TC 9-62
1-50. When ball number 1 is removed from the tube, a hole is left. This hole is then
filled by ball number 2, which leaves still another hole. Ball number 3 then moves into the
hole left by ball number 2. This causes still another hole to appear where ball 3 was. Notice
the holes are moving to the right side of the tube. This action continues until all the balls
have moved one space to the left in which time the hole moved eight spaces to the right
and came to rest at the right-hand end of the tube.
1-51. In the theory just described, two-current carriers (the negative electron and the
positive hole) were created by the breaking of covalent bonds. These carriers are referred
to as electron-hole pairs. Since the semiconductor we have been covering contains no
impurities, the number of holes in the electron-hole pairs is always equal to the number of
conduction electrons. Another way of describing this condition where no impurities exist is
by saying the semiconductor is INTRINSIC. The term intrinsic is also used to distinguish
the pure semiconductor that we have been working with from one containing impurities.
DOPING PROCESS
1-52. The pure semiconductor already mentioned is basically neutral. It contains no free
electrons in its conduction bands. Even with the application of thermal energy, only a few
covalent bonds are broken, yielding a relatively small current flow. A much more efficient
method of increasing current flow in semiconductors is by adding very small amounts of
selected additives to them, generally no more than a few parts per million. These additives
are called impurities and the process of adding them to crystals is referred to as DOPING.
The purpose of semiconductor doping is to increase the number of free charges that can be
moved by an external applied voltage. When an impurity increases the number of free
electrons, the doped semiconductor is NEGATIVE or N-TYPE. The impurity that is added
is known as an N-type impurity. However, an impurity that reduces the number of free
electrons, causing more holes, creates a POSITIVE or P-TYPE semiconductor, and the
impurity that was added to it is known as a P-type impurity. Semiconductors that are doped
in this manner, either with N- or P-type impurities, are referred to as EXTRINSIC
semiconductors.
N-Type Semiconductor
1-53. The N-type impurity easily loses its extra valence electron when added to a
semiconductor material. This also increases the conductivity of the material by contributing
a free electron. This type of impurity has five valence electrons and is called a
PENTAVALENT impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent
impurities. Since these materials give or donate one electron to the doped material, they are
also called DONOR impurities.
1-54. When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will
form covalent bonds with the germanium atoms. Figure 1-10 shows an arsenic atom in a
germanium lattice structure. Notice the arsenic atom in the center of the lattice. It has five
valence electrons in its outer shell but uses only four of them to form covalent bonds with
the germanium atoms, leaving one electron relatively free in the crystal structure. Pure
germanium may be converted into a N-type semiconductor by "doping" it with any donor
impurity having five valence electrons in its outer shell. Since this type of semiconductor
(N-type) has a surplus of electrons, the electrons are considered MAJORITY carriers,
while the holes, being few in number, are the MINORITY carriers.
1-14
TC 9-62
23 June 2005
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