SPACE-CHARGE DISTRIBUTION IN ELECTRON-BEAM CHARGED DIELECTRