Development of a Model for Voltage Degradation of Various Dielectric Materials

Results of accelerated life tests On various voltagestressed dielectrics in the literature, e.g., tantalum and film capacitors, SiO2layers in MOS circuits, and cryogenic cable insulation, can be fitted by an exponential dependence on voltage v of the lifetime t, i.e., t^-1 exp ( ), rather than by the power relation t^{-1 n}. The exponential -dependence is attributed to the fieldassisted diffusion of "detrimental" ions in the high field approximation of t^-1 sinh ( ). On the other hand, the well-known power relation between the lifetime t of barium titanate chip capacitors and the applied stress voltage , e.g., t^{-1 2.6}as observed by Baker et al. at 170°C, cannot be fitted by an exponential field acceleration factor. Instead, it may be approximated by a PooleFrenkel current i versus field E relation at E 3.5 x 10⁵V/cm assuming t^-l i. The high field in the depletion layer near the grain surface pulls off electrons from traps by the Poole-Frenkel process, and hence lowers the Schotty barrier at the grain boundary and eventually leads to the breakdown of ceramic chip capacitors at some microscopic locations.