256 Chapter 5. Solid State Detectors
Si Si Si SiSi Si SiSi Si Si SiSi Si Si SiCovalent BondEnergy
(a)
(b)
Valence BandConduction BandElectronP
Donor LevelsFigure 5.1.3: (a)Addition of phosphorus to the
silicon lattice. Phosphorus having five available
electrons for bonding leaves an extra electron,
which can move freely in the lattice. (b)Addition
of a donor impurity in a semiconductor creates
new energy levels near to the conduction band,
which effectively extends the valence band to
lower energy. As a result the band gap is de-
creased with electrons as major charge carriers.Although in semiconductor detectors we are mainly interested in the ionization
process but the process of lattice excitations also has a significant impact on the
statistics of electron-hole pair production. Before we look at the statistics of the
charge pair production, let us first have a closer look at the ionization mechanism
in semiconductors.
In a perfect semiconductor material at a temperature below the band gap energy,
all the electrons are in valence band. The conduction band in such a situation is
completely empty. The outer shell electrons, taking part in the covalent bonding
between lattice atoms, are not free to move around in the material. However as the
temperature is raised, some of the electrons may get enough thermal excitation to
leave the valence band and jump to the conduction band. This creates an electron
deficiency or a net positive charge in the valence band and is generally referred to
as ahole. This process can also occur, albeit at a much higher rate, when radiation
passes through the material. Any radiation capable of delivering energy above a
material-specific threshold is capable of creating electron-hole pairs along its track
in the material. This threshold is higher than the band gap energy of material as
some of the energy also goes into crystal excitations. For silicon the threshold is
very low (3.62eV), which makes it highly desirable for use in radiation detectors
(see Table 5.1.1).