Module 11—Microwave Principles
Pages i - ix
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AI-1 to AI-6
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Assignment 1 - 1-8
Assignment 2 - 9-16
Q-53. What limits
the usefulness of high-gain, tunnel-diode frequency converters?
Q-54. The varactor is a PN junction that
acts as what type of electronic device?
Q-55. The underlying principle of operation of the parametric
amplifier is based on what property?
Q-56. What is the most important feature of the parametric
Q-57. How is amplification achieved in the circuit shown in figure 2-43?
Q-58. What is the purpose
of the pump in a parametric amplifier?
Q-59. The pump signal frequency must be of what value when
compared to the input signal of a simple parametric amplifier?
Q-60. What is the primary difference
between the pump signal of a simple parametric amplifier and the pump signal of a nondegenerative parametric
Q-61. In a nondegenerative parametric amplifier the difference between the input frequency and the pump
frequency is called what?
SEMICONDUCTORS are unlike normal pn-junction diodes in both construction and operation. Some types have
no junctions and the processes necessary for operation occur in a solid block of semiconductor material. Other
types have more than one junction but still use bulk-effect action. Bulk-effect devices are among the latest of
developments in the field of microwave semiconductors and new applications are being developed rapidly. They seem
destined to revolutionize the field of high- power, solid-state microwave generation because they can produce much
larger microwave power outputs than any currently available pn-junction semiconductors. Bulk-effect semiconductors
are of two basic types: the transferred-electron devices and the avalanche transit-time devices.
TRANSFERRED-ELECTRON SEMICONDUCTORS.—The discovery that microwaves could be generated by applying a
steady voltage across a chip of n-type gallium-arsenide (GaAs) crystal was made in 1963 by J.B. Gunn. The device
is operated by raising electrons in the crystal to conduction-band energy levels that are higher than the level
they normally occupy. The overall effect is called the transferred-electron effect.
In a gallium-arsenide
semiconductor, empty electron conduction bands exist that are at a higher energy level than the conduction bands
occupied by most of the electrons. Any electrons that do occupy the higher conduction band essentially have no
mobility. If an electric field of sufficient intensity is applied to the semiconductor electrons, they will move
from the low-energy conduction band to the high- energy conduction band and become essentially immobile. The
immobile electrons no longer contribute to the current flow and the applied voltage progressively increases the
rate at which the electrons move from the low band to the high band. As the curve in figure 2-48 shows, the
maximum current rate is reached and begins to decrease even though the applied voltage continues to increase. The
point at which the current on the curve begins to decrease is called the THRESHOLD. This point is the beginning of
the negative-resistance region. Negative resistance is caused by electrons moving to the higher conduction band
and becoming immobile.
Figure 2-48.—Characteristic curve for a bulk-effect semiconductor.
If an increase in voltage is applied to a gallium-arsenide semiconductor, which is biased to operate in
the negative-resistance region, it divides into regions of varying electric fields. A tiny region, known as a
DOMAIN, forms that has an electric field of much greater intensity than the fields in the rest of the
semiconductor. The applied voltage causes the domain to travel across the semiconductor chip from the cathode to
the anode. The high field intensity of the domain is caused by the interaction of the slow electrons in the
high-energy band and the faster electrons in the low-energy band. The electrons in the low-energy band travel
faster than the moving domain and continually catch up during the transit from cathode to anode. When the fast
electrons catch up to the domain, the high field intensity forces them into the higher band where they lose most
of their mobility. This also causes them to fall behind the moving domain. Random scattering causes the electrons
to lose some energy and drop back into the lower, faster, energy band and race again after the moving domain. The
movement from the low-energy band to the high-energy band causes the electrons to bunch up at the back of the
domain and to provide the electron- transfer energy that creates the high field intensity in the domain. The
domains form at or near the cathode and move across the semiconductor to the anode, as shown in figure 2-49. As
the domain disappears at the anode, a new domain forms near the cathode and repeats the process.
Figure 2-49.—Gallium-arsenide semiconductor domain movement.
The GUNN OSCILLATOR is a source of microwave energy that uses the bulk-effect, gallium-
arsenide semiconductor. The basic frequency of a Gunn oscillator is inversely proportional to the transit time of
a domain across the semiconductor. The transit time is proportional to the length of
and to some extent, the voltage applied. Each domain causes a pulse of current at the output; thus, the output is
a frequency determined by the physical length of the semiconductor chip.
The Gunn oscillator can deliver
continuous power up to about 65 milliwatts and pulsed outputs of up to about 200 watts peak. The power output of a
solid chip is limited by the difficulty of removing heat from the small chip. Much higher power outputs have been
achieved using wafers of gallium-arsenide as a single source.
AVALANCHE TRANSIT-TIME DIODES.—Avalanche transit-time diodes, also called IMPATT (Impact
Avalanche and Transit-Time) diodes, are multilayer diodes of several different types used to generate microwave
power. The earliest of the avalanche transit-time diodes consists of four layers in a PNIN arrangement. The
intrinsic (I) layer has neither p nor n properties.
