Module 7—Introduction to Solid-State Devices and Power Supplies
i - ix
, 1-1 to 1-10
1-11 to 1-20
, 1-21 to 1-30
1-31 to 1-40
, 1-41 to 1-47
2-1 to 2-10
, 2-11 to 2-20
2-21 to 2-30
2-31 to 2-40
, 2-41 to 2-50
2-51 to 2-54
, 3-1 to 3-10
3-11 to 3-20
, 3-21 to 3-30
3-31 to 3-40
, 3-41 to 3-50
3-51 to 3-54
4-1 to 4-10
, 4-11 to 4-20
4-21 to 4-30
, 4-31 to 4-40
4-41 to 4-50
, 4-51 to 4-62
referred to as inert or inactive atoms. However, if the valence shell of an atom lacks the required number of
electrons to complete the shell, then the activity of the atom increases.
Silicon and germanium, for
example, are the most frequently used semiconductors. Both are quite similar in their structure and chemical
behavior. Each has four electrons in the valence shell. Consider just silicon. Since it has fewer than the
required number of eight electrons needed in the outer shell, its atoms will unite with other atoms until eight
electrons are shared. This gives each atom a total of eight electrons in its valence shell; four of its own and
four that it borrowed from the surrounding atoms. The sharing of valence electrons between two or more atoms
produces a COVALENT BOND between the atoms. It is this bond that holds the atoms together in an orderly structure
called a CRYSTAL. A crystal is just another name for a solid whose atoms or molecules are arranged in a
three-dimensional geometrical pattern commonly referred to as a lattice. Figure 1-7 shows a typical crystal
structure. Each sphere in the figure represents the nucleus of an atom, and the arms that join the atoms and
support the structure are the
Figure 1-7.—A typical crystal structure.
As a result of this sharing process, the valence electrons are held tightly together. This can best be
illustrated by the two-dimensional view of the silicon lattice in figure 1-8. The circles in the figure represent
the nuclei of the atoms. The +4 in the circles is the net charge of the nucleus plus the inner shells (minus the
valence shell). The short lines indicate valence electrons. Because every atom in this pattern is bonded to four
other atoms, the electrons are not free to move within the crystal. As a result of this bonding, pure silicon and
germanium are poor conductors of electricity. The reason they are not insulators but semiconductors is that with
the proper application of heat or electrical pressure, electrons can be caused to break free of their bonds and
move into the conduction band. Once in this band, they wander aimlessly through the crystal.
Figure 1-8.—A two-dimensional view of a silicon cubic lattice.
Q13. What determines the chemical activity of an atom?
Q14. What is the term used to
describe the sharing of valence electrons between two or more atoms?
As stated earlier, energy can be added to electrons by applying heat. When enough energy is absorbed by the
valence electrons, it is possible for them to break some of their covalent bonds. Once the bonds are broken, the
electrons move to the conduction band where they are capable of supporting electric current. When a voltage is
applied to a crystal containing these conduction band electrons, the electrons move through the crystal toward the
applied voltage. This movement of electrons in a semiconductor is referred to as electron current flow.
There is still another type of current in a pure semiconductor. This current occurs when a covalent bond is broken
and a vacancy is left in the atom by the missing valence electron. This vacancy is commonly referred to as a
"hole." The hole is considered to have a positive charge because its atom is deficient by one electron, which
causes the protons to outnumber the electrons. As a result of this hole, a chain reaction begins when a nearby
electron breaks its own covalent bond to fill the hole, leaving another hole. Then another electron breaks its
bond to fill the previous hole, leaving still another hole. Each time an electron in this process fills a hole, it
enters into a covalent bond. Even though an electron has moved from one covalent bond to another, the most
important thing to remember is that the hole is also moving. Therefore, since this process of conduction resembles
the movement of holes rather than electrons, it is termed hole flow (short for hole current flow or conduction by
holes). Hole flow is very similar to electron flow except that the holes move toward a negative potential and in
an opposite direction to that of the electron. Since hole flow results from the breaking of covalent bonds, which
are at the valence band level, the electrons associated with this type of conduction contain only valence band
energy and must remain in the valence band. However, the electrons associated with electron flow have conduction
band energy and can, therefore, move throughout the crystal. A good analogy of hole flow is the movement of a hole
through a tube filled with balls (figure 1-9).
