2-54
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.
Q-62. 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?
Q-69. What 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
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.
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