The Gunn Diode Oscillator Lives On…and Prospers

by Pasternack

For most designers, GaAs, GaN, and other semiconductor technologies represent the future of both commercial and defense applications, while the Gunn diode is a relic best suited for mundane applications such as automatic door openers. However, 60 years after it was conceived, the market for Gunn diodes and oscillators continues to grow by about 5% per year, and as remote sensing and other applications operating at hundreds of gigahertz are finally being developed, the ability of Gunn diode oscillators based on GaAs or indium phosphide (InP) to deliver RF power far into the milimeterWave region is very appealing. So, it’s a good time to reexamine the Gunn diode, the interesting story of how it was developed, and why it will remain a viable technology in the future.

Credit for the Gunn diode and the “Gunn effect” goes primarily (although not exclusively) to John Battiscombe Gunn, who died in 2008 (Figure 1). Gunn never liked his birth name so early in his life he chose not to use it, instead preferring to be called Ian or Iain, the Scottish name for John. Regardless, he is best known as J.B., providing a hint into his eccentricities as well as those of some of his family.

Gunn was a British citizen born in Cairo, Egypt, while his archeologist father was on a dig. His mother, Lillian Florence (Meena) Meacham Hughes Gunn, was a psychoanalyst who studied under Sigmund Freud, and as a member of the clique centered around George Bernard Shaw and H.G. Wells, she dabbled in the occult.

His father Battiscombe was a leading Egyptologist who was curator of the Egyptian Section of the museum at the University of Pennsylvania and later professor of Egyptology at Oxford University. As an interpreter of ancient languages, he translated inscriptions of some of Egypt’s most important excavations. His brother, Patrick “Spike” Hughes, is considered England’s earliest jazz composer, and his aunt Wendy Wells was a leading campaigner for Scottish independence. His grandfather, Samuel Peploe Wood, was an English sculptor and painter whose works are still on display throughout England.

The family moved from England to Glen Riddle, PA, when Battiscombe accepted the position at the university, but three years later moved back to England when his father took the position at Oxford. As a side note, the Pennsylvania home Gunn grew up in was bought by Ken Iverson, the inventor of the APL programming language and among other accolades, received the Turing Award in 1979.

According to his brother Patrick, Gunn’s interest in “how things worked” was evident at age four when he insisted on taking apart the family’s radio. After an education at Trinity College, Cambridge and passing through several jobs, he worked for the Royal Radar Establishment and then returned to the U.S. to take a position at the IBM Thomas J. Watson Research Center in Yorktown, NY, where he spent the rest of his career.

In the 1960s, there was considerable attention to two-terminal devices that at the time could deliver more RF power at higher frequencies than transistors. The core characteristic of two-terminal solid-state devices is their inherent negative resistance. That is, the real part of their impedance is negative over a range of frequencies. Passive devices with resistances absorb power, while active devices with negative resistances generate it, and when certain conditions are met, they can increase it. As will (hopefully) become clear, this is a very important characteristic.

A few years before Gunn created the effect and diode that bear his name, British solid-state physicists Brian Ridley, Tom Watkins, and Cyril Hilsum discovered a mechanism by which differential negative resistance can be created in certain bulk compound semiconductor materials such as GaAs and indium phosphide (InP) when a voltage is applied to terminals on the material.

Devices like this are called bulk devices because microwave amplification and oscillation are derived from the bulk negative-resistance property of semiconductors rather than from the junction negative resistance between two different semiconductors, which is the case with the tunnel diode, among others.

The effect, appropriately called the Ridley–Watkins–Hilsum theory (RWH), defines the transfer of conduction electrons in a semiconductor from a high mobility energy “valley” to lower mobility, higher energy valleys. The band structure of a solid describes the range of energy levels that electrons may have  within it and the ranges of energy it may not have, which are called band gaps. Only some compound semiconductor materials satisfy these requirements, and GaAs and InP are among them.

