Physics and Engineering of Radiation Detection

250 Chapter 5. Solid State Detectors

5.1.A Structure of Semiconductors

We know from Quantum Physics that electrons in an atom can occupy only discrete
energy levels. In fact this discreteness orquantizationis not in any way limited to
isolated atoms. For example the covalent bonding between atoms in semiconductors
creates allowed discrete energy levels. However these energy levels are lumped to-
gether in the so called twobands: valence band and conduction band. Valence band
represents a large number of very closely spaced energy levels at lower energies as
compared to the conduction band, which contains levels at higher energies. These
two bands are separated by aforbiddengap: a region in the energy level diagram
containing no energy levels. This essentially means that electrons can not assume
any energy that lies in this band. We’ll later see that this holds for only ideal semi-
conductors with no impurities and any practical semiconductor does have at least
one energy level in the band gap.
The electrons in the valence band are tightly bound to the atoms and need at least
an energy equal to the band gap to move to the conduction band. The conduction
band electrons, on the other hand, are very loosely bound and are almost free to
move around. These electrons take part in the electrical conduction process. In an
ideally pure semiconductor in the ground state all the electrons would populate the
valence band while conduction band would be empty.
Actually this band structure is not typical of just semiconductors. Insulators and
conductors also have similar structures. The distinguishing feature between them is
the band gap, since it represents the energy barrier that must be overcome by bound
electrons to become free and take part in the electrical conduction process. Fig. 5.1.1
compares the three types of solids in terms of energy level diagram. The band gap
in insulator and conductors are exactly opposite to each other, being very large for
insulators and non-existent for conductors. Semiconductors, on the other hand, have
a small band gap, so small that even a small thermal excitation can provide enough
energy to electrons in the valence band to jump up to the conduction band.

valence band

conduction band

E

band gap ~ 6 eV band gap ~ 1 eV

Insulator Semiconductor Conductor

Figure 5.1.1: Simplified energy

band structure diagrams for insula-

tors, semiconductors, and conduc-

tors.

When an electron from the valence band jumps to the conduction band, it leaves a
net positive charge behind. This effective positive charge, called ahole, behaves like
a real particle and takes part in electrical conduction process. However it should
be noted that by movement of a hole we mean the shift of a net positive charge
from one site to another due to the movement of an electron. A hole should not be
considered a localized positive charge having defined mass.