5.1. Semiconductor Detectors 275
Now, according to equation 5.1.33, the percentage decrease in intrinsic charge
concentration is given byn =T 13 /^2 e−Eg^1 /^2 kBT^1 −T 23 /^2 e−Eg^2 /^2 kBT^2
T 13 /^2 e−Eg^1 /^2 kBT^1× 100
=
6. 536 × 10 −^7 − 6. 40 × 10 −^8
6. 536 × 10 −^7
× 100
=90.2%. (5.1.37)
This example clearly demonstrates the advantage of operating a silicon detec-
tor at low temperatures.G.2 Germanium (Ge)........................
Use of germanium detectors inγ-ray spectroscopy is well established. Their high
resolution and wide dynamic range make them highly suitable for spectroscopic
purposes. However, in other applications, such as particle tracking, they are not
preferred over silicon based detectors. In this section we will look at some of the
important properties of germanium and compare them with those of silicon.
The crystal structure of germanium is the same as silicon but its atomic density
is slightly lower. The most distinguishing feature of germanium is its low band gap
energy (0.661eV), which is almost half that of silicon. The energy band structure
of germanium is shown in Fig.5.1.15.
EΓ 2 = 3.22 eVΓ (^1) Eg= 0.66 eV
E = 0.8 eV
E EL= 0.85 eV
X= 1.2 eV
Wavevector
Holes
Energy
<100> <111>
Figure 5.1.15: Band structure diagram of germanium showing energy
versus wavenumber (reproduced from (47)). The subscripts ofErepre-
sent different energy levels. The number in brackets (100 and 111) are
the Miller indices. A Miller index represents the orientation of an atomic
plane in a crystal lattice.