5.1. Semiconductor Detectors 267
silicon and germanium. This property makes them suitable for operation at room
temperature, which completely eliminates the need for colling system and is a big
advantage in terms of operating cost. GaAsandCdZnTebased detectors have
therefore gained a lot of popularity in recent years.
Table 5.1.2: Comparison of some basic properties at room temperature of semicon-
ductor materials commonly used in radiation detectors (note that the actual values
may differ slightly from these nominal values due to manufacturing and structural
differences) (47).
Property Si Ge GaAs CdZnTe
Weight Density (gcm−^3 ) 2.329 5.323 5.32 5.78
Dielectric Constant 11.7 16 12.8 10.9
Energy Gap (eV) 1.12 0.661 1.424 1.56
Intrinsic Carrier
Concentration (cm−^3 ) 1 × 1010 2 × 1013 2. 1 × 106 2. 0 × 105
W-value (eV) 3.62 2.95 4.2 4.64
Intrinsic Resistivity (Ωcm) 3. 2 × 105 46 3. 3 × 108 3. 0 × 1010
G.1 Silicon (Si)
For radiation detection, silicon is by far the most commonly used material. It
is relatively cheaper than other semiconductor materials and is easily available in
purified form. These factors and the fact that silicon has moderate intrinsic charge
concentration and intrinsic resistivity makes it suitable for use as detection medium.
In Fig.5.1.1 we saw the simplified sketch of the band structure of a semiconductor
material. In reality the energy levels are not so well behaved. Fig.5.1.8 shows the
actual energy level diagram for silicon.
A good thing about silicon is that its forbidden energy gap is neither very low (as
of germanium) nor very high (as of gallium arsenide). This makes it a good candidate
for manipulation by adding impurities so that the desired properties, such as high
resistivity, are achieved. As with all semiconductor materials, the energy gap for
silicon has a moderate temperature dependence, which can be described by (47)
Eg=1. 17 − 4. 73 × 10 −^4T^2
T+ 636
, (5.1.29)
where temperatureTis in absolute units andEgis ineV. This equation has been
plotted in Fig.5.1.9. It is apparent that small changes in temperature can cause the
band gap to shorten or widen. This is certainly not a desirable feature, since it could
induce non-linearities in the detector response. Shortening of band gap means more
electron hole pairs will be generated with the same deposited energy while a wider
band gap would make it harder for the electrons in the valence band to jump to the
conduction band.