Theory of Radiation-Induced and Carrier-Enhanced Conductivity: Space Charge and Contact Effects

Numerous experiments to determine conductivity in dielectrics are reported. It is often necessary to use a theoretical model to correctly interpret them. A carrier model is described here, based on steady-state solutions of general kinetic equations for electrons and holes. An assumption is made that the holes are instantaneously trapped into deep traps, while the electrons hop from shallow trap to shallow trap and are described as quasi-free with a lowered "trap-modulated" effective mobility. This simplifies the description of the system to the Poisson equation plus a single transport equation for the electrons. Parameters required by the model include mobility, recombination rate, pair generation rate, and excesscharge deposition rate. Raw data on a 6.4-μm biased sample of Kapton, penetrated by a 28-keV incident electron beam, are considered for interpretation. A number of solutions yielding valuable insights are discussed. Current measurements at zero bias can be interpreted in terms of the shape of the excess-charge deposition profile. Measurements at high bias are matched by the model with an appropriate choice for the trap-modulated electron mobility (about 7 × 10-15 m2/V-s), provided injection is assumed to occur at the cathode contact.