Biomedical Engineering Reference
In-Depth Information
A second, nonbonding state is formed with parallel spins. These two energy levels
are degenerate at large
R
, where the wave functions of the two atomic electrons do
not overlap appreciably.
Molecular spectra are very complicated. Changes in the rotational motion of
molecules accompany the emission or absorption of photons in the far infrared.
Vibrational changes together with rotational ones usually produce spectra in the
near infrared. Electronic transitions are associated with the visible and ultraviolet
part of molecular spectra. Electronic molecular spectra have a fine structure due
to the vibrational and rotational motions of the molecule. Molecular spectra also
show isotopic structure. The presence of the naturally occurring
35
Cl and
37
Cl iso-
topes in chlorine, for example, gives rise to two sets of vibrational and rotational
energy-level differences in the spectrum of HCl.
2.9
Solids and Energy Bands
We briefly discuss the properties of solids and the origin of energy bands, which are
essential for understanding how semiconductor materials can be used as radiation
detectors (Chapter 10).
Solids can be crystalline or noncrystalline (e.g., plastics). Crystalline solids, of
which semiconductors are an example, can be put into four groups according to
the type of binding that exists between atoms. Crystals are characterized by regular,
repeated atomic arrangements in a lattice.
In a
molecular
solid the bonds between molecules are formed by the weak, attrac-
tive van der Waals forces. Examples are the noble gases, H
2
,N
2
,andO
2
, which are
solids only at very low temperatures and can be easily deformed and compressed.
All electrons are paired and hence molecular solids are poor electrical conductors.
In an
ionic
solid all electrons are also paired. A crystal of NaCl, for example,
exists as alternating charged ions, Na
+
and Cl
-
, in which all atomic shells are filled.
These solids are also poor conductors. The electrostatic forces between the ions
are very strong, and hence ionic solids are hard and have high melting points.
They are generally transparent to visible light, because their electronic absorption
frequencies are in the ultraviolet region and lattice vibration frequencies are in the
infrared.
A
covalent
solid is one in which adjacent atoms are covalently bound by shared
valence electrons. Such bonding is possible only with elements in Group IVB of
the periodic system; diamond, silicon, and germanium are examples. In diamond,
a carbon atom (electronic configuration 1s
2
2s
2
p
2
) shares one of its four L-shell elec-
trons with each of four neighbors, which, in turn, donates one of its L electrons for
sharing. Each carbon atom thus has its full complement of eight L-shell electrons
through tight binding with its neighbors. Covalent solids are very hard and have
high melting points. They have no free electrons, and are therefore poor conduc-
tors. Whereas diamond is an insulator, Si and Ge are semiconductors, as will be
discussed in Chapter 10.
