266 Chapter 5. Solid State Detectors
where we have used the relationsρe=eneandρh=enhwithneandnhbeing the
number density of electrons and holes respectively.
If now this material is placed in an ionizing radiation field, electron hole pairs
will be created and consequently the number of free charge pairs in the bulk of the
material will increase. The result of this will be a change in the conductivity of
the material. Ifn′represents the number of charge pairs created by the incident
radiation, then the change in the conductivity will be given by
δσ=e(μe+μh)n′. (5.1.28)This change in conductivity is proportional to the energy delivered by the incident
radiation provided all other conditions remain constant. Hence measuring the change
in conductivity is equivalent to measuring the delivered energy if the detector has
been properly calibrated. Such a measurement can be done by placing the detector
in an external circuit, which could measure the change in current caused by change
in conductivity.
It should be mentioned here that both electrons and holes take finite amount
of time to recombine and hence the change in conductivity has a time profile that
extends up to the lifetime of the slowest charge carrier. Therefore, even for a localized
radiation interaction that could be represented by a delta function, the output signal
actually has a shape with a finite rise and decay times.
5.1.G Materials Suitable for Radiation Detection
Not all semiconductors can be used in radiation detectors. The choice depends on
many factors such as resistivity, mobility of charges, drift velocity, purity, operating
temperature, and cost. Silicon has traditionally been the most commonly used ma-
terial in particle detectors, a trend that is now changing. Other commonly used ma-
terials are germanium (Ge), gallium arsenide (GaAs), and cadmium-zinc-tellurium
(CdZnTe). The need of a new generation of radiation hard silicon detectors is now
pushing the researchers to develop more complex semiconductor structures. In the
following we will look at some of the commonly used semiconductor materials and
study their properties relevant to their use as radiation detectors. The purpose of
this activity is to supply the reader with enough information so that a good compar-
ison of merits and demerits of these materials with respect to their use as detection
media could be performed.
A quick comparison of basic properties of common semiconductor devices can be
done from Table.5.1.2. However, the reader is encouraged to go through the details
of each material as given in the following sections to develop a working knowledge
of advantages and disadvantages associated with each device type. One should also
note that there are other novel semiconductor devices besides the ones discussed
here and more are being constantly designed and developed. It is not the intention
here to give the reader a comprehensive list of materials available, rather to give a
broader perspective of the basic properties that are essential for these devices to be
used as efficient semiconductor detectors.
A very important property of semiconductor materials is their intrinsic carrier
concentration since it can be used to estimate the signal to noise ratio at the room
temperature. It is evident from Table.5.1.2 thatGaAsandCdZnTehave intrin-
sic carrier concentrations that are several orders of magnitude lower than those of