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, SiO
2
layers 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
alpha
exp (
beta
upsi
), rather than by the power relation t
^-1
alpha upsi
ⁿ
. The exponential
upsi
-dependence is attributed to the fieldassisted diffusion of "detrimental" ions in the high field approximation of t
^-1
alpha
sinh (
beta upsi
). On the other hand, the well-known power relation between the lifetime t of barium titanate chip capacitors and the applied stress voltage
upsi
, e.g., t
^-1
alpha beta
^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
simeq
3.5 x 10
⁵
V/cm assuming t
^-l
alpha
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.