Bulk Negative Differential Conductivity in a Near Cathode Region in n-Type Germanium

Sinusoidal current oscillations up to 0.2 A (rms) are observed in n-type germanium at 77°K. The oscillations occur at a frequency near 1 GHz and for an average threshold electric field of approximately 1 kV/cm. The germanium samples were cut from single crystal n-type germanium with carrier concentration 6×1014/cm3, and were approximately 2.0×0.5×0.5 mm. Using a capacitive probe, the oscillation is found to occur in a region of bulk negative differential conductivity which causes traveling high electric field domains. The domain movement confines itself to a 0.3-mm region starting 0.1 mm beyond the cathode contact. The electric field in this region, at the threshold applied voltage, is approximately 2800 V/cm, the same as that previously measured for bulk negative differential conductivity in germanium. A discussion of the effect of a cathode boundary electric field is included. It is shown that cathode boundary conditions can lead to a stable high electric field at the cathode, which in turn gives rise to traveling high-field domains which are confined to a region near the cathode.