Design of Germanium for Thermometric Applications

Germanium doped with certain impurities has a strongly temperature-dependent conductivity below the liquid air range, suitable for use in secondary thermometry. Two mechanisms are responsible for this behavior: (1) Down to about 10°K, the conduction is provided by free current carriers (electrons or holes) liberated from impurity atoms. (2) Control of the conductivity below ∼10°K is exercised by the relatively feeble ``impurity conduction'' process. For both processes, conductivity depends not only on the primary doping atoms but also on the density of compensating impurities; indeed, low temperature impurity conduction can be inhibited unless some compensating centers are present. In order that germanium thermometers should have resistance-temperature characteristics which ``scale'' reasonably well from one unit to another, it is desirable to deliberately incorporate a number of compensators, sufficient to suppress any influence of uninvited impurities. Moreover, by control of both the primary and compensating impurity densities, a resistance-temperature characteristic can be controlled to emphasize a desired range. The transition between free carrier conduction and impurity conduction is relatively sharp for some impurities (e.g., gallium) but occurs over a wide range of temperature for other impurities (e.g., arsenic). This influences the suitability for thermometry in various ranges. Characteristics of wide range and narrow range thermometer materials are illustrated.