Summary of the IMPATT Diode
The IMPATT or IMPact Avalanche Transit Time diode is an RF semiconductor
device that is used for generating microwave radio frequency signals. With
the ability to operate at frequencies between about 3 and 100 GHz or more,
one of the main advantages is their relatively high power capability.
These diodes are used in a variety of applications from low power radar
systems to alarms. The main drawback of generators using IMPATT diodes is the
high level of phase noise they generate. This results from the statistical
nature of the avalanche process. Nevertheless these diodes make excellent
microwave generators for many applications.
Construction
There is a variety of structures that are used for the IMPATT diode. All are
variations of a basic PN junction and usually there is an instrinsic layer, i.e.
a layer without any doping that is placed between the P type and N type regions.
Typically the N type layer is around one or two micron thick and the intrinsic
layer between 3 and 20 microns. In the very high frequency versions of the
diodes the instrinsic layer will be very much thinner and dimensions of only 0.5
microns are not unknown.
A variety of semiconductor materials are used for the fabrication of these
diodes. Silicon and gallium arsenide are the most commonly used semiconductors,
although germanium, indium phosphide and other mixed group semiconductors can be
employed.
Operation
In terms of its operation the device can be considered to consist of two areas,
namely the avalanche region or injection region, and secondly the drift region.
These two areas provide different functions. The avalanche or injection
region creates the carriers which may be either holes of electrons, and the
drift region is where the carriers move across the diode taking a certain amount
of time dependent upon its thickness.
The IMPATT diode is operated under reverse bias conditions. These are set so
that avalanche breakdown occurs. This occurs in the region very close to the P+
(i.e. heavily doped P region). The electric field at the p-n junction is very
high because the voltage appears across a very narrow gap creating a high
potential gradient. Under these circumstances any carriers are accelerated very
quickly.
As a result they collide with the crystal lattice and free other carriers.
These newly freed carriers are similarly accelerated and collide with the
crystal lattice freeing more carriers. This process gives rise to what is termed
avalanche breakdown as the number of carriers multiplies very quickly. For this
type of breakdown only occurs when a certain voltage is applied to the junction.
Below this the potential does not accelerate the carriers sufficiently.
Once the carriers have been generated the device relies on negative
resistance to generate and sustain an oscillation. The effect does not occur in
the device at DC, but instead, here it is an AC effect that is brought about by
phase differences that are seen at the frequency of operation. When an AC signal
is applied the current peaks are found to be 180 degrees out of phase with the
voltage. This results from two delays which occur in the device: injection
delay, and a transit time delay as the current carriers migrate or drift across
the device.
The voltage applied to the diode has a mean value that means the diode is on
the verge of avalanche breakdown. The voltage varies as a sine wave, but the
generation of carriers does not occur in unison with the voltage variations. It
might be expected that it would occur at the peak voltage. This arises because
the generation of carriers is not only a function of the electric field but also
the number of carriers already in existence.
As the electric field increases so does the number of carriers. Then even
after the field has reached its peak the number of carriers still continues to
grow as a result of the number of carriers already in existence. This continues
until the field falls to below a critical value when the number of carriers
starts to fall. As a result of this effect there is a phase lag so that the
current is about 90 degrees behind the voltage. This is known as the injection
phase delay.
When the electrons move across the N+ region an external current is seen, and
this occurs in peaks, resulting in a repetitive waveform.
Practical operation
The main application for IMPATT diodes is in microwave generators. An
alternating signal is generated simply by applying a DC supply when a suitable
tuned circuit is applied. The output is reliable and relatively high when
compared to other forms of diode. In view of its high levels of phase noise it
is used in transmitters more frequently than as a local oscillator in receivers
where the phase noise performance is generally more important.
To run an IMPATT diode, a relatively high voltage, often as high as 70 volts
or higher may be required. This often limits their application as potentials of
this order are not always easy to use in some pieces of equipment. Nevertheless
they still make a very useful source of microwave energy for many applications.
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