5.1. Semiconductor Detectors 279
Figure 5.1.18: Dependence of
electron drift velocity on elec-
tric field intensity in germa-
nium at two different temper-
atures (20). Solid and dashed
lines represent theoretical pre-
dictions while the points repre-
sent experimental data. 100 and
111 are the two crystallographic
directions in which the electric
field was applied.velocity would also be proportional to the applied electric field. This is true to
a certain extent, though. Actually the behavior of hole mobility differs from the
behavior of electron mobility in germanium. Even though it decreases with increase
in temperature but variation is not as linear on double logarithmic scale as it is for
electron mobility. The hole mobility for germanium shows a temperature dependence
given by (44)
μh∝T−^2.^33. (5.1.44)G.3 Gallium Arsenide (GaAs)
Gallium arsenide is another semiconductor material that is extensively used as de-
tection medium. The distinguishing feature ofGaAsis its higher photon absorption
efficiency as compared to silicon, which has allowed the development of extremely
thin ( 100-200μm) x-ray detectors. Another advantage ofGaAsis that it can
be operated at room temperature, which simplifies the detector design considerably
and also cuts down the cost of development and operation.
The band structure diagram of gallium arsenide is shown in Fig.5.1.20 and its
basic properties are listed in Table 5.1.2. It can be seen that, as far as atomic and
weight densities are concerned, there is no significant difference between germanium
and gallium arsenide. However since the band gap ofGaAsis more than twice that
of germanium and significantly higher than silicon, therefore its intrinsic carrier
concentration is several orders of magnitude lower than the two materials. The
most dramatic difference is the intrinsic resistivity of gallium arsenide, which is
about eight orders of magnitude higher than that of germanium and three orders of