IMPATT diode

An IMPATT diode (IMPact ionization Avalanche Transit-Time) is a form of high power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields.

They operate at frequencies between about 3 and 100 GHz or more. A main advantage is their high power capability. These diodes are used in a variety of applications from low power radar systems to alarms. A major drawback of using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process. Nevertheless these diodes make excellent microwave generators for many applications.

Device structure

The IMPATT diode family includes many different junctions and metal semiconductor devices. The first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Because of the strong dependence of the ionization coefficient on the electric field, most of the electron–hole pairs are generated in the high field region. The generated electron immediately moves into the region, while the generated holes drift across the p region. The time required for the hole to reach the contact constitutes the transit time delay.

The original proposal for a microwave device of the IMPATT type was made by Read and involved a structure. The Read diode consists of two regions as illustrated in figure. (i) The Avalanche region (p1 – region with relatively high doping and high field, ), in which avalanche multiplication occurs and (ii) the drift region (p2 – region with essentially intrinsic doping and constant field, , in which the generated holes drift towards the - contact. Of course, a similar device can be built with the configuration, in which electrons generated from the avalanche multiplication drift through the intrinsic region.

A fabricated IMPATT diode generally is mounted in a micro wave package. The diode is mounted with its high – field region close to the Copper heat sink so that the heat generated at the junction can be conducted away readily. Similar microwave packages are used to house other microwave devices.

Principle of operation

At breakdown, the n – region is punched through and forms the avalanche region of the diode. The high resistivity i – region is the drift zone through which the avalanche generated electrons move toward the anode.

Now consider a dc bias V_B, just short of that required to cause breakdown, applied to the diode in the figure. Let an ac voltage of sufficiently large magnitude be superimposed on the dc bias, such that during the positive cycle of the ac voltage, the diode is driven deep into the avalanche breakdown. At t=0, the ac voltage is zero, and only a small pre-breakdown current flows through the diode. As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairs are produced by impact ionization. As long as the field in the avalanche region is maintain above the breakdown field , the electron-hole concentration grow exponentially with t. Similarly this concentration decay exponentially with time when the field is reduced below at the negative swing of the ac voltage. The holes generated in the avalanche region disappear in the p+ region and are collected by the cathode. The electrons are injected into the i – zone where they drift toward the n+ region. Then, the field in the avalanche region reaches its maximum value and the population of the electron-hole pairs starts building up. At this time, the ionization coefficients have their maximum values. Although follow the electric field instantaneously the generated electron concentration does not because it also depends on the number of electron-hole pairs already present in the avalanche region. Hence, the electron concentration at will have a small value. Even after the field has passed its maximum value, the electron-hole concentration continues to grow because the secondary carrier generation rate still remains above its average value. For this reason, the electron concentration in the avalanche region attains its maximum value at , when the field has dropped to its average value. Thus, it is clear that the avalanche region introduces a 90^o phase shift between the ac signal and the electron concentration in this region.

With a further increase in t, the ac voltage becomes negative, and the field in the avalanche region drops below its critical value. The electrons in the avalanche region are then injected into the drift zone which induces a current in the external circuit which has a phase opposite to that of the ac voltage. The ac field, therefore, absorbs energy from the drifting electrons as they are decelerated by the decreasing field. It is clear that an ideal phase shift between the diode current and the ac signal is achieved if the thickness of the drift zone is such that the bunch of electron is collected at the n⁺ - anode at when the ac voltage goes to zero. This condition is achieved by making the length of the drift region equal to the wavelength of the signal. This situation produces an additional phase shift of 90^o between the ac voltage and the diode current. The waveforms of the ac voltage, they injected electron charge, and the current induced in the external circuit.

Further reading

* D. Christiansen, C.K. Alexander, and R.K. Jurgen (eds.) "Standard Handbook of Electronic Engineering" (5th edition). McGraw Hill. p. 11.107-11.110 (2005). ISBN 0-07-138421-9.
* M.-S. Gupta,Gandhe,Ankur: "Large-Signal Equivalent Circuit for IMPATT-Diode Characterization and Its Application to Amplifiers". 689-694 (Nov 1973). Microwave Theory and Techniques. IEEE Transactions Volume: 21. Issue: 11. ISSN 0018-9480
*R. L. Jonston, B. C. DeLoach Jr., and B. G. Cohen: "A Silicon Diode Oscillator". Bell Systems Technical Journal. 44, 369 (1965)
*H. Komizo, Y. Ito, H. Ashida, M. Shinoda: "A 0.5-W CW IMPATT diode amplifier for high-capacity 11-GHz FM radio-relay equipment". 14-20 (Feb 1973). IEEE Journal Volume: 8. Issue: 1. ISSN 0018-9200
* W. T . Read, Jr., "A proposed high-frequency, negative-resistance diode," Bell Systems Technical Journal, vol.: 7, pp. 401-446, March 1958.
* S. M. Sze: "Physics of Semiconductor Devices". second edition. John Wiley & Sons. 566-636 (1981). ISBN 0-471-05661-8
* M. S. Tyagi: "Introduction to Semiconductor Materials and Devices". John Wiley & Sons. 311-320 (1991). ISBN 0-471-60560-3