How does an IMPATT Diode Work

IMPATT Diode Tutorial Includes:
IMPATT diode     How does an IMPATT diode work     IMPATT diode structure     TRAPATT diode     BARITT diode    

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The structure of an IMPATT diode is very similar to a standard Schottky or PIN diode but when looking at how an IMPATT diode works, it can be seen to be very different.

The IMPATT microwave diode uses avalanche breakdown combined and the charge carrier transit time to create a negative resistance region which enables it to 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

The IMPATT diode has a very similar I-V characteristic to any other form of PN junction diode. It conducts in the forward direction once the turn on voltage has been reached. In the reverse direction it blocks current flow, until the diode breakdown voltage is reached. It this point avalanche breakdown occurs and current flows in the reverse direction.

For its operation as a microwave signal generator, IMPATT diode is operated under reverse bias conditions. These are set so that avalanche breakdown occurs.

Breakdown occurs in the region very close to the P+ (i.e. heavily doped P region). The electric field at the PN 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.

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.

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 diode circuits

IMPATT diodes are generally used at frequencies above about 3 GHz or more. 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.

The IMPATT diode is driven from a supply through a current limiting resistor. The value of this limits the current to the value required. The current is passed through an RF choke to isolate the DC from the radio frequency signal. The IMPATT microwave diode is placed across the tuned circuit. Typically the diode may be mounted in a waveguide cavity that provides the required tuned circuit. Once the supply voltage is applied the circuit will oscillate.

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.

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