Figure 1-9.—Analogy of hole flow.
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
spaces to the right
and came to rest at the right-hand end of the tube.
In the theory just described, two current carriers
were created by the breaking of covalent bonds: the negative electron and the positive hole. These carriers are
referred to as electron-hole pairs. Since the semiconductor we have been discussing 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
Q15. Name the two types of current flow in a semiconductor.
Q16. What is the name
given to a piece of pure semiconductor material that has an equal number of electrons and holes?
The pure semiconductor mentioned earlier 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, and 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 which are doped in this manner — either with N- or P-type impurities — are referred to as
The N-type impurity loses its
extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity
of the material by contributing a free electron. This type of impurity has 5 valence electrons and is called a
PENTAVALENT impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these
materials give or donate one electron to the doped material, they are also called DONOR impurities.
pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalent bonds with the germanium
atoms. Figure 1-10 illustrates this by showing an arsenic atom (AS) in a germanium (GE) lattice structure. Notice
the arsenic atom in the center of the lattice. It has 5 valence electrons in its outer shell but uses only 4 of
them to form covalent bonds with the germanium atoms, leaving 1 electron relatively free in the crystal structure.
Pure germanium may be converted into an N-type semiconductor by "doping" it with any donor impurity having 5
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.
Figure 1-10.—Germanium crystal doped with arsenic.
The second type of impurity, when added to a semiconductor
material, tends to compensate for its deficiency of 1 valence electron by acquiring an electron from its neighbor.
Impurities of this type have only 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium,
gallium, and boron are trivalent impurities. Because these materials accept 1 electron from the doped material,
they are also called ACCEPTOR impurities.
A trivalent (acceptor) impurity element can also be used to dope
germanium. In this case, the impurity is 1 electron short of the required amount of electrons needed to establish
covalent bonds with 4 neighboring atoms. Thus, in a single covalent bond, there will be only 1 electron instead of
2. This arrangement leaves a hole in that covalent bond. Figure 1-11 illustrates this theory by showing what
happens when germanium is doped with an indium (In) atom. Notice, the indium atom in the figure is 1
electron short of the required amount of electrons needed to form covalent bonds with 4 neighboring
atoms and, therefore, creates a hole in the structure. Gallium and boron, which are also trivalent impurities,
exhibit these same characteristics when added to germanium. The holes can only be present in this type
semiconductor when a trivalent impurity is used. Note that a hole carrier is not created by the removal of an
electron from a neutral atom, but is created when a trivalent impurity enters into covalent bonds with a
tetravalent (4 valence electrons) crystal structure. The holes in this type of semiconductor (P-type) are
considered the MAJORITY carriers since they are present in the material in the greatest quantity. The electrons,
on the other hand, are the MINORITY carriers.
Figure 1-11.—Germanium crystal doped with indium.
Q17. What is the name given to a doped germanium crystal with an excess of free holes?
Q18. What are the majority carriers in an N-type semiconductor?
If we join a section of N-type semiconductor material with a similar section of P-type semiconductor
material, we obtain a device known as a PN JUNCTION. (The area where the N and P regions meet is appropriately
called the junction.) The usual characteristics of this device make it extremely useful in electronics as a diode
rectifier. The diode rectifier or PN junction diode performs the same function as its counterpart in electron
tubes but in a different way. The diode is nothing more than a two-element semiconductor device that makes use of
the rectifying properties of a PN junction to convert alternating current into direct current by permitting
current flow in only one direction. The schematic symbol of a PN junction diode is shown in figure 1-12. The
vertical bar represents the cathode (N-type material) since it is the source of electrons and the arrow represents
the anode. (P-type material) since it is the destination of the electrons. The label "CR1" is an alphanumerical
code used to identify the diode. In this figure, we have only one diode so it is labeled CR1 (crystal rectifier
number one). If there were four diodes shown in the diagram, the last diode would be labeled CR4. The heavy dark
line shows electron flow. Notice it is against the arrow. For further clarification, a pictorial diagram of a PN
junction and an actual semiconductor (one of many types) are also illustrated.
