5 Solid State Detectors
139
5.2 Basic Detection Process of Single Photons
in Semiconductors
The simplest detector is a reverse-biased planar diode (Fig. 5.1). Photons interacting
in silicon will, dependent on their energy, produce one or more electron-hole pairs
close to their points of interaction. Charged particles will generate pairs along their
path within the semiconductor. An average energy of 3.6 eV is needed for creation
of a pair in silicon with a band gap of 1.12 eV at room temperature. This should
be compared with the ionization energy of gases which is more than an order of
magnitude higher. Electrons and holes will be separated by the electric field within
the space charge region and collected at the electrodes on opposite sides of the diode.
The small band gap and the corresponding large signal charge generated in
the photon absorption process is the principal cause for the excellent properties
of semiconductor radiation detectors manifesting themselves in particular in very
good spectroscopic resolution down to low energies. Further reasons are the high
density and corresponding low range of delta-electrons which makes very precise
position measurement possible. High charge carrier mobilities combined with small
detector volume leads to short charge collection time and makes the use of detectors
in high rate environment possible. The excellent mechanical rigidity makes the use
of gas containment foils superfluous and allows operation in the vacuum. Therefore
very thin entrance windows can be constructed and high quantum efficiency can be
reached down to low photon energies. Position dependent doping of semiconductors
allows construction of detectors with sophisticated electric field configurations and
intrinsically new properties.
Fig. 5.1 Schematic structure of a reverse-biased semiconductor diode used as photon detector.
The region heavily doped with acceptors is denoted p + , and n-bulk and n + the regions lightly and
heavily doped with donors, respectively
139
5.2 Basic Detection Process of Single Photons
in Semiconductors
The simplest detector is a reverse-biased planar diode (Fig. 5.1). Photons interacting
in silicon will, dependent on their energy, produce one or more electron-hole pairs
close to their points of interaction. Charged particles will generate pairs along their
path within the semiconductor. An average energy of 3.6 eV is needed for creation
of a pair in silicon with a band gap of 1.12 eV at room temperature. This should
be compared with the ionization energy of gases which is more than an order of
magnitude higher. Electrons and holes will be separated by the electric field within
the space charge region and collected at the electrodes on opposite sides of the diode.
The small band gap and the corresponding large signal charge generated in
the photon absorption process is the principal cause for the excellent properties
of semiconductor radiation detectors manifesting themselves in particular in very
good spectroscopic resolution down to low energies. Further reasons are the high
density and corresponding low range of delta-electrons which makes very precise
position measurement possible. High charge carrier mobilities combined with small
detector volume leads to short charge collection time and makes the use of detectors
in high rate environment possible. The excellent mechanical rigidity makes the use
of gas containment foils superfluous and allows operation in the vacuum. Therefore
very thin entrance windows can be constructed and high quantum efficiency can be
reached down to low photon energies. Position dependent doping of semiconductors
allows construction of detectors with sophisticated electric field configurations and
intrinsically new properties.
Fig. 5.1 Schematic structure of a reverse-biased semiconductor diode used as photon detector.
The region heavily doped with acceptors is denoted p + , and n-bulk and n + the regions lightly and
heavily doped with donors, respectively