small capacitive load of the readout electrode to the readout amplifier,
drift detectors are well suited for high precision energy measurement.