IMPATT Microwave Diode Tutorial
- the IMPATT diode , IMPact ionisation Avalanche Transit Time microwave diode is used for many microwave RF applications where it is one of the highest power microwave diodes.
The IMPATT diode or to give it the complete name, IMPact ionisation 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 (often ten watts and more) when compared to other forms of microwave diode.
The IMPATT diode has many advantages, although with highly stable frequency synthesizers offering high levels of performance and other semiconductor technologies improving, in recent years it has been used less widely.
IMPATT diodes are ideal where small cost effective microwave radio sources are needed. 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 diodes make excellent signal sources for many RF microwave applications.
Typically the device is used in a number of applications including:
- Detectors using RF technology
Discovery & development
The original idea for the diode was put forward by Shockley in 1954. He thought of the idea of creating negative resistance using a transit time delay mechanism. The method of injection for the carriers was a forward biased PN junction. He published this in the Bell Systems Technical Journal in 1954 in an item entitled: 'Negative resistance arising from transit time in semiconductor diodes.'
However it was not until 1958 that W.T. Read of Bell Laboratories proposed the p+ n i n+ diode structure which was later called the Read diode. This diode used the avalanche multiplication as the injection mechanism. Again this was published in the Bell Systems Technical Journal in 1958 under the title: A proposed high-frequency, negative resistance diode.'
Although the injection mechanism and diode had been postulated, it was not until 1965 that the first practical operating diodes were made that enabled oscillations to be observed. The diode used for this demonstration was fabricated using silicon and had a p+ n structure.
After this, operation of the Read diode was demonstrated and then in 1966 a p i n diode was also demonstrated to work.
In many respects the IMPATT diode is an unusual diode in that it is able to provide high power RF signals at microwave frequencies using a structure that is not that far different from the basic PN junction. However it has been developed to enable its different mode of operation to be utilised properly.
- Theory &operation: The IMPATT diode relies upon a negative resistance effect caused by the transit time of the carriers. This negative resistance enables the diode to act as an oscillator, creating signals at microwave frequencies. Read more about the IMPATT theory & operation
- Fabrication & structure: There are a number of structures and fabrication methods used for IMPATT diodes. Each one has its own advantages and disadvantages. Read more about the IMPATT structure & fabrication
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 microwave 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. It is also used in applications where phase noise performance is unlikely to be of importance.
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 voltages of this order are not always easy to use in some pieces of equipment. Nevertheless IMPATT diodes are particularly attractive option for microwave diodes for many areas.
By Ian Poole
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