1-11
referred to as inert or inactive atoms. However, if the valence shell of an atom lacks the required number
of electrons to complete the shell, then the activity of the atom increases.
Silicon and germanium, for example, are the most frequently used semiconductors. Both are quite
similar in their structure and chemical behavior. Each has four electrons in the valence shell. Consider just
silicon. Since it has fewer than the required number of eight electrons needed in the outer shell, its atoms
will unite with other atoms until eight electrons are shared. This gives each atom a total of eight electrons
in its valence shell; four of its own and four that it borrowed from the surrounding atoms. The sharing of
valence electrons between two or more atoms produces a COVALENT BOND between the atoms. It is
this bond that holds the atoms together in an orderly structure called a CRYSTAL. A crystal is just
another name for a solid whose atoms or molecules are arranged in a three-dimensional geometrical
pattern commonly referred to as a lattice. Figure 1-7 shows a typical crystal structure. Each sphere in the
figure represents the nucleus of an atom, and the arms that join the atoms and support the structure are the
covalent bonds.
Figure 1-7.—A typical crystal structure.
As a result of this sharing process, the valence electrons are held tightly together. This can best be
illustrated by the two-dimensional view of the silicon lattice in figure 1-8. The circles in the figure
represent the nuclei of the atoms. The +4 in the circles is the net charge of the nucleus plus the inner
shells (minus the valence shell). The short lines indicate valence electrons. Because every atom in this
pattern is bonded to four other atoms, the electrons are not free to move within the crystal. As a result of
this bonding, pure silicon and germanium are poor conductors of electricity. The reason they are not
insulators but semiconductors is that with the proper application of heat or electrical pressure, electrons
can be caused to break free of their bonds and move into the conduction band. Once in this band, they
wander aimlessly through the crystal.
