2-53
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
semiconductor material, 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.
