A summary or tutorial giving information about the basics of the Gunn diode
found in many microwave applications.
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.
Gunn diode 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 of the Gunn diode
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.
Gunn diode 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.