Gunn Devices

H. Eisele and R. Kamoua, Submillimeter-wave InP Gunn devices, IEEE Transactions on Microwave Theory and Techniques, vol. 52, pp. 2371, 2004. 

Gunn devices belong to a group called transferred electron oscillators and are the ones most often encountered in MMW spectrometry, as they offer the lowest noise figure. They rely on a bulk property of gallium arsenide and indium phosphide when a DC voltage is applied across the end contacts of the n-type material. As the voltage is increased, the current initially increases linearly and then starts to oscillate, with a period closely related to the transit time of the carriers between the contacts across the bulk material. The device is housed in a cavity coupled to a transmission line and is used as a source of MMW radiation, the frequency of which can be tuned mechanically and electronically. 

Figure 3.1 Stylised current vs. voltage curves for a typical Gunn device. The working point would be chosen in the quasi-linear region of negative resistance. The power dissipated is quite large, into a small volume, which is why heat sinking is so important...

An estimate of the highest frequency of oscillation suggests that GaAs Gunn devices would work up to 70 GHz when in a suitable mount. InP exhibits similar properties to GaAs, and an InP Gunn device should work up to about 160 GHz... 

This complex relationship means that if the cavity is held at resonance and the spectral line is swept, e.g. by Stark or Zeeman modulation, although other modulation schemes are possible, the cavity impedance changes and the reflected power incident on the Gunn device changes in sympathy with the spectral scan. This causes a current to flow in the Gunn oscillator circuit related to the spectral absorption profile, and therefore to its amplitude and area. That current can be readily transformer-coupled out of the Gunn bias circuit and detected S)mchro-nously with the modulation frequency. 

Solid state sources are not able presently to achieve the required power output over such a wide band 10% of the centre frequency would represent a typically good -3 dB power bandwidth. For single component measurements it is not necessary to scan the entire waveguide band, the 6-8 GHz FWHM of typical lines at 50-200 GHz, could be adequately covered by a Gunn device. This would be an attractive method for dedicated measurements at atmospheric pressure of flame combustion products in smoke stack and engine exhaust effluents, e.g. oxygen and carbon monoxide whose concentrations at >1 ppm are particularly important. 

Rugg-Gunn, A. J., Maguire, A., Gordon, P. FI., McCabe, J. F., and Stephenson, G. (1998). Comparison of erosion of dental enamel by four drinks using an intra-oral device. Caries Res. 32,337-343. 

This chapter focuses on the properties of deep states in wide band-gap III-V semiconductors. Deep states are important in semiconductors in general and very often determine the recombination properties. Wide band-gap III-V compounds are now commonly used for many important optoelectronic devices (for example, lasers, light-emitting diodes, and Gunn diodes), and are being suggested for additional interesting applications (for example, solar cells). In view of the importance of such work there are a number of earlier... 

The lower-powered microwave signals used by communication transmitters are usually produced by solid-state devices. The Gunn diode is an example. When supplied with voltage from a well-regulated power supply these devices reliably produce a few watts of microwave signal. 

The most commonly encountered MMW sources are now the Gunn and Impatt devices although the Backward Wave Oscillator (BWO) is still used for wideband spectroscopic studies. The Gunn and Impatt devices exhibit the property of negative resistance that makes them well suited as MMW oscillators. 

Figure 3.7 Schematic diagram of a phase-sensitive detector at 20 MHz. The transformer is replaced by an active circuit in the HP8709A synchroniser, and probably most high precision configurations. The phase error voltage output is amplified and used in the spectrometer to control the YIG oscillator magnetic field and hence lock the YIG source frequency to the synthesiser frequency. An identical device locks the Gunn MMW source to the YIG frequency (Adapted from Connor )...

Figure 5.16 A schematic diagram of the current voltage curve for a Gunn diode. The device will not operate in a stable mode anywhere in the negative resistance region as marked. For the load line indicated the circuit has two stable operating points as shown.

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