Physics and Engineering of Radiation Detection

268 Chapter 5. Solid State Detectors

EΓ 2 = 4.2 eV

EL= 2.0 eV

EΓ 1 = 3.4 eV

Eg= 1.12 eV

EX= 1.2 eV

Wavevector

<111>

Holes

Energy

<100>

Figure 5.1.8: Band structure diagram of silicon showing energy ver-

sus 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.

T (K)

250 260 270 280 290 300 310

(eV)g

E

1.122

1.124

1.126

1.128

1.13

1.132

1.134

1.136

1.138

Figure 5.1.9: Variation of sili-

con band gap energy with abso-

lute temperature.

Silicon detectors are generally operated at low temperatures of around− 30 C
to− 100 C^1. The reason is to suppress the thermal agitation, which can produce
electron hole pairs even at room temperature. Lowering of the temperature has two
effects: widening of the band gap and decrease in the thermal agitation. Both of
these compliment one another to suppress the noise in the detector. It should, how-
ever be noted that, even though operating silicon detectors at low temperatures is a

(^1) The choice of operating temperature is mainly based on noise considerations. Some silicon detectors are
even operated at as low as− 400 C.