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 TM 9-8000
them out of their orbits. The replaced electrons repeat
c. Insulators (B, Fig. 11-3). Whenever there are
this process in nearby atoms until the movement is
transmitted throughout the entire length of the conductor,
more than four electrons in the outer orbits of the atoms
thus creating a current flow. Copper is an example of a
of a substance, these electrons will tend to be bound,
good conductor because it only has one free electron.
causing restriction of free electron movement, making it
This electron is not held very strongly in its orbit and can
an insulator.
Common insulating substances in
get away from the nucleus very easily. Silver is a better
automotive applications are rubber, plastic, Bakelite,
conductor of electricity but it is too expensive to be used
varnish, and fiberboard.
in any great quantity. Because of this, copper is the
conductor used most widely in automotive applications.
Section II. SEMICONDUCTOR DEVICES
electrons in its outer shell), will yield one free electron
11-5. Fundamental Principles.
per molecule, thus making the material an electrical
conductor. The process of adding impurities to a
a. Description. Paragraph 11-4 explains that any
semiconductor is called doping. Any semiconductor
substance whose atoms contain less than four electrons
material that is doped to yield free electrons is called N-
in their outermost orbits is classified as an electrical
type material.
conductor. It also is explained that any substance whose
atoms contain more than four electrons in their
(3) When boron, which has three electrons in its
outermost orbits is classified as an electrical insulator. A
outer ring, is used to dope the silicon crystal, the
special case exists, however, when a substance contains
resultant covalent bonding yields seven electrons in the
four electrons in the outermost orbits of its atoms. This
outer shell. This leaves an opening for another electron
type of substance is known as a semiconductor and is
and is illustrated in figure 11-6. This space is called a
the basis for all modern electronic equipment. The most
hole and can be considered a positive charge just as the
popular of all semiconductors is silicon.
extra electrons that exist in N-type semiconductor
material are considered a negative charge. Materials
b. Characteristics of Semiconductors. in its pure
that have holes in their outermost electron shells are
state, silicon is neither a good conductor or insulator.
called positive or P-type materials.
In order to
But by processing silicon in the following ways, its
understand the behavior of P-type semiconductors, it is
conductive or insulative properties can be adjusted to
necessary to look upon the hole as a positive current
suit just about any need.
carrier, just as the free electron in N-type semiconduc-
tors are considered negative current carriers. Just as
(1) When a number of silicon atoms are jammed
electrons move through N-type semiconductors, holes
together in crystalline (glasslike) form, they form a
move from atom-to-atom in P-type semiconductors.
covalent (sharing) bond. Therefore, the electrons in the
Movement of holes through P-type semiconductors,
outer ring of one silicon atom join with the outer ring
however, is from the positive terminal to the negative
electrons of other silicon atoms, resulting in a sharing of
terminal. For this reason, any circuit analysis of solid-
outer ring electrons between all of the atoms. It can be
state circuitry is done on the basis of positive to negative
seen in figure 11-4 that covalent sharing gives each
(conventional) current flow.
atom eight electrons in its outer orbit, making the orbit
complete. This makes the material an insulator because
c. Hole Movement Theory (Fig. 11-7). When a
it contains more than four electrons in its outer orbit.
source voltage, such as a battery, is connected to N-type
material, an electric current will flow through it. The
(2) When certain materials such as phosphorus are
current flow in the N-type semiconductor consists of the
added to the silicon crystal in highly controlled amounts
movement of free
the resultant mixture becomes a conductor (fig. 11-5).
This is because phosphorus, which has five electrons in
forming a covalent bond with silicon (which has four
11-4
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