CHARGE-INJECTION INSTABILITY IN PERFECT INSULATORS

We show that in a macroscopic perfect insulator, charge injection at a field-enhancing defect is associated with an instability of the insul ating state or with bistability of the insulating and the charged stat e. The effect of a nonlinear carrier mobility is emphasized. The forma tion of the charged state is governed by two different processes with clearly separated time scales. First, due to a fast growth of a charge -injection mode, a localized charge cloud forms near the injecting def ect (or contact). Charge injection stops when the field enhancement is screened below criticality. Secondly, the charge slowly redistributes in the bulk. The linear instability mechanism and the final charged s teady state are discussed for a simple model and for cylindrical and s pherical geometries. The theory explains an experimentally observed in crease of the critical electric field with decreasing size of the inje cting contact. Numerical results are presented for de and ac biased in sulators.