More than fifty years after the publication of the early work on conduction and dielectric breakdown of solids, we are still unable to describe quantitatively the electrical response of these materials. During this period of time, concepts derived from semiconductor physics have been transposed to the case of insulating solids, and among them, to polymers. Alternative descriptions have been proposed as well. In spite of this, there is still no agreement on how to describe charge transport and there is still some controversy as regards the applicability of semiconductors physics to the case of disordered insulating materials and in particular to polymers used in electrical engineering applications. The last twenty years have been marked by the publication of excellent review papers summarizing the physical concepts available to describe charge transport. Enormous steps forward have been achieved as regards computing facilities and our ability to spatially map the space charge, quantitatively, inside dielectric materials. We consider these two factors as fundamental in providing possibilities for developing sound models of charge transport, by using the basis of fundamental knowledge that has been accumulated in the previous years, and by coupling up-to-date techniques in experiments and in simulation. In this paper, which is not a review of either the published work on modeling or of new concepts in dielectric physics, we emphasize recent progress in the field of atomistic and macroscopic modeling and we discuss challenges based on such approaches that, we think, constitute a direction for future research.

Charge transport modeling in insulating polymers: from molecular to macroscopic scale

Charge Transport Modeling in Insulating Polymers : From Molecular to Macroscopic Scale

Transient and dc photoconductivity measurements are presented for two low‐mobility dielectrics, poly (vinylidene fluoride) and polyethylene terephthalate. In each case, the data are analyzed using the present models for dispersive transport, and the inadequacies of these theories when applied to these low‐mobility dielectrics are illustrated. In this paper we report an anomalously small transient photoresponse in poly (vinylidene fluoride); from our analysis, we attribute this result to the dispersive transport limited drift mobility in this material. Polymer structure and electric field contributions to the radiation‐induced conductivity in these dielectrics are also discussed.

Radiation-induced photoconductivity in polymers: Poly(vinylidene fluoride) compared with polyethylene terephthalate

A space‐charge mapping technique was used to measure the space charge and internal electric fields in metal–poly(ethylene terephthalate)–metal samples subjected to electric field stressing. From these results the role of electronic injection and transport in the observed electrical properties of capacitor structures could be inferred. Below breakdown fields, there was no evidence that the dark current can be attributed to electronic processes. As breakdown fields were approached, a decrease in the injected space charge trapped adjacent to the electrode was observed. This behavior was attributed to the onset of tunneling into high‐energy electronic states in the polymer. This process may produce large current densities leading to dielectric breakdown.

Properties of the metal–polymer interface observed with space‐charge mapping techniques

Charge Storage and Transport in Electron-Irradiated and Corona-Charged Dielectrics

Reversible changes (switching) in the magnitude of the leakage current through a metal‐insulator‐metal (semiconductor) structure can be induced by the excitation of electron‐hole pairs in the insulator. The switching behavior in metal‐polyethyleneterephthalate (PET) metal capacitors is given as a function of the variables: excitation rate, applied electric field, film thickness, and metal work function. The switching is interpreted by a simple model which requires knowledge of the bulk transport of electrons and holes in the insulator, and a high density of hole (or electron) traps near the contact only. The switching occurs when the trapped space charge induces injection from the contact, and saturation is achieved by recombination of the injected carriers with the trapped carriers. Both the time dependence and magnitude of switching is predicted for published switching data in MOS devices with good agreement. Since the model requires knowledge of the current‐voltage characteristic for the contact in que...

R. C. Hughes

Papers

The electronic properties of the metal‐insulator contact: Space‐charge induced switching