The Turnover Phenomenon in Thermistors and in Point-Contact Germanium Rectifiers

Thermistors are semiconducting devices which display a voltage maximum in their steady-state current voltage characteristics. The properties of this `turnover' phenomenon can be calculated from the dependence of the thermistor conductance on the temperature of the element and for an exponential relation of the form I ∝ V exp (- b/T ) it is found that the turnover power increases as the square of the absolute ambient temperature. The similarity in form between the reverse characteristic of a point-contact germanium rectifier and the static characteristic of a thermistor suggests a thermal hypothesis for rectifier turnover. However, in the latter, the turnover power decreases approximately linearly with the ambient temperature. Even if it be assumed that the isothermal characteristic is not linear and that the energy term b is a function of applied voltage such as would be expected in a number of physical models, the theoretical turnover power increases with increasing ambient temperature. A number of possible forms for the dependence of rectifier current on the voltage and contact temperature are examined and a general type of function which displays the essential features of the observed turnover behaviour is discussed. The physical basis for such a dependence does not conform with existing theories of rectification and a radical revision of these is apparently required under the conditions of elevated contact temperature and high field which exist at turnover