_________________________________________________________________ Semiconductor Diodes
1-36. If an electron absorbs a sufficient amount of energy, it is possible for that electron
to be completely removed from the influence of the atom. This is called IONIZATION. It
is said that an atom is ionized when it loses electrons or gains electrons in this process of
electron exchange. For ionization to take place, there must be a transfer of energy that
results in a change in the internal energy of the atom. An atom having more than its normal
amount of electrons acquires a negative charge and is called a NEGATIVE ION. The atom
that gives up some of its normal electrons is left with fewer negative charges than positive
charges and is called a POSITIVE ION. Therefore, we can define ionization as the process
by which an atom loses or gains electrons.
1-37. So far we have covered only isolated atoms. When atoms are spaced far enough
apart, as in a gas, they have very little influence upon each other and are very much like
lone atoms. However, atoms within a solid have a marked affect upon each other. The
forces that bind these atoms together greatly modify the behavior of the other electrons.
One result of this close proximity of atoms is to cause the individual energy levels of an
atom to break up and form bands of energy. Discrete (separate and complete) energy levels
still exist within these energy bands. However, there are many more energy levels than
there were with the isolated atom. In some cases, energy levels will have disappeared.
Figure 1-5 shows the difference in the energy arrangement between an isolated atom and
the atom in a solid. Notice that the isolated atom (such as in gas) has energy levels while
the atom in a solid has energy levels grouped into ENERGY BANDS.
1-38. The upper band in Figure 1-5 is called the CONDUCTION BAND because
electrons in this band are easily removed by the application of external electric fields.
Materials that have a large number of electrons in the conduction band act as good
conductors of electricity.
1-39. Below the conduction band is the FORBIDDEN BAND or energy gap. Electrons
are never found in this band. However, they may travel back and forth through it, provided
they do not come to rest in the band.
1-40. The bottom band or VALENCE BAND is composed of a series of energy levels
containing valence electrons. Electrons in this band are more tightly bound to the
individual atom than the electrons in the conduction band. However, the electrons in the
valence band can still be moved to the conduction band with the application of energy
(usually thermal energy). There are more bands below the valence band but they are not
important to the understanding of semiconductor theory.
1-41. The concept of energy bands is particularly important in classifying materials as
conductors, semiconductors, and insulators. An electron can exist in either of two energy
bands (the conduction band or the valence band). All that is necessary to move an electron
from the valence band to the conduction band, so it can be used for electric current, is
enough energy to carry the electron through the forbidden band. The width of the
forbidden band or the separation between the conduction and valence bands determines
whether a substance is an insulator, semiconductor, or conductor. Figure 1-6 uses energy
level diagrams to show the difference between insulators, semiconductors, and conductors.
23 June 2005
TC 9-62
1-9
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