Gunn Devices Research Articles

Factors Limiting the Signal-to-Noise Ratio of Negative-Conductance Amplifiers and Oscillators in FM/FDM Communications Systems (Short Papers)

A derivation is presented for the signal-to-noise ratio of negative-conductance amplifiers and oscillators in FM/frequency division multiplexing (FDM) communications applications. Results indicate the limiting value of signal-to-noise ratio depends on the semiconductor properties and channel loading only. This means circuit adjustments, such as Q, cannot increase the signal-to-noise ratio without bounds. Typical specifications are given. Limiting values of signal-to-noise ratio for Gunn and Si IMPATT devices are given in typical applications. Results indicate that Gunn devices have a clear advantage over Si IMPATT'S in a signal-to-noise sense.

Pulsed Gunn-diode oscillator: 40 W at 16 GHz

Very high peak power levels at microwave frequencies can be generated from several semiconductor devices (Gunn diodes, l.s.a. mode devices, TRAPATTs, etc.), but some of these devices are only a little removed from laboratory devices, and are limited in frequency. The Gunn device is the most practical and proven device, but, at 16 GHz, is only capable of powers of about 10 W peak. A simple technique is described that results in the addition of the powers and which does not degrade the other parameters, such as r.f. risetime.

Static Negative Resistance in Highly Doped Gunn Diodes and Application to Switching and Amplification

Stationary distributions of the field and the electron density of the trapped domain in highly doped Gunn devices are calculated by a computer simulation and a simple numerical analysis. From these results, characteristics of the bistable switching due to the static negative resistance are clarified against the donor density of the active region. The results of the computer simulation also demonstrates possibilities of the wide band reflection amplification with the frequencies independent of the transit time in the overcritically doped Gunn diodes.

An integrated wide-band varactor-tuned Gunn oscillator

A report is made of a thin-film varactor-tuned Gunn effect oscillator in X band employing unencapsulated diodes. The Gunn device, which is an inverted mesa type, is mounted off the substrate on a copper header and this arrangement can dissipate 7 W without the Gunn device suffering thermal damage. The varactor is a beam-lead silicon p+nn+IMPATT diode connected in series with the Gunn device. A circulator is integrated with the oscillator to provide load isolation, and tuning ranges in excess of 2 GHz are reported. Good agreement exists between calculated and measured values of oscillator tuning range.

Equivalent circuit analysis of noise in bulk semiconductor devices

Equivalent transmission-line analogs may be developed to advantage for the analysis of noise in bulk semiconductor devices. We discuss first a transmission-line analog for the law of conduction and diffusion of a single species of charge carriers that experience small disturbances from equilibrium. Through the use of Nyquist's theorem it is possible to obtain the power spectra of the noise sources of the equivalent transmission-line circuit at thermal equilibrium. Next, an equivalent circuit can be obtained if there is more than one charge carrier. This circuit is generalized to the case of drifted carriers and applied to the metal-semiconductor-metal (MSM) diode and to the small-signal analysis of the Gunn effect. We show how the equivalent circuit for the Gunn effect gives, by inspection, a lower bound on the noise measure achievable with a Gunn diode. We find that the Gunn diode has noise measures exceeding the lower bound. A traveling-wave Gunn device achieves the lower limit.

Semiconductors for microwave frequencies

Microwaves can now be processed by semiconductor structures, which are either a further development of the transistor for the lower microwave frequency ranges, or which use the non-linear behaviour of junction devices for mixing and parametric effects, particularly for low noise levels, or which exploit transit-time effects to create an effective negative conductance gd such as impatt and Gunn devices. The important role of material parameters is discussed New experimental results on gd vs. microwave voltage amplitude are presented for Gunn diodes as an illustration of the type of device characterization required.

Current spreading at contacts to planar Gunn devices

The implications of current constriction at a planar contact to GaAs are inferred from the results of a simpler case (current flow to a small conducting cylinder). It is argued that a number of the pathological properties of Gunn diodes can be explained in terms of this mechanism.

The behaviour of a Gunn oscillator in the domain-delayed mode†

The maximum negative conductance realizable by a Gunn device operating in the domain-delayed mode is found analytically over a range of frequencies. The effect of finite domain nucleation and extinction times, of domain charge and of velocity modulation of the domain, are considered. Comparison is made with experimentally determined values. An explanation in terms of the negative conductance curves is offered for tuning-curve hysteresis.

Comments on "Couple-mode analysis for nonuniform crossed-field drift beams"

Howes [l] has described the effects of the encapsulation on Gunn device performance, and has derived theoretical tuning characteristics from an analysis of the microwave circuit. We have carried out similar studies, and obtained results that differ in certain respects from those of Howes. Our results will be briefly summarized here, with comments on the reasons for the apparent discrepancies. Like Howes, we have derived the input impedance of the packaged Gunn device, although we have employed the results of the detailed measurements on the S4 microwave package that are reported upon elsewhere [2]. We find three resonances that in order of increasing frequency are series, parallel, and series. The input impedance of the package may now be equated to the complex conjugate of the input reactance of the waveguide resonator combined with the load resistance. It is important to decide whether the load is in series or in parallel with the resonator, since close to the package resonances the solutions for the waveguide cavity length are entirely different-the resonance condition may be met either with a low reactance in series with a resistance, or by a high reactance in parallel with a resistance. It is on this point that our studies differ from those of Howes. The analysis given by Howes implies a load resistance in series with the resonator. R‘e suggest that this is inconsistent with his microwave circuit, which is more appropriately represented by the parallel load configuration. This approach is supported by the work

Circuit considerations in the design of wide-band tunable transferred electron oscillators

Studies of the effects of the encapsulation on Gunn device performance have been carried out. Results are compared with those of Howes [1], and certain apparent discrepancies are explained.

