2-51
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 amplifier?
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 amplifier?
Q-61. In a nondegenerative parametric amplifier the difference between the input frequency and the
pump frequency is called what?
Bulk-Effect Semiconductors
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.
