What are Gunn Diode Oscillators?

The “Gunn diode” is the semiconductor used in the Gunn oscillator. The Gunn diode, also referred to as a “Transferred Electron Device,” was invented by J. B. Gunn in 1963. It is a two-terminal “negative” resistance device. Gunn oscillators have been playing a unique role in replacing the tube to generate low to medium level microwave power in the frequency range of 2 to 140 GHz. This became especially true in millimeter wave frequencies, long before the inception of pHEMT and other more advanced microwave three-terminal devices reached maturity and became more practical.

Engineers today may have limited opportunities to get to know more about Gunn oscillators, because they are an old technology. There are numerous other three-terminal based devices such as MOSFET, pHEMT, and HBT. The oscillators based on these devices are playing the dominate role as signal generators in the microwave industry for various system applications. This is especially true in the microwave frequency domain all the way up to 40 GHz. There are three main advantages of using these three-terminal based devices: high power-added-efficiency (PAE), ease-of-system-integration (ESI), and friendly-surface-mount volume manufacturing.

However, when higher millimeter wave signal generation is required for lab use, system concept approval, and prototyping and production test sets, they become cumbersome and expensive. They often involve the use of expensive synthesizer or DRO/PLO, filters, frequency multipliers, and amplifiers. Alternatively, the Gunn oscillator utilizes a single Gunn diode and a waveguide cavity which offers a simple circuit solution. Therefore, Gunn oscillators still play an important role in providing high-performance solutions to both microwave and millimeter wave system applications. Furthermore they are small, lightweight, display less harmonic contents, and have a substantially lower cost compared to three-terminal based signal generators.

Mechanically Tuned Gunn Oscillators and Varactor tuned Gunn Oscillators both use GaAs and InP compound material Gunn diodes. Even though InP Gunn diodes offer an inherently higher operation frequency, the manufacturer discontinued the product line a decade ago due to a lack of business. Therefore, Gunn oscillators generally offered with GaAs Gunn diodes, but some InP Gunn diode based oscillators exist, often for high frequency applications.

The fundamental operation frequency limit for GaAs diode based oscillators is around 65 GHz, but for InP it is up to 140 GHz. The standard cavity configurations for the fundamental operations are front-iris-coupled and half-wavelength-backshort defined. The front-iris-coupled oscillator exhibits less load pull effect and good frequency stability, but the operation frequency is limited to 40 GHz or below due to its mechanical realization limits. This type of oscillator however can reach upper millimeter wave frequencies by combining it with a passive diode multiplier. The half-wavelength-backshort defined oscillators can reach 65 GHz, but these types of oscillators show poor frequency stability and high load pull effect. Therefore, adding an isolator is highly recommended for reducing the load pull effects and a heater for improving the frequency stability for this type of oscillator.

The second harmonic Gunn Oscillator is widely used for high millimeter wave frequency applications and offer incomparable features against its counterpart, the fundamental oscillator. The oscillation is established within its cavity, below the cut-off frequency of the waveguide, therefore the oscillator has an inherently high isolation between the oscillator and the load. This makes the isolator unnecessary while performing system integration. Second harmonic Gunn Oscillators are mainly offered in the frequency range of 60 to 140 GHz.

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A Gunn Diode oscillator is simply an oscillator built around a Gunn diode device. A Gunn diode is a type of diode that uses two negatively doped regions with a slightly less negatively doped region in between. This construction presents a negative resistance over a certain threshold voltage, and behaves as a transferred electron device (TED). With a negative resistance, instability and oscillations can readily occur.

As Gunn Diodes can be built using semiconductors with very high electron mobility and frequency response, terahertz oscillators have been built using this technology. Gallium Arsenide (GaAs) and gallium nitride (GaN) semiconductors are commonly used to make Gunn diodes that operate into the gigahertz to terahertz. Gunn diode oscillators are known for being able to produce extremely high energy levels at high frequencies. Hence, their common use in microwave, millimeter-wave, and terahertz systems.

There are several commercially available implementations of Gunn Diode oscillators. These include waveguide, microwave cavity, and yttrium-iron-garnet (YIG) sphere Gunn Diode oscillators. The frequency of oscillation of a Gunn Diode-based oscillator is controlled by a resonant structure. For waveguide and microwave cavity Gunn diode oscillators, the construction geometries and cavities of the waveguide and microwave cavity determine the resonant frequencies. For a YIG sphere-based oscillator, the magnetically controlled resonant frequency of the YIG sphere determines the resonant frequency.