Thermal and Electrothermal Instabilities

Abstract

Thermal effects in solids have been treated in great detail over the past 60 years (see, e.g., Carslaw and Jaeger, 1959). Of particular interest has been the myriad of phenomena associated with thermal runaway induced by Joule heating and the associated breakdown or switching processes often observed (see e.g., Fock, 1927; Lueder and Spenke, 1935; Becker, 1936; Franz, 1956; Skanavi, 1958; Böer et al., 1961; Stocker et al., 1970; Shousha, 1971; Altcheh et al., 1972; Thoma, 1976). Reviews have been given, e.g., by Klein (1969, 1978, 1983), Shaw and Yildirim (1983), and Madan and Shaw (1988). These instabilities often result in regions of NDC appearing in the I(Φ) characteristics of a variety of materials. Indeed, it is now well known that NDC can appear in the static and dynamic characteristics of common materials and devices in which the current level is determined not only by the applied voltage, but also by the temperature. One reason for this is that the Joule heating of the sample often causes the average temperature to rise above that of the ambient temperatures, T _a . Figure 8–1 shows how this might arise. Linear I(Φ) characteristics are sketched for isothermal cases where the ambient temperature T _a4 > T_a3 > T _a2> T _a1. These are the characteristics that would result were the heat sinking sufficient to maintain the system at the ambient levels shown. However, when the heat sinking is insufficient to remove heat fast enough, then, e.g., if the ambient is T _a1, it is possible that the steady state average T can correspond to a point on the T _a4 line.