The PN junction for the PNIN diode, shown in figure
2-50, is strongly reverse biased to cause an avalanche in its depletion layer when the positive half cycle of a
microwave signal is applied. The avalanche effect causes the electrons in the n region, which is very thin, to
cross over to the intrinsic layer. The intrinsic layer is constructed so that the drift transit time causes the
current to lag the signal voltage by more than 90 degrees at the desired frequency. Such a lag represents a
negative resistance at the desired frequency. The PNIN avalanche transit-time diode, when inserted in a microwave
cavity with the proper dc bias, amplifies microwave signals introduced to the cavity.
Figure 2-50.—Avalanche transit time for a PNIN diode.
More recent research has shown that pin-junction diodes and simple PN-junction diodes can show negative
resistance and amplification at microwave frequencies when they are reverse biased into an avalanche condition.
The negative resistance in a simple PN-junction or pin diode is the result of a more complicated internal
mechanism than in the PNIN diode. The avalanche region and the drift region of the PNIN diode are physically
separate. Diodes of the PN and pin type must use the same physical region for both avalanche and drift-time
control. In all types of avalanche transit-time diodes, the negative-resistance property causes dc bias energy to
be absorbed by electrons in the avalanche process and given up to the applied microwave field.
What is the output frequency of an upper-sideband parametric-frequency converter?
Q-63. What is the
primary advantage of bulk-effect devices over normal PN-junction semiconductors?
Q-64. What happens to
the electrons of a gallium-arsenide semiconductor when they move from the
normal low-energy conduction band to
the high-energy conduction band?
Q-65. The point on the current curve of a gallium-arsenide
semiconductor at which it begins to exhibit negative resistance is called what?
Q-66. The domain in a
gallium-arsenide semiconductor has what type of electrical field when compared to the other regions across the
body of a semiconductor?
Q-67. What characteristic of a Gunn oscillator is inversely proportional to the
transit time of the domain across the semiconductor?
Q-68. What is the junction arrangement of the original avalanche transit-time diode?
causes dc bias energy to be absorbed by avalanche electrons and given up to the microwave field applied to an
avalanche transit-time diode?
The Point-Contact Diode
DIODES, commonly called CRYSTALS, are the oldest microwave semiconductor devices. They were developed
during World War II for use in microwave receivers and are still in widespread use as receiver mixers and
Unlike the PN-junction diode, the point-contact diode depends on the pressure of contact between a
point and a semiconductor crystal for its operation. Figure 2-51A and B, illustrate a point-contact diode. One
section of the diode consists of a small rectangular crystal of n-type silicon. A fine beryllium-copper,
bronze-phosphor, or tungsten wire called the CATWHISKER presses against the crystal and forms the other part of
the diode. During the manufacture of the point contact diode, a relatively large current is passed from the
catwhisker to the silicon crystal. The result of this large current is the formation of a small region of p-type
material around the crystal in the vicinity of the point contact. Thus, a PN-junction is formed which behaves in
the same way as a normal PN-junction.
Figure 2-51A.—Point-contact diode. DIAGRAM.
Figure 2-51B.—Point-contact diode. P REGION AROUND POINT.
Figure 2-51C.—Point-contact diode. CUT AWAY VIEW.
Figure 2-51D.—Point-contact diode. SCHEMATIC SYMBOL.
The pointed wire is used instead of a flat metal plate to produce a high-intensity electric field at the
point contact without using a large external source voltage. It is not possible to apply large voltages across the
average semiconductor because of the excessive heating.
The end of the catwhisker is one of the terminals
of the diode. It has a low-resistance contact to the external circuit. A flat metal plate on which the crystal is
mounted forms the lower contact of the diode with the external circuit. Both contacts with the external circuit
are low-resistance contacts. The characteristics of the point-contact diode under forward and reverse bias are
somewhat different from those of the junction diode.
With forward bias, the resistance of the point-contact diode is higher than that of the junction
diode. With reverse bias, the current flow through a point-contact diode is not as independent of the voltage
applied to the crystal as it is in the junction diode. The point-contact diode has an advantage over the junction
diode because the capacitance between the catwhisker and the crystal is less than the capacitance between the two
sides of the junction diode. As such, the capacitive reactance existing across the point- contact diode is higher
and the capacitive current that will flow in the circuit at high frequencies is smaller. A cutaway view of the
entire point-contact diode is shown in figure 2-51C. The schematic symbol of a point-contact diode is shown in
Schottky Barrier Diode
The SCHOTTKY BARRIER DIODE is actually a
variation of the point-contact diode in which the metal semiconductor junction is a surface rather than a point
contact. The large contact area, or barrier, between the metal and the semiconductor in the Schottky barrier diode
provides some advantages over the point-contact diode. Lower forward resistance and lower noise generation are the
most important advantages of the Schottky barrier diode. The applications of the Schottky barrier diode are the
same as those of the point-contact diode. The low noise level generated by Schottky diodes makes them especially
suitable as microwave receiver detectors and mixers.
