Other diodes: Diode types
The IMPATT diode or IMPact ionisation Avalanche Transit Time diode is an RF semiconductor device that is used for generating microwave radio frequency signals.
The IMPATT diode technology is able to generate signals typically from 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) which is much higher than many other forms of microwave diode.
Although the IMPATT diode is not as widely used these days as other technologies have been able to provide higher levels of performance, it nevertheless fits a niche in the microwave signal generation market, especially where relatively cost effective sources are needed.
IMPATT discovery & development
The original idea for IMPATT diode technology was proposed by William Shockley in 1954. He developed the concept for creating a negative resistance using a transit time delay mechanism. In his proposal, the method of injection for the carriers was a forward biased PN junction.
Shockley published his idea in the Bell Systems Technical Journal in 1954 in a paper entitled: 'Negative resistance arising from transit time in semiconductor diodes.'
The idea was not developed further until 1958 when W.T. Read of Bell Laboratories proposed his P+ N I N+ diode structure which was later called the Read diode. This diode used avalanche multiplication as the injection mechanism. Again a paper was published in the Bell Systems Technical Journal, this time in 1958 under the title: A proposed high-frequency, negative resistance diode.'
Although the injection mechanism and diode had been postulated, it was not possible to realise the diode at first. It took until 1965 before the first practical operating diodes were made and the first oscillations were 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 PIN diode was also demonstrated to work.
IMPATT diode basics
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 a totally different mode of operation to be utilised.
- Theory & operation: The IMPATT microwave diode relies upon a negative resistance effect caused by the transit time of the carriers. When a negative resistance occurs, reducing the voltage increases the current and vice versa. This negative resistance enables the diode to act as an oscillator, creating signals at microwave frequencies. Read more about the How does an IMPATT diode work?
- 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
IMPATT diode operation
The IMPATT diode is generally used in microwave generators. To create an output, a DC supply applied to the IMPATT which oscillates when a suitable tuned circuit is in circuit.
The IMPATT circuit output is reliable and relatively high when compared to other forms of microwave diode. However it also generates high levels of phase noise as a result of its operation and this means that it is used in simple transmitters more frequently than as a local oscillator in receivers where the phase noise performance is generally more important. The phase noise issue also means that it has to be used in relatively undemanding applications where the phase noise performance is unlikely to be of importance.
The IMPATT diode requires a relatively high voltage for its operation. Often this may be as high as 70 volts or possibly more. Along with the phase noise, this can limit the applications in which the diode can be used. Nevertheless IMPATT diodes are particularly attractive option for microwave diodes for many areas.
IMPATT diode applications
IMPATT diodes are used in a number of applications where a compact cost effective ,means of generating microwave power is required. They have the drawback that on their own they are free running, and also in view of the way in which they operate, they generate relatively high levels of phase noise. Nevertheless there are several applications for which IMPATT diode technology is applicable.
- Intruder alarms
- Basic forms of radar
- General detectors using RF technology
There are many areas in which the IMPATT diode can be used, but with other alternatives now becoming available, along with its high phase noise and the high voltage needed, its use has declined. Nevertheless it is still a form of diode that can be useful in some applications.