The Gunn effect in polar semiconductors

The main features of the Gunn effect can be accounted for by the transferred electron model of Ridley and Watkins, which predicts a bulk differential negative resistance and subsequent domain formation if electrons can be transferred sufficiently rapidly from the lowest conduction-band minimum to lower-mobility subsidiary minima. Experimental results for -GaAs in verification of such a bulk negative resistance are presented. In the longer samples the current-time waveform consists of sharp spikes separated by flat valleys, as expected from the motion of domains. The voltage across the domains is found to scale with sample length as predicted; the value of the electric field inside the domain is estimated to be ≥60 000 V/cm, while the field outside is about 1500 V/cm. Gunn effect oscillations have also been observed in -CdTe, but not in -InSb or -InAs. The absence of an instability in InSb and InAs is consistent with the transferred electron model, since the subsidiary minima in these materials are believed to be too high in energy to be populated before carrier multiplication occurs. Finally, it will be shown that unless the separation of the conduction-band minima is very small, the critical electric field at which the Gunn effect occurs is reasonably well predicted by the one-band polar optical mode runaway field.