Gunnovy diody?

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vid nizsie...

Vladimír Anděl wrote:

>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
>
>  
>


    Summary of the Gunn Diode

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.

*Construction*
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.

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.

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.

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.

*Operation*
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.

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.

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.

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.

*Tuning*
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.

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.

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.


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