Charge motion and trapping in insulators: surface and bulk effects

The electrical currents that flow in and on insulators are small, but they may be important in applications and they may indicate the mechanism of carrier motion. The processes which come into play include mobility and ionisation, trapping and detrapping, and injection and blocking. Some of these phenomena are intrinsic to the material, while others depend on the amount and nature of the impurities. In this paper, I concentrate on problems in data analysis and in the interpretation of charge measurements in the bulk and on surfaces. Some materials and some processes are well understood, while others remain obscure, particularly the identities of carriers and traps. A considerable investment of time and money will be required to solve these problems