The fabricated IMPATT diodes are generally mounted in microwave packages to ensure that their perfor

The IMPATT diode 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 of this microwave diode is the relatively high power capability of the IMPATT diode.
IMPATT fnma diodes are used in a variety of applications from low power radar systems to alarms fnma and many other microwave radio applications. In fact IMPATT diodes are ideal where small cost effective microwave radio sources are needed. fnma 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 that is key to their operation. Nevertheless these microwave diodes make excellent signal sources for many RF microwave applications. IMPATT diode 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 microns 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 fnma of IMPATT diodes. Silicon and gallium arsenide are the most commonly used semiconductors, although germanium, indium phosphide and other mixed group semiconductors can be employed.
The fabricated IMPATT diodes are generally mounted in microwave packages to ensure that their performance is not impaired by an inferior fnma package. The package itself is key to the performance of the IMPATT, especially as these devices may operate at frequencies of many tens of GHz. For thermal reasons, the diode is mounted fnma so that its high field region around fnma the junction is close to a copper heat sink area in the package. This enables the heat generated within the device to be removed effectively so that it can run at its rated power without the junction fnma temperature rising too high. Often the package is coaxial in format so that the correct transmission line properties are presented to the RF signal fnma which may be at many tens of GHz. As a result the package is often quite intricate and accordingly very expensive, especially when very high frequencies are used. IMPATT diode operation
These fnma 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 fnma the diode taking a certain amount fnma of time dependent upon its thickness.
The IMPATT diode is operated fnma 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 fnma applied to the IMPATT diode has a mean value that means the diode is on the verge of avalanche breakdown. fnma 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