Gunn Effect in Compound Semiconductors

A theoretical and experimental study of the Gunn effect is presented. It appears that this effect, originally observed by Gunn as a time variation in the current through ohmic samples of n-GaAs when the sample voltage exceeded a critical value, can be accounted for by the transferred electron model of Ridley and Watkins. This model is based on a transfer of electrons from a low-mass, high-mobility conduction band that is lowest in energy to a higher-mass, low-mobility band as the electron temperature is increased by the applied electric field. If the transfer occurs rapidly enough as the electric field is increased, a bulk differential negative resistance will be realized, which then leads to the formation of domains of different electrical conductivity which move through the sample, giving rise to a time-varying current. Most of the experimental results for n-GaAs verify this interpretation. The shape of the current vs time waveform, sharp spikes in current separated by flat valleys in current, for the longer samples (l ~ 100 to lOOOmicrons) and the observed independence of threshold electric field (2300 to 4000 volts/cm) on sample length are shown to be consistent with a negative resistance model. The value of electric field which characterized the regions of high conductivity, about 1500 volts/cm, is found to be independent of sample length as expected. In addition, the voltage across the high electric field domain is found to scale with sample length, also as expected, and the value of electric field which characterizes the regions of low conductivity is estimated to be ^-60,000 volts/cm. The effects of temperature on the threshold electric field and on the threshold electron drift velocity are consistent with the transferred electron model. For short samples (i ~ 25 to 100 microns), a sinusoidal current vs time waveform is seen, and for samples in the 100-micron length range, the sinusoidal mode is seen near threshold, and the spike mode is seen well above threshold. Although the sinusoidal mode is not predicted by the simplest form of the model, the effects of magnetic field and termination impedance on this mode are consistent with the interpretation of this mode as a longitudinal disturbance caused by a negative resistance. The Gunn effect has also been observed in n-CdTe, and resistance vs hydrostatic pressure experiments show that the transferred electron model is a reasonable explanation for this material as well. Finally, the absence of an instability in n-lnSb and n-lnAs is shown to be consistent with the transferred electron model. The higher conduction band minima in these materials are probably sufficiently separated from the lowest minimum that other effects, such as earner multiplication, will occur before transfer, and no negative resistance is to be expected. Accepted for the Air Force Stanley J. Wisniewski Lt Colonel, USAF Chief, Lincoln Laboratory Office *This report is based on a thesis of the same title submitted to the Department of Electrical Engineering at the Massachusetts Institute of Technology on 14 May 1965, in partial fulfillment of the requirements for the degree of Doctor of Science.