ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 2
designed tubes render extensive service, the life expectancy of
transistors is better than three to four times that of ordinary
electron tubes. There is no known failure mechanism (such as an
open filament in a tube) to limit the semiconductor's life.
However, semiconductor devices do have some limitations. They
are usually affected more by temperature, humidity, and
radiation than tubes are.
c. Semiconductor Theory. To understand why solidstate devices
function as they do, we will have to examine the composition and
nature of semiconductors. This entails theory, which is
fundamental to the study of solidstate devices.
(1) Energy Bands. Energy bands are groups of energy levels
which result from the close proximity of atoms in a solid. The
three most important energy bands are the CONDUCTION BAND,
FORBIDDEN BAND, and VALENCE BAND. Each of these bands will be
discussed briefly in the following paragraphs. While reading
through these paragraphs refer to figure 72 on the following
page.
(a) Conduction Band. The upper band in figure 72 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.
(b) Forbidden Band. Below the conduction band is the
forbidden band or energy gap. Electrons are never found in this
band, but may travel back and forth through it, provided they do
not come to rest in the band.
(c) Valence Band. The last 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
and will not be discussed.
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