As part of this work, Gunn and other solid-state physicists focused on these compound semiconductors to learn how their properties could best be used, and while exposing thin disks of n-type GaAs and n-type InP GaAs to pulsed high electric fields, he found it perplexing that they emitted oscillations that could not be defined as noise. This led to his discovery that when the applied voltage to the material exceeded a certain critical (threshold) value, the current passing through n-type bulk GaAs would fluctuate. That is, in a “normal” diode, current increases with voltage, but the current for a Gunn diode (Figure 2) starts to increase to the point at which once the threshold voltage is reached, it begins once again to fall, but then rises again. This is the negative resistance region and explains why a Gunn diode oscillates.

While Gunn was the first to discover microwave oscillation in bulk GaAs samples with ohmic contacts, a year later physicist Herbert Kroemer noted that Gunn’s observations (and thus the Gunn effect) could be explained by the RWH theory, effectively making them more or less the same. Kroemer would later win the Nobel Prize and the IEEE Medal of Honor and created the heterojunction bipolar transistor.

In short, while Gunn discovered the effect that bears his name, Kroemer was the first to explain it fully, and others such as physicist John A. Copeland later verified that the microwave oscillation observed by Gunn was caused by the negative resistance property of the material. Negative differential resistance the timing properties of the intermediate layer of the material are responsible for the Gunn diode’s ability to become an oscillator.

Although the oscillation frequency is determined in part by its middle layer, the diode can be controlled externally by using a resonator, typically a waveguide or microwave cavity in which the diode is mounted (Figure 3), as well as a yttrium iron garnet (YIG) sphere. In the case of the resonator, the diode cancels the loss resistance of the resonator, producing only oscillations at its resonant frequency. With the cavity, the frequency is changed by adjusting its size, and with a YIG sphere by changing its magnetic field.

Ahead of its Time

It’s important to note at the time these research efforts were taking place, only vacuum tubes were capable of producing RF power in the microwave region, so the ability to achieve this with a semiconductor was a major breakthrough. Of course, unlike vacuum tubes, Gunn diode oscillators produce only a few watts of RF power at best, but when connected to a horn antenna that produces forward gain of 30 dBi, the result is a significant amount of radiated power.

In addition, these oscillators were easy to build and could operate from batteries at low voltages. By applying a DC voltage to bias the diode, its negative differential resistance cancels the positive resistance of the load, creating a circuit with no differential resistance that spontaneously produces oscillations. It didn’t take long for the Gunn diode oscillator to find its way into a wide variety of systems, from airborne collision avoidance radar to pedestrian safety systems, motion detectors, radar detectors, automatic door openers and traffic gates, as well as burglar alarms and as sensors to prevent to train derailment, vibration, rotational speed, and moisture content, among others.

For radio amateurs, the Gunn diode was a dream come true, as it allowed experimentation at frequencies never possible before at a relatively low cost. However, most amateurs who wanted to communicate at 10 GHz and above had to scavenge their own parts and construct a transmitter and receiver from scratch. To make that easier, the Gunn diode-based transceiver, or Gunnplexer (Figure 4), was born.

The term Gunnplexer was coined by Jim Fisk, the editor of Ham Radio magazine and previously editor of 73 magazine. As he explained it, while writing the editorial for the March 1977 issue of Ham Radio that announced the availability of commercial Gunn diode transceivers, he used the term Gunnplexer to describe them. When Microwave Associates CEO Dana Atchley read it, he contacted Fisk and asked if the company could trademark it, after which Microwave Associates (now MACOM) began to make them. Since then, hams have created hundreds of variations on the theme, many of which are available on the Web.

At a minimum, the Gunnplexer combines a Gunn diode oscillator with a Schottky diode mixer, and the entire system is embedded in a cavity that connects to a waveguide horn antenna. An audio amplifier, microphone, and a wideband FM receiver complete the transmit/receive package. More details about Gunnplexers can be found in “For Further Reading” at the end of this article.

A Reemergence?

