IMPATT Operation & Theory
- the IMPATT diode is very similar in basic structure to other forms of diode, but as the theory shows, it uses avalanche transit time delays to provide a negative resistance region.
While much of the structure of the IMPATT diode is similar to a standard Schottky or PIN diode in its basic outline, the theory and method of operation are very different.
The diode utilises avalanche breakdown combined with the transit times of the charge carriers to enable it to provide a negative resistance region and therefore act as an oscillator.
As the nature of the avalanche breakdown is very noisy, and signals created by an IMPATT diode have high levels of phase noise.
IMPATT diode theory basics
Like any other diode, an IMPATT has a relatively standard IV characteristic. In the forward direction it will conduct after it has reached the forward conduction point. In the reverse direction it will block current.
However at a certain voltage the diode will break down and current will flow in the reverse direction.
Graphical representation of the IMPATT diode IV characteristic
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.
In terms of its operation the IMPATT diode 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 two types of carrier drift in opposite directions.
Charge carrier movement within an IMPATT diode
IMPATT diode operation
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° 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.
IMPATT diode voltage & current waveforms
The voltage applied to the IMPATT diode has a mean value where it 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° 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.
IMPATT diodes are generally used at frequencies above around 3 GHz. It is found that when a tuned circuit is applied along with a voltage around the breakdown voltage to the IMPATT, and oscillation will occur.
Compared to other devices that use negative resistance and are available for operation at these frequencies, the IMPATT is able to produce much higher levels of power. Typically figures of ten or more watts may be obtained, dependent upon the device.
One of the main drawbacks of the IMPATT diode in its operation is the generation of high levels of phase noise as a result of the avalanche breakdown mechanism. It is found the devices based around Gallium Arsenide technology are much better than those using Silicon. This results from the much closer ionisation coefficients for holes and electrons.
By Ian Poole
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