Imperfections in Solids
One type of imperfection that all crystals have in common is atomic thermal vibration.
Aperfect single crystal contains atoms at particular lattice sites, the atoms separated
from each other by a distance we have assumed to be constant. The atoms in a
crystal, however, have a certain thermal energy, which is a function of temperature.
The thermal energy, causes the atoms to vibrate in a random manner about an eauilibrium
lattice point. This random thermal motion causes the distance between atoms
to randomly fluctuate, slightly disrupting the perfect geometric arrangement of atoms.
This imperfection, called lattice vibrations, affects some electrical parameters, as we
will see later in our discussion of semiconductor material characteristics.
Another type of defect is called a point defect. There are several of this type that
we need to consider. Again, in an ideal single-crystal lattice, the atoms are arranged
in a perfect periodic arrangement. However, in a real crystal, an atom may be missing
from a particular lattice site. This defect is referred to as a vacancy; it is schematically
shown in Figure 1.17a. In another situation, an atom may be located between lattice
sites. This defect is referred to as an interstitial and is schematically shown in Figure
1.17b. In the case of vacancy and interstitial defects, not only is the perfect geometric
arrangement of atoms broken, but also the ideal chemical bonding between
atoms is disrupted, which tends to change the electrical properties of the material. A
vacancy and interstitial may be in close enough proximity lo exhibit an interaction
between the two point defects. This vacancy-interstitial defect, also known as a
Frenkel defect, produces different effects than the simple vacancy or interstitial.
The point defects involve single atoms or single-atom locations. In forming
single-crystal materials, more complex defects may occur. A line defect. for example,
occurs when an entire row of atoms is missing from its normal lattice site. This defect
is referred to as a line dislocation and is shown in Figure 1.18. As with a point
defect, a line dislocation disrupts both the normal geometric periodicity of the lattice
and the ideal atomic bonds in the crystal. This dislocation can also alter the electrical
properties of the material, usually in a more unpredictable manner than the simple
point defects.
Other complex dislocations can also occur in it crystal lattice. However. this introductory
discussion is intended only to present a few of the basic types of defect,
and to show that a red crystal is not necessarily a perfect lattice structure. The effect
of these imperfections on the electrical properties of a semiconductor will be considered as imperfection in solids
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Aperfect single crystal contains atoms at particular lattice sites, the atoms separated
from each other by a distance we have assumed to be constant. The atoms in a
crystal, however, have a certain thermal energy, which is a function of temperature.
The thermal energy, causes the atoms to vibrate in a random manner about an eauilibrium
lattice point. This random thermal motion causes the distance between atoms
to randomly fluctuate, slightly disrupting the perfect geometric arrangement of atoms.
This imperfection, called lattice vibrations, affects some electrical parameters, as we
will see later in our discussion of semiconductor material characteristics.
Another type of defect is called a point defect. There are several of this type that
we need to consider. Again, in an ideal single-crystal lattice, the atoms are arranged
in a perfect periodic arrangement. However, in a real crystal, an atom may be missing
from a particular lattice site. This defect is referred to as a vacancy; it is schematically
shown in Figure 1.17a. In another situation, an atom may be located between lattice
sites. This defect is referred to as an interstitial and is schematically shown in Figure
1.17b. In the case of vacancy and interstitial defects, not only is the perfect geometric
arrangement of atoms broken, but also the ideal chemical bonding between
atoms is disrupted, which tends to change the electrical properties of the material. A
vacancy and interstitial may be in close enough proximity lo exhibit an interaction
between the two point defects. This vacancy-interstitial defect, also known as a
Frenkel defect, produces different effects than the simple vacancy or interstitial.
The point defects involve single atoms or single-atom locations. In forming
single-crystal materials, more complex defects may occur. A line defect. for example,
occurs when an entire row of atoms is missing from its normal lattice site. This defect
is referred to as a line dislocation and is shown in Figure 1.18. As with a point
defect, a line dislocation disrupts both the normal geometric periodicity of the lattice
and the ideal atomic bonds in the crystal. This dislocation can also alter the electrical
properties of the material, usually in a more unpredictable manner than the simple
point defects.
Other complex dislocations can also occur in it crystal lattice. However. this introductory
discussion is intended only to present a few of the basic types of defect,
and to show that a red crystal is not necessarily a perfect lattice structure. The effect
of these imperfections on the electrical properties of a semiconductor will be considered as imperfection in solids
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