Time-Dependent Space-Charge Currents in Insulators Involving Two Species with Widely Differing Mobilities

The space-charge-modified transport of two oppositely charged carriers under the influence of a constant applied field is discussed. A Lagrangian approach is adopted to avoid spurious numerical diffusion effects. This paper considers the situation where one of the species is so much less mobile than the other that it may be regarded as effectively at rest. The motion which ensues when a constant field is applied to a specimen of insulating material in the form of a slab consists of two stages. In the first the more mobile species is swept along by its own time-dependent space charge field and an approximately fixed field produced by the less mobile species and the applied field together. When all the more mobile charges have left the specimen, the second stage begins in which the less mobile charges are themselves driven from the slab by the fixed applied field and their own space charge field. The method of analysis required for the first stage is discussed in this paper. Another problem which is treated in a similar way arises when, in addition to the fast and slow species already referred to, there is a fixed polarization ation charge distribution corresponding to a deeply trapped charge distribution.