Pulse generation in planar Gunn devices with varying nL product

Measurements of the current reduction in planar Gunn devices by domain nucleation are reported. The devices were made of epitaxially grown GaAs layers. The measurements showed a decrease of the current reduction, relative to the threshold current Ith, with decreasing nL product, beginning at about nL = 1013 cm−2, where n is the electron concentration and L is the length of active channel.

Simple integrated circuit with Gunn devices

This letter describes the structure and electrical performance of a simple integrated digital circuit on GaAs using Gunn devices, which is capable of operating at up to 2 Gbit/s. A directed transfer of domain pulses within the circuit, which is almost free of reactive effects, is achieved by employing Schottky-barrier diodes.

1/4 W continuous-wave Q band Gunn oscillator

C.W. Gunn devices using integral-heatsink techniques have been produced with a power output of more than ¼ W at 31.5 GHz. Conversion efficiencies of up to 4.6% are reported. A tuning range greater than 8 GHz has been obtained.

Frequency/temperature characteristics of Gunn devices

The variability and large magnitude of the frequency/temperature coefficient of Gunn oscillators has limited the application of these devices. The letter outlines a program of experiments and computer simulations aimed at achieving an understanding of the basic ∂f/∂T mechanism and thus producing a deep stable oscillator. Our simulations successfully predict observed trends in the ∂f/∂T behaviour of Gunn devices with both alloyed metal and n+ regrown contacts. The results of our study show that stable oscillators with a small loaded Q factor and near maximum power output are realisable.

Planar Schottky-gate Gunn devices

Gunn devices, with a Schottky diode as control electrode, were fabricated and investigated. The experimental results are compared with calculations, which are also outlined, and agreement between theory and experiment is found to be satisfactory. A jitter in domain nucleation is shown to be due to fluctuations in the anode voltage.

Double cavity tuning of Gunn oscillators at millimeter wavelengths

Experiments have been performed confirming the advantage of double cavity tuning of Gunn devices in the 35-GHz band. Output power improvements achieved were of 20-40 percent. The tunability of devices was over 5 GHz with output power above 5 mW. Maximum achievable frequency is with dual cavity about 7-8 percent lower than with the simple basic device.

Circuit characterisation of waveguide-mounted Gunn-effect oscillators

A lumped equivalent circuit is proposed to characterise the load impedance presented to a Gunn device mounted in a waveguide cavity. Using analytical expressions for the elements of the circuit, theoretical predictions for the properties of Gunn oscillators with and without a coupling iris are made, and are compared with experiments.

An observed r.f. hysteresis characteristic of particular gunn diodes

Certain Gunn diodes, when mounted in half-wave resonant cavities, exhibited marked hysteresis in r.f. power when the bias voltage was increased above threshold and then reduced. Observations and measurements are described of the dependence on circuit loading of this phenomenon, and the associated I-V curve of the diode. Explanations of the phenomenon are proposed, based on recent investigations into the effect of imperfect cathode boundary conditions on the behaviour of Gunn devices. Some possible practical applications of the hysteresis are suggested, assuming that controlled manufacture of such devices might eventually be possible.

Pulse discrimination by Gunn-effect switching

Starting with simple equations for the domain dynamics the switching behaviour of a Gunn device with an ohmic load resistance is investigated, in a way similar to tunnel diode switching. An appropriate load line shall define three bias states on the current voltage characteristic, i.e. a homogeneous low-field state I ( E I < E peak), a homogeneous high-field state III ( E III > E valley) and a steady-state domain state IV. Then triggering the device from the only d.c. stable state I by (rectangular and triangular) input pulses of various heights and lengths leads to three types of switching processes namely direct return to I, return via state III or via state IV, these transitions yielding three typical output pulses. Discrimination curves for the input-pulse parameters are presented. Simple limiting cases for the three switching processes are treated in some detail yielding the switching times typical for each transition. Finally, the long-time stability of the bias states I, III and IV is discussed.

Design of complex microstrip circuits by measurement and computer modelling

Measurement techniques have been developed which allow the complex admittance existing at a nominal microstrip open circuit to be accurately characterised. This information, together with the line loss and wavelength, has enabled the electrical defining plane of a shunt stub to be determined. By using a circuit-design technique based on computer modelling it is possible to allow for all these factors. When this approach was applied to the design of a filter with 13 elements, the computer element lengths, when fabricated in microstrip, produced a filter with a bandpass characteristic to within 1% of the computer-modelled response. The need for cut-and-try development is therefore virtually eliminated.The technique of exact computer modelling has also been applied to oscillator circuits, using varactortuned Gunn devices. These circuits are extremely complicated, since they employ waveshaping of the voltage applied to the device. The second harmonic is trapped in a carefully controlled independent resonant circuit. For certain devices, this technique can increase the microwave output power by a factor of 4 or 5. Without the use of cut-and-try methods, devices have operated only 5MHz away from the designed output frequency of 6GHz.