Indium Phosphide (InP) versus GaAs for Gunn Diode Oscillators
Gunn Diodes have been used to build upper microwave, millimeter-wave, and even terahertz oscillators for a wide range of applications over 40 years. To this day, Gunn Diode oscillators are competitive with monolithic microwave integrated circuit (MMIC) oscillator solutions. Hence, Gunn Diodes have been the premier solid-state coherent power generation device at frequencies above ~10 GHz. Both gallium arsenide (GaAs) and indium phosphide (InP) Gunn Diode oscillators exhibit low phase-noise and adequate output power levels in these frequencies to be well suited for millimeter-wave radar and imaging applications. Gunn Diode devices have been used to realize local oscillators (LOs), voltage-controlled oscillators (VCOs), power amplifiers (PAs), and even power combiners to over 140 GHz.
A Gunn Diode, in general, can be fabricated from semiconductor materials that are formed into multiple closely spaced energy valleys within their conduction band. For instance, the original Gunn Diodes were GaAs, InP, and Cadmium Telluride (CdTe). Since then, additional semiconductor materials have been used to manufacture Gunn Diodes, including gallium nitride (GaN), Cadmium Sulfide (CdS), Indium Arsenide (InAs), Indium Antimonide (InSb), and Zinc Selenide (ZnSe). Of these, GaAs, GaN, and InP are the most commonly used semiconductors for Gunn Diodes.
GaAs and InP Gunn Diodes have historically been the most common, with GaN Gunn Diodes becoming increasingly common for the frequency generation capability well into the terahertz frequencies. Hence, the main competition has been between GaAs and InP Gunn Diodes. Typically, InP Gunn Diodes are considered to yield higher output power at millimeter-wave frequencies with greater efficiency. Moreover, InP Gunn Diodes likely exhibit less amplitude modulation (AM) noise than GaAs Gunn Diodes with comparable performance. On the other hand, InP Gunn Diodes do not support the use of graded-gap hot electron injection techniques, which results in InP Gunn Diodes exhibiting inferior temperature stability compared to GaAs Gunn diodes that support these techniques.
For these reasons, InP Gunn Diode Oscillators have largely replaced GaAs Gunn Diode Oscillators at higher millimeter-wave frequencies, though the cost advantages of GaAs diodes at lower frequencies still exist. Given the wide range of applications Gunn Diode Oscillators are used, there are many other considerations beyond power, frequency, and noise performance to consider which may result in GaAs being a better choice than InP, even at higher millimeter-wave frequencies.
An example of this is using a Gunn Diode Oscillator as a VCO for a frequency modulated continuous wave (FMCW) radar. Though power at high frequency with minimal noise is important, consistent voltage tuning characteristics over a wide temperature range are also critical to prevent the introduction of nonlinearity in the FMCW choir signal. Any non-linearities in the chirp will directly result in reduced radar range resolution. Therefore, a GaAs Gunn Diode Oscillator with graded-bandgap hot electron injection could offer a more temperature stable VCO solution compared to InP.
As in virtually all RF/Microwave applications, finding the best solution for a given application depends on a wide range of parameters that must be carefully considered and weighed beyond the simple text in a datasheet.
Learn more about Pasternack’s Gunn Diode Oscillator offering: Waveguide Gunn Diode Oscillators
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