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vid nizsie...<br>
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Vladimír Anděl wrote:
<blockquote cite="mid003a01c5f6a4$d8d2a4c0$0200a8c0@pocitac1"
type="cite">
<pre wrap="">Jen bych se zeptal jako totalni neznalec: Na jakem principu pracuje Gunnova
dioda? Je to snad jeste neco jineho nez tunelova dioda? Pokud si ze skoly
jeste pamatuju (s mikrovlnama nedelam) tak tunelova dioda by mela byt
Esakiho dioda. Nebo ne?
Andel
</pre>
</blockquote>
<br>
<br>
<h2>Summary of the Gunn Diode</h2>
<p>Gunn diodes
electronics semiconductor diodes that form a cheap and easy method of
producing relatively low power radio signals at microwave frequencies.
Gunn diodes are a form of semiconductor component able to operate at
frequencies from a few Gigahertz up to frequencies in the region of 100
GHz. As such they are used in a wide variety of units requiring low
power RF signals.</p>
<p><strong>Construction</strong><br>
Gunn diodes are fabricated from a single piece of n-type silicon.
Within the device there are three main areas, which can be roughly
termed the top, middle and bottom areas.</p>
<p>The top and bottom areas of the device are heavily doped to give N+
material. This provides the required high conductivity areas that are
needed for the connections to the device. The device is mounted on a
conducting base to which a wire connection is made. It also acts as a
heat-sink for the heat which is generated. The connection to the other
terminal of the diode is made via a gold connection deposited onto the
top surface. Gold is required because of its relative stability and
high conductivity.</p>
<p>The centre area of the device is the active region. It normally
around ten microns thick its thickness will vary because this is one of
the major frequency determining elements. This region is also less
heavily doped and this means that virtually all the voltage placed
across the device appears across this region.</p>
<p>In view of the fact that the device consists only of n type material
there is no p-n junction and in fact it is not a true diode, and it
operates on totally different principles.</p>
<p><strong>Operation</strong><br>
The operation of the Gunn diode can be explained in basic terms. When a
voltage is placed across the device, most of the voltage appears across
the inner active region. As this is particularly thin this means that
the voltage gradient that exists in this region is exceedingly high.</p>
<p>It is found that when the voltage across the active region reaches a
certain point a current is initiated and travels across the active
region. During the time when the current pulse is moving across the
active region the potential gradient falls preventing any further
pulses from forming. Only when the pulse has reached the far side of
the active region will the potential gradient rise, allowing the next
pulse to be created.</p>
<p>It can be seen that the time taken for the current pulse to traverse
the active region largely determines the rate at which current pulses
are generated, and hence it determines the frequency of operation.</p>
<p>A clue to the reason for this unusual action can be seen if the
voltage and current curves are plotted for a normal diode and a Gunn
diode. For a normal diode the current increases with voltage, although
the relationship is not linear. On the other hand the current for a
Gunn diode starts to increase, and once a certain voltage has been
reached, it starts to fall before rising again. The region where it
falls is known as a negative resistance region, and this is the reason
why it oscillates.</p>
<p><strong>Tuning</strong><br>
The frequency of the signal generated by a Gunn diode is chiefly set by
the thickness of the active region. However it is possible to alter it
somewhat. Often Gunn diodes are mounted in a waveguide and the whole
assembly forms a resonant circuit. As a result there are a number of
ways in which the resonat frequency of the assembly can be altered.
Mechanical adjustments can be made by placing an adjusting screw into
the waveguide cavity and these are used to give a crude measure of
tuning.</p>
<p>However some form of electrical tuning is normally required as well.
It is possible to couple a varactor diode into the Gunn oscillator
circuit, but changing the voltage on the varactor, and hence its
capacitance, the frequency of the Gunn assembly can be trimmed.</p>
<p>A more effective tuning scheme can be implemented using what is
termed a YIG. It gains its name from the fact that it contains a
ferromagnetic material called Yttrium Iron Garnet. The Gunn diode is
placed into the cavity along with the YIG which has the effect of
reducing the effective size of the cavity. This is achieved by placing
a coil outside the waveguide. When a current is passed through the coil
it has the effect of increasing the magnetic volume of the YIG and
hence reducing the electrical size of the cavity. In turn this
increases the frequency of operation. This form of tuning, although
more expensive, produces much lower levels of phase noise, and the
frequency can be varied by a much greater degree.</p>
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