Figure 1-12.—The PN junction diode.
Merely pressing together a section of P material and a section of
N material, however, is not sufficient to produce a rectifying junction. The semiconductor should be in one piece
to form a proper PN junction, but divided into a P-type impurity region and an N-type impurity region. This can be
done in various ways. One way is to mix P-type and N-type impurities into a single crystal during the
manufacturing process. By so doing, a P-region is grown over part of a semiconductor’s length and N- region is
grown over the other part. This is called a GROWN junction and is illustrated in view A of figure 1-13. Another
way to produce a PN junction is to melt one type of impurity into a semiconductor of the opposite type impurity.
For example, a pellet of acceptor impurity is placed on a wafer of N-type germanium and heated. Under controlled
temperature conditions, the acceptor impurity fuses into the wafer to form a P-region within it, as shown in view
B of figure 1-13. This type of junction is known as an ALLOY or FUSED-ALLOY junction, and is one of the most
commonly used junctions. In figure 1-14, a POINT-CONTACT type of construction is shown. It consists of a fine
metal wire, called a cat whisker, that makes contact with a small area on the surface of an N-type semiconductor
as shown in view A of the figure. The PN union is formed in this process by momentarily applying a high-surge
current to the wire and the N-type semiconductor. The heat generated by this current converts the material nearest
the point of contact to a P-type material (view B).
Figure 1-13.—Grown and fused PN junctions from which bars are cut.
Figure 1-14A.—The point-contact type of diode construction.
Figure 1-14B.—The point-contact type of diode construction.
Still another process is to heat a section of semiconductor material to near melting and then diffuse
impurity atoms into a surface layer. Regardless of the process, the objective is to have a perfect bond everywhere
along the union (interface) between P and N materials. Proper contact along the union is important because, as we
will see later, the union (junction or interface) is the rectifying agent in the diode.
Q19. What is the
purpose of a PN junction diode?
Q20. In reference to the schematic symbol for a diode, do electrons flow
toward or away from the arrow?
Q21. What type of PN diode is formed by using a fine metal wire and a section of N-type
PN JUNCTION OPERATION
Now that you are familiar with P-
and N-type materials, how these materials are joined together to form a diode, and the function of the diode, let
us continue our discussion with the operation of the PN junction. But before we can understand how the PN junction
works, we must first consider current flow in the materials that make up the junction and what happens initially
within the junction when these two materials are joined together.
Current Flow in the N-Type
Conduction in the N-type semiconductor, or crystal, is similar to conduction in a copper wire. That is, with
voltage applied across the material, electrons will move through the crystal just as current would flow in a
copper wire. This is shown in figure 1-15. The positive potential of the battery will attract the free electrons
in the crystal. These electrons will leave the crystal and flow into the positive terminal of the battery. As an
electron leaves the crystal, an electron from the negative terminal of the battery will enter the crystal, thus
completing the current path. Therefore, the majority current carriers in the N-type material (electrons) are
repelled by the negative side of the battery and move through the crystal toward the positive side of the battery.
Figure 1-15.—Current flow In the N-type material.
Current Flow in the P-Type Material
Current flow through the P-type material is
illustrated in figure 1-16. Conduction in the P material is by positive holes, instead of negative electrons. A
hole moves from the positive terminal of the P material to the negative terminal. Electrons from the external
circuit enter the negative terminal of the material and fill holes in the vicinity of this terminal. At the
positive terminal, electrons are removed from the covalent bonds, thus creating new holes. This process continues
as the steady stream of holes (hole current) moves toward the negative terminal.
Figure 1-16.—Current flow In the P-type material.
Notice in both N-type and P-type materials, current flow in the external circuit consists of electrons
moving out of the negative terminal of the battery and into the positive terminal of the battery. Hole flow, on
the other hand, only exists within the material itself.