The Gunn diode oscillator has a storied history that has been overshadowed by advanced compound semiconductors such a GaAs and GaN, but its original primary benefit still holds: the ability to generate RF power at extremely high frequencies. Researchers have demonstrated that Gunn diode-based oscillators can produce RF energy to at least 1 THz and perhaps higher. This makes them an excellent candidate for use in a variety of terahertz imaging applications.

Terahertz imaging is a nondestructive evaluation technique that is being used in a growing number of applications in the pharmaceutical, biomedical, security, materials characterization, and aerospace industries, and has demonstrated its effectiveness for inspection of layers in paint and coatings, detecting structural defects in ceramic and composite materials, and imaging the physical structure of paintings and manuscripts. Other uses include biomedical diagnostics, semiconductor device diagnostics, trace gas analysis, moisture analysis, inter-satellite communications, and tissue burn reflectometry.

GaAs or InP-based Gunn diode technology currently allows for frequency-multiplied sources to operate up to 1 THz. Although output power decreases rapidly with frequency, it’s likely that future devices will improve in performance and operate at even higher frequencies that in imaging achieve higher spatial resolutions.

Negative-differential-resistance GaN is also being explored for use in high-frequency imaging as it has the potential to produce higher RF power than GaAs or InP because of its well-known properties such as its large bandgap, high breakdown voltage, and high-power density. At the moment, researchers are working to improve GaN epitaxial growth and device fabrication for these higher frequencies, and the problem of heat dissipation as the threshold electric field effect is nearly 50 times that of GaAs.

Summary

The Gunn diode oscillator proved its worth more than a half century ago, and even while overshadowed by more “modern” semiconductor technologies, there seems to be no end to what this technology can achieve. There is little reason to doubt that this will continue in the future, as its ability to produce RF power at extremely high frequencies opens a world of possibilities for applications that are just now emerging.

For Further Reading

1. Thermal Modeling of the GaN-based Gunn Diode at Terahertz Frequencies, Ying Wang, Jinping Ao, Shibin Liu, Yue Hao, Applied Sciences, December 2018.

2. Electrical 4U, Gunn Diode Oscillator: What is it? (Theory & Working Principle), October 27, 2020.

3. Gunnplexers, Tom Williams, WA1MBA, QST, March 2002. http://www.arrl.org/files/file/Technology/microwave/0203096.pdf

4. The Microwave Associates 10 GHz Transceiver, Microwave-museum.org.

5. Fabrication and characterization of planar Gunn diodes for Monolithic Microwave Integrated Circuits, Simone Montanari, Forschungszentrum Jülich GmbH, / Information Technology Band, Volume 9, 2005.

6. Terahertz Oscillations in an InGaS Submicron Planar Gunn Diode, Ata Khalid, et al., University of Glasgow, Journal of Applied Physics, American Institute of Physics, April 2014.

7. Advanced physical modeling of step graded Gunn Diode for high power terahertz sources, Thesis submission, Amir Faisal, School of Electrical and Electronic Engineering, University of Manchester, 2001.

8. Gunn Diode: microwave diode tutorial, electronic notes, https://www.electronics-notes.com/articles/electronic_components/diode/gunn-microwave-diode.php

9. Gunn Diode: Working, Characteristics & Applications, Electronics Projects Focus, https://www.elprocus.com/gunn-diode-working-characteristics-and-its-applications/

10. Gunnplexers & Microwaves, Physics Open Lab, March 2016. https://physicsopenlab.org/2016/03/10/gunnplexer-microwaves/

11. A Communications System Using Gunnplexer Transceivers, John V. Bellantoni and Steven P. Powell, communications Quarterly,

12. MA-87127 Series, Varactor Tuned Gunnplexer Transceiver “Front end” for Amateur Applications, Microwave associates, Bulletin 7624A, April 1977.

13. The Gunnplexer Cookbook: A Microwave Primer for Radio Amateurs and Students, Robert M. Richardson, Ham Radio Publishing Group, 1981.

https://www.n5dux.com/ham/files/pdf/Gunnplexer%20Cookbook.pdf

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