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Module 11 - Microwave Principles |
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Module 11 - Microwave Principles
Pages 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 1−50, 1−51 to 1−60, 1−61 to 1−68, 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 2−60, 2−61 to 2−66, 3−1 to 3−10, 3−11 to 3−20, AI−1 to AI−6, Index−1 to Index−2, Assignment 1 - 1−8, Assignment 2 - 9−16 BULK-EFFECT 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. 2-51
Figure 2-48. - Characteristic curve for a bulk-effect semiconductor. 2-52
Figure 2-49. - Gallium-arsenide semiconductor domain movement. 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. 2-53
Figure 2-50. - Avalanche transit time for a PNIN diode. Q-63. What is the
primary advantage of bulk-effect devices over normal PN-junction semiconductors? POINT-CONTACT 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 detectors. 2-54 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.
2-55
Figure 2-51C. - Point-contact diode. CUT AWAY VIEW.
Figure 2-51D. - Point-contact diode. SCHEMATIC SYMBOL. 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. 2-56 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 figure 2-51D. 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. 2-57 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). 2-58 Q-73. At frequencies above 100 megahertz, the intrinsic (i) region causes a pin diode to act as what?
Microwave Transistors
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 multistage amplifiers. 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. SUMMARY 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. 2-59
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. 2-60 NEETS Table of Contents
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