Техническое чтение для энергетиков. Бухарова Г.П. - 47 стр.

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49
The covalently bonded carbon atoms, in the diamond modification, are shown
in Fig. Since each carbon contributes four valence electrons, and it is tetrahedrally
bonded to four neighboring carbons, all of the electrons are used up in forming the
covalent bonds. In this situation no net flow of electrons through the solid is possible,
and the material is an insulator.
If an extra electron is added to the structure, however, no empty bonds are
available, and the electron is free to wander through the solid. It will move through
the crystal in the opposite direction from an applied electric field, and can thus
contribute to the electrical conductivity. This situation is shown in Fig. Electrons
which are not bound in the valence bonds, and thus free to move in this way, are
called conduction electrons. If conduction electrons can be produced in some manner
in sufficient quantity, the material Is no longer an insulator, but shows appreciable
electrical conductivity.
There is n second way in which the total number of electrons fails to match the
number of available bonding sites, i.e., when there are too few electrons. There is
then only one electron in some of the bonds, as shown in Fig. This missing bonding
electron is called a "hole". It, like the conduction electron, is free to wander through
the crystal. As shown in Fig., an electron in a bond adjacent to the one-electron bond
where the "hole" is localized, can jump into the empty position, leaving a vacancy
behind as it goes. As this process is repeated, the net effect is for the hole to move
through the crystal under the influence of an electric field; it can be seen that the hole
will move in the opposite! direction from the conduction electron, since the motion of
the hole is opposite to that of the valence electrons.
IMPURITY SEMICONDUCTORS
Using germanium, which crystallizes in the diamond lattice, as an example, the
effect of adding certain foreign atoms in substitutional positions in the crystal may be
seen easily. If a germanium atom is replaced by an atom of an element from Group
V, such as arsenic there are five valence electrons from the arsenic to be disposed of.
Four of these are shared with the four adjacent germanium atoms, to form covalent
bonds similar to those existing between adjacent germanium atoms. The fifth electron
will not be held in any chemical bond, as there are no empty sites available. It will be
attracted weakly by the arsenic, however, by coulombic forces, as its removal to large
distances leaves the arsenic with a net positive charge. When the electron is at large
distances the only available energy state is in the conduction band. The energy
required to remove the electron is called the impurity ionization energy, and the term
donor derives from the fact that the arsenic can "donate" a conduction electron to the
lattice.
The replacement of a germanium atom by an element of Group III, indium for
example, leads to a deficiency in valence electrons. Referring to the schematic picture
of Fig., the three electrons contributed by the indium form covalent bonds with three
of the four adjacent germanium atoms, but the fourth bond remains a one-electron
bond, i.e., a hole is formed. Just as the electron was weakly held to the arsenic by

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