Q22. What are the majority carriers in a P-type
Q23. Conduction in which type of semiconductor material is similar to conduction in a
Although the N-type material has an excess of free
electrons, it is still electrically neutral. This is because the donor atoms in the N material were left with
positive charges after free electrons became available by covalent bonding (the protons outnumbered the
electrons). Therefore, for every free electron in the N material, there is a corresponding positively charge atom
to balance it. The end result is that the N material has an overall charge of zero.
By the same reasoning,
the P-type material is also electrically neutral because the excess of holes in this material is exactly balanced
by the number of electrons. Keep in mind that the holes and electrons are still free to move in the material
because they are only loosely bound to their parent atoms.
It would seem that if we joined the N and P
materials together by one of the processes mentioned earlier, all the holes and electrons would pair up. On the
contrary, this does not happen. Instead the electrons in the N material diffuse (move or spread out) across the
junction into the P material and fill some of the holes. At the same time, the holes in the P material diffuse
across the junction into the N material and are filled by N material electrons. This process, called JUNCTION
RECOMBINATION, reduces the number of free electrons and holes in the vicinity of the junction. Because there is a
depletion, or lack of free electrons and holes in this area, it is known as the DEPLETION REGION.
of an electron from the N-type material created a positive ion in the N material, while the loss of a hole from
the P material created a negative ion in that material. These ions are fixed in place in the crystal lattice
structure and cannot move. Thus, they make up a layer of fixed charges on the two sides of the junction as shown
in figure 1-17. On the N side of the junction, there is a layer of positively charged ions; on the P side of the
junction, there is a layer of negatively charged ions. An electrostatic field, represented by a small battery in
the figure, is established across the junction between the oppositely
charged ions. The diffusion of electrons and holes across the junction will continue until the
magnitude of the electrostatic field is increased to the point where the electrons and holes no longer have enough
energy to overcome it, and are repelled by the negative and positive ions respectively. At this point equilibrium
is established and, for all practical purposes, the movement of carriers across the junction ceases. For this
reason, the electrostatic field created by the positive and negative ions in the depletion region is called a
Figure 1-17.—The PN junction barrier formation.
The action just described occurs almost instantly when the junction is formed. Only the carriers in the
immediate vicinity of the junction are affected. The carriers throughout the remainder of the N and P material are
relatively undisturbed and remain in a balanced condition.
FORWARD BIAS.—An external
voltage applied to a PN junction is call BIAS. If, for example, a battery is used to supply bias to a PN junction
and is connected so that its voltage opposes the junction field, it will reduce the junction barrier and,
therefore, aid current flow through the junction. This type of bias is known as forward bias, and it causes the
junction to offer only minimum resistance to the flow of current.
Forward bias is illustrated in figure
1-18. Notice the positive terminal of the bias battery is connected to the P-type material and the negative
terminal of the battery is connected to the N-type material. The positive potential repels holes toward the
junction where they neutralize some of the negative ions. At the same time the negative potential repels electrons
toward the junction where they neutralize some of the positive ions. Since ions on both sides of the barrier are
being neutralized, the width of the barrier decreases. Thus, the effect of the battery voltage in the forward-bias
direction is to reduce the barrier potential across the junction and to allow majority carriers to cross the
junction. Current flow in the forward-biased PN junction is relatively simple. An electron leaves the negative
terminal of the battery and moves to the terminal of the N-type material. It enters the N material, where it is
the majority carrier and moves to the edge of the junction barrier. Because of forward bias, the barrier offers
less opposition to the electron and it will pass through the depletion region into the P-type material. The
electron loses energy in overcoming the opposition of the junction barrier, and upon entering the P material,
combines with a hole. The hole was produced when an electron was extracted from the P material by the positive
potential of the battery. The created hole moves through the P material toward the junction where it combines with
Introduction to Matter, Energy, and Direct Current, Introduction
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading, Introduction to Generators and Motors,
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies,
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits, Introduction to Wave Propagation, Transmission
Lines, and Antennas, Microwave Principles,
Modulation Principles, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros,
Introduction to Test Equipment, Radio-Frequency
Communications Principles, Radar Principles, The Technician's Handbook,
Master Glossary, Test Methods and Practices, Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics