An experimental study of the influence of boundary conditions on the Gunn effect

The electrical characteristics of long inhomogeneous bulk negative differential mobility (NDM) semiconductor elements (n-GaAs and n-InP) exhibiting various modes of current instabilities at low microwave frequencies have been experimentally observed. Measurements were made of the time-dependent sample current and voltage and of the prethreshold electric-field distribution within the sample. The measurements were made under conditions in which the configuration of the circuit, the properties of the boundary of the NDM element, the sample geometry, the temperature, and the magnetic field were varied. We have obtained excellent agreement between the experimental results and a model in which the sample is assumed to have an assigned value for the electric field at the cathode boundary. Different modes of instability may be accurately modeled by choosing an appropriate value for this field. Three regions of behavior may be identified which correspond to three regions for the cathode boundary field. These regions are 1) cathode boundary field E
c
less than the threshold electric field E
p
for the onset of NDM; the samples are likely to operate as bulk oscillators, 2)
E_{c} > E_{p}
but less than the electric field E
v
corresponding to the onset of the saturated electron-drift velocity; the samples operate in the Gunn domain mode with a peak-to-valley ratio determined by E
c
, and 3)
E_{c} > E_{v}
the samples yield saturating current versus voltage curves and only weak oscillations are possible.