An Introduction to Bonding in Solids - Models for Bonding in Chemistry - page 141

Figure 3.11 Band structure of intrinsic and doped semiconductors

bands is extremely small, their energy being readily distributed over the
entiremetal because of fullyMOdelocalization. Furthermore, ametal can
absorb light of any wavelength. The highest valence band is only partially
filled, and electrons may flow easily under the action of an external field,
except for the collisions with the positive ions of the lattice. Increasing
temperature increases lattice vibrations and electron collisions, so that
electrical conductivity decreases.
Semiconductors have a small band gap, 60-100 kJ mol 1 , namely
0.6-1 eV for Ge or Si (Figure 3.11). The electrons of the last valence
band are easily excited to the conduction band (empty) under the effect of
temperature , the latter effect being

known as photoconductivity. The electronic population in the conduction
band will increase with temperature according to the statistical equilib-
rium described by the Fermi-Dirac statistics, so that conductivity will
increase with temperature (the opposite of what was found for metals).
Besides conduction due to the electrons thermally excited to the
conduction band (n-type, negative charge), there may be conduction due
to vacancies occurring in the valence band (p-type, where p stands for a
positive hole). Germanium and silicon are typical intrinsic semiconduc-
tors (left-hand side of Figure 3.11), whose properties are due to the pure
elements. But also of great importance are the so-called impurity semi-
conductors, where small amounts of impurity in a perfect crystal lattice
can modify the structure of the Brillouin zones, giving products whose
properties may be of commercial interest. The 'doping' of silicon or
germanium can be done using elements with one more electron in their
valence shell, such as phosphorus or arsenic, or elements with one less

ð kT « C « F Þ or light ð h n « C « F Þ