A Schottky-Diode Model of the Nonlinear Insulation Resistance Effects in SPRTs—Part 1: Theory

The decreasing insulation resistance of standard platinum resistance thermometers (SPRT) above about 850 ◦Cmakes amajor contribution to uncertainties in measured temperatures. In principle, the insulation breakdown ought to be easily modeled as a temperature-dependent shunt resistance depending principally on the insulator materials and dimensions. However, the phenomenon exhibits a complex nonlinear behavior that, to date, has defied explanation. The lack of an explanation is a major obstacle to improvements in SPRT design and to assessments of uncertainty caused by the insulation breakdown. This article suggests that the nonlinear effects are due to metal–semiconductor diodes, also known as Schottky-barrier or point-contact diodes, formed at the points of contact between the fused-silica insulators and the platinum of the SPRT sensing element and lead wires. The article presents an overview of the theory underlying Schottky diodes, shows that this model qualitatively explains the observations, and suggests practical experiments to verify the model