Introduction to Gunn Diode Oscillators

A Gunn diode, though very unlike other common semiconductor diodes, is a transferred electron device (TED) that presents a negative resistance after its threshold voltage is adequately exceeded. The controllable negative resistance properties of a Gunn Diode allows it to be used as a microwave oscillator, and these devices are often used in microwave communications, radar guns, and microwave sensors.

Gunn Diode Oscillators are built with only negatively doped semiconductor regions, two heavily N-doped regions with one lightly N-doped region separating the other two regions. The current/voltage (IV) curve of a Gunn Diode appears linear with increasing current and voltage, until a threshold voltage is reached. At this threshold voltage, the conductive properties of the lightly N-doped middle region change to enhanced resistance, which causes the current to decrease at higher voltages. This function enables microwave amplification, but can also be driven while unstable to produce oscillations when properly biased.

With the proper DC bias, which drives the Gunn Diode into the negative resistance region, when this negative differential resistance matches, and cancels out, the positive differential resistance of the load circuit, a zero differential resistance circuit is created. At this stage spontaneous oscillation will occur, which can be controlled by external circuitry or resonators. The geometries of the Gunn Diode Oscillator also contribute to the electrical behavior of the diode, including oscillating frequencies.

With a resonator, a Gunn Diode Oscillator will generate an oscillating signal at the resonant frequency of the resonator. Some types of resonators can be manipulated to control their resonant frequency, such as cavity resonators and yttrium-iron-garnet (YIG) resonators, and can be used to create tunable oscillators with Gunn Diodes. The type of resonator tuning limits the frequency range at which a Gunn Diode Oscillator can be tuned. The quality of the resonator will also determine the phase noise, frequency stability, and output power of the oscillator.

Gallium arsenide (GaAs) and gallium nitride (GaN) are the most common Gunn Diode materials, where GaAs Gunn Diodes can be used to a couple hundred gigahertz, and GaN Gunn Diodes can hit operating frequencies of a few terahertz. Waveguide Gunn Diode Oscillators, are often designed to take advantage of extremely reliable mechanical tuning methods that leverage precision machined cavity resonators. Given the appropriate material choices and construction, waveguide Gunn Diode Oscillators can achieve good electrical characteristics, high reliability, and stability over a wide temperature range.

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