Physics and Engineering of Radiation Detection

5.1. Semiconductor Detectors 257

The underlying physical processes involved in the creation of electron-hole pairs
in semiconductors are the same as in other solids, which we have already discussed
in Chapter 2 and therefore will not be repeated here. As in gases, in semiconductors
too, the average energy needed to create an electron hole pair is independent of the
type of radiation and depends on the semiconductor material and its temperature.
The process is similar to the ionization process in gases except that the energy
needed in semiconductors is approximately 4 to 8 times less than in gases. This
implies that the number of charge carriers produced by radiation in a semiconductor
is much higher than in gases. Although one would expect that the noise equivalent
charge in semiconductors would also be higher by approximately the same amount,
but as we will see later, this is not necessarily the case. Because of this property the
semiconductor detectors are considered to be far superior than gaseous detectors in
terms of resolution and sensitivity.

Table 5.1.1: Densities and average ionization energies of common semiconductor
materials.

Material Density (g/cm^3 ) Ionization EnergyeV

Silicon 2.328 3.62

Germanium 5.33 2.8

Silicon Dioxide 2.27 18

Gallium Arsenide 5.32 4.8

Fig.5.1.4 shows the mechanism of production of charge pairs by incident photons
and by thermal agitation. Note that the energy levels in the forbidden gap produced
by crystal imperfections and impurities enhance the production of charge pairs. This
is certainly not a very desirable channel since it can produce non-linearities in the
detector response through an effect called charge trapping. What happens is that
an electron jumping to an impurity level may get trapped there for some time. This
electron can then do two things: it can either jump up to the conduction band and
complete the process of electron hole pair generation or it can fall back into the
valence band and recombine with the hole. The former introduces a time delay in
the charge pair production while in latter no charge pair is produced. The excess
energy in this case is not enough to create an electron hole pair (since it will be
equal toEimpand notEg) and is absorbed by the lattice.
To understand the statistics of the electron-hole pair production, let us assume
that the energy deposited by the incident radiation goes into causing lattice excita-
tions and ionization. Ifiandxrepresent the average energies needed to produce
ionization and excitation respectively, then the total deposited energy can be written
as
Edep=ini+xnx, (5.1.12)

whereniandnxrepresent the total number of ionization and excitations produced
by the radiation. If we now assume that these processes follow Gaussian statistics,