As the stress in solids is increased and approaches the breakdown stress,
the current is found to increase exponentially, but does not vary so markedly
with time for steady voltage.1
* This increased current at high stresses is
generally believed to result from the injection of carriers from an electrode or
from electron multiplication in the bulk of the material or both. In addition,
if impurities or structural defects are present they may cause local allowed
energy levels (traps) in the forbidden band, and electrons may pass through
the insulator by jumping from one trap to another (hopping effect).