140
G. Lutz and R. Klanner
Reaching all these good detector properties requires a readout electronics which
is well matched to the detectors. Here we notice a point specific to silicon which
is also the basic material of most of present day electronics. For that reason it is
natural to integrate the sensitive front-end part of electronics into the detector. This
is the case, for example, in CCDs and drift diodes [3] with very high spectroscopic
resolution. A further device (DEPFET) [3] combines the function of detector
and amplifier in the basic structure. In MAPS (Monolithic Active Pixel Sensors)
sophisticated readout electronics is directly integrated on the silicon chip of the
sensor.
Dependent on the field of application different aspects of semiconductors are in
the focus of interest. In particle physics tracking requires high position resolution
and often high speed capabilities while energy resolution is of less importance.
Recently at the CERN LHC also a timing accuracy of a few tens of picoseconds in
combination with precision tracking became a requirement. In X-ray spectroscopy
and imaging, as well as in X-ray astronomy both energy and position resolution are
of importance. For light detection, high photon-detection efficiency and resolving
single photons are typically more important than position accuracy.
5.3 Basics of Semiconductor Physics
After these introductory remarks on semiconductor detectors we will look into the
underlying mechanisms in a little more detail.
Most commonly used semiconductors are single crystals with diamond (Si and
Ge) or zinc blende (GaAs and other compound semiconductors) lattice. Each
atom in the crystal shares their outermost (valence) electrons with the four closest
neighbours. At very low temperature all electrons are bound to their respective
locations and the material is an insulator. At elevated temperature thermal vibrations
will sometimes break a bond and both the freed electron and the hole (the empty
place left behind to be filled by a neighbouring electron) are available for electrical
conduction. The density of free electrons/holes is called intrinsic carrier density n i .
For silicon its value at room temperature is about 10 10 cm –3 , resulting in an intrinsic
resistivity of about 350 k·cm.
Creation of electron–hole pairs can also be accomplished by electromagnetic
radiation or by the passage of charged particles knocking out of their covalent bond
some of the valence electrons. This is the mechanism used in the detection process.
These free charge carriers will then be moved by an applied electrical field (drift)
and redistribute due to concentration variations (diffusion) until finally reaching an
external electrode connected to the readout electronics.
So far we have only dealt with intrinsic semiconductors, perfect crystals without
foreign atoms. One may, however, replace a small fraction of atoms with some
having either one more, called donors (e.g. P in Si) or one less, called acceptors (e.g.
B in Si) valence electron. The additional electron or the missing electron (hole) is
only weakly bound, resulting in states in the silicon band gap located about 40 meV
G. Lutz and R. Klanner
Reaching all these good detector properties requires a readout electronics which
is well matched to the detectors. Here we notice a point specific to silicon which
is also the basic material of most of present day electronics. For that reason it is
natural to integrate the sensitive front-end part of electronics into the detector. This
is the case, for example, in CCDs and drift diodes [3] with very high spectroscopic
resolution. A further device (DEPFET) [3] combines the function of detector
and amplifier in the basic structure. In MAPS (Monolithic Active Pixel Sensors)
sophisticated readout electronics is directly integrated on the silicon chip of the
sensor.
Dependent on the field of application different aspects of semiconductors are in
the focus of interest. In particle physics tracking requires high position resolution
and often high speed capabilities while energy resolution is of less importance.
Recently at the CERN LHC also a timing accuracy of a few tens of picoseconds in
combination with precision tracking became a requirement. In X-ray spectroscopy
and imaging, as well as in X-ray astronomy both energy and position resolution are
of importance. For light detection, high photon-detection efficiency and resolving
single photons are typically more important than position accuracy.
5.3 Basics of Semiconductor Physics
After these introductory remarks on semiconductor detectors we will look into the
underlying mechanisms in a little more detail.
Most commonly used semiconductors are single crystals with diamond (Si and
Ge) or zinc blende (GaAs and other compound semiconductors) lattice. Each
atom in the crystal shares their outermost (valence) electrons with the four closest
neighbours. At very low temperature all electrons are bound to their respective
locations and the material is an insulator. At elevated temperature thermal vibrations
will sometimes break a bond and both the freed electron and the hole (the empty
place left behind to be filled by a neighbouring electron) are available for electrical
conduction. The density of free electrons/holes is called intrinsic carrier density n i .
For silicon its value at room temperature is about 10 10 cm –3 , resulting in an intrinsic
resistivity of about 350 k·cm.
Creation of electron–hole pairs can also be accomplished by electromagnetic
radiation or by the passage of charged particles knocking out of their covalent bond
some of the valence electrons. This is the mechanism used in the detection process.
These free charge carriers will then be moved by an applied electrical field (drift)
and redistribute due to concentration variations (diffusion) until finally reaching an
external electrode connected to the readout electronics.
So far we have only dealt with intrinsic semiconductors, perfect crystals without
foreign atoms. One may, however, replace a small fraction of atoms with some
having either one more, called donors (e.g. P in Si) or one less, called acceptors (e.g.
B in Si) valence electron. The additional electron or the missing electron (hole) is
only weakly bound, resulting in states in the silicon band gap located about 40 meV