Thermal pinching at low fields in n-type germanium

We report studies of thermal breakdown in n-type germanium at fields considerably below those necessary for avalanche breakdown. A sustained negative resistance is observed at points above the threshold for thermal breakdown. The heating causes enhanced electrical conduction along the current channel, producing a constriction of the current, and thus forming a thermal pinch. We feel this is the first significant report of thermal pinching in which an initial current constriction due to self-pinching was not involved. Thermal breakdown exists when the power input per unit length exceeds P = 2πrK (dT/dr) ∼ 2πKT, where P is the power loss per unit length due to heat conduction, K is the thermal conductivity, and T is the ambient temperature. Rectangular parallelpipeds of n-type, 3 ohm-cm Ge, provided with non-injecting contacts were pulsed with essentially rectangular pulses of duration 0.30-0.95 microseconds. The repetition rate was one per second. The samples were cooled to 77°K to reduce residual heating effects. The current pulses were obtained by discharging a coaxial line across the sample. A threshold power of 230 watts/cm was observed and is in close agreement with calculated values. The form of the negative resistance characteristic above threshold agrees within experimental error to theoretical expectations.