The Schottky barrier diode is sometimes called the
HOT-ELECTRON or HOT-CARRIER DIODE because the electrons flowing from the semiconductor to the metal have a higher
energy level than the electrons in the metal. The effect is the same as it would be if the metal were heated to a
higher temperature than normal. Figure 2-52 is an illustration of the construction of a Schottky barrier diode.
Figure 2-52.—Schottky-barrier diode.
The pin diode consists of two narrow, but highly doped, semiconductor
regions separated by a thicker, lightly-doped material called the intrinsic region. As suggested in the name, pin,
one of the heavily doped regions is p-type material and the other is n-type. The same semiconductor material,
usually silicon, is used for all three areas. Silicon is used most often for its power-handling capability and
because it provides a highly resistive intrinsic (I) region. The pin diode acts as an ordinary diode at
frequencies up to about 100 megahertz, but above this frequency the operational characteristics change.
large intrinsic region increases the transit time of electrons crossing the region. Above 100 megahertz, electrons
begin to accumulate in the intrinsic region. The carrier storage in the intrinsic region causes the diode to stop
acting as a rectifier and begin acting as a variable resistance. The equivalent
circuit of a pin diode at microwave frequencies is shown in figure 2-53A. A resistance versus voltage
characteristic curve is shown in figure 2-53B.
Figure 2-53A.—Diode equivalent circuit (PIN).
Figure 2-53B.—Diode equivalent circuit (PIN).
When the bias on a pin diode is varied, the microwave resistance changes from a typical value of 6
kilohms under negative bias to about 5 ohms when the bias is positive. Thus when the diode is mounted across a
transmission line or waveguide, the loading effect is insignificant while the diode is reverse biased, and the
diode presents no interference to power flow. When the diode is forward biased, the resistance drops to
approximately 5 ohms and most power is reflected. In other words, the diode acts as a switch when mounted in
parallel with a transmission line or waveguide. Several diodes in parallel can switch power in excess of 150
kilowatts peak. The upper power limit is determined by the ability of the diode to dissipate power. The upper
frequency limit is determined by the shunt capacitance of the PN junction, shown as C1 in figure 2-53A. Pin diodes
with upper limit frequencies in excess of 30 gigahertz are available.
Q-70. During the manufacture of a
point-contact diode, what is the purpose of passing a relatively large current from the catwhisker to the silicon
Q-71. What is the capacitive reactance across a point-contact diode as compared to a normal
Q-72. What are the most important advantages of the Schottky barrier diode?
Q-73. At frequencies above 100 megahertz, the intrinsic (i) region causes a pin diode to act as what?
Q-74. The pin diode is primarily used for what purpose?
Transistors, like vacuum tubes, have had a very limited application in the microwave range. Many of
the same problems encountered with vacuum tubes, such as transit-time effects, also limit the upper frequency
range of transistors. However, research in the area of microwave transistors, and especially MICROWAVE INTEGRATED
CIRCUITS (ICs), is proceeding rapidly.
GALLIUM-ARSENIDE FET AMPLIFIERS have been
developed which provide low-noise amplification up to about 30 dB in the 7- to 18-gigahertz range. The power
output of many of these amplifiers is relatively low, approximately 20 to 200 milliwatts, but that is satisfactory
for many microwave applications. Research has extended both the frequency range and the power output of
gallium-arsenide FET amplifiers to frequencies as high as 26.5 gigahertz and power levels in excess of 1 watt in
SILICON BIPOLAR-TRANSISTOR AMPLIFIERS in integrated circuit form
have been developed that provide up to 40 watts peak power in the 1- to 1.5-gigahertz range. Other types of
microwave transistor amplifiers combined into multistage modules are capable of providing power outputs
approaching 100 watts.
Microwave transistor amplifiers, because of their stability, light weight, and long
life, are rapidly replacing microwave tubes in the first stages of high-powered radar and communications
transmitters. In the future new systems will be almost completely solid state.
The information that follows summarizes the important points presented in this chapter. The use of microwave
frequencies forced the development of special tubes to offset the limitations caused by interelectrode
capacitance, lead inductance, and electron transit-time effects in conventional tubes. Microwave tubes, such as
the klystron and TWT, take advantage of transit-time effects through the use of VELOCITY MODULATION to amplify and
generate microwave energy.
The KLYSTRON is a velocity-modulated tube which may be used as an amplifier or
oscillator. The klystron, when used as an amplifier, requires at least two resonant cavities, the buncher and the
catcher. A diagram of a basic klystron is shown at the right.
The REFLEX KLYSTRON, shown at the right, is used only as an oscillator and uses only one cavity to bunch
and collect the electrons. The frequency is determined by the size and shape of the cavity. The reflex klystron
has several possible modes of operation which are determined by electron transit time. Electron transit time is
controlled by the REPELLER voltage.
Introduction to Matter, Energy, and Direct Current,
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,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics