_________________________________________________________________ Semiconductor Diodes
1-42. The energy diagram for the insulator shows the insulator with a very wide energy
gap. The wider the gap, the greater the amount of energy required moving the electron
from the valence band to the conduction band. Therefore, an insulator requires a large
amount of energy to obtain a small amount of current. The insulator "insulates" because of
the wide forbidden band or energy gap.
1-43. The semiconductor has a smaller forbidden band and requires less energy to move
an electron from the valence band to the conduction band. Therefore, for a certain amount
of applied voltage, more current will flow in the semiconductor than in the insulator.
1-44. The last energy level diagram (see Figure 1-6) is that of a conductor. Notice that
there is no forbidden band or energy gap and the valence and conduction bands overlap.
With no energy gap, it only takes a small amount of energy to move electrons into the
conduction band. Consequently, conductors pass electrons very easily.
COVALENT BONDING
1-45. The number of electrons in its valence shell determines the chemical activity of an
atom. When the valence shell is complete, the atom is stable and shows little tendency to
combine with other atoms to form solids. Only atoms that possess eight valence electrons
have a complete outer shell. These atoms are referred to as inert or inactive atoms.
However, if the valence shell of an atom is short the required number of electrons to
complete the shell, then the activity of the atom increases.
1-46. For example, silicon and germanium 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. The arms that join the atoms and support the structure are the covalent
bonds.
1-47. As a result of this sharing process, the valence electrons are held tightly together.
Figure 1-8 shows a two-dimensional view of the silicon lattice. 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. Since
every atom in this pattern is bonded to four other atoms, then 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 because
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.
23 June 2005
TC 9-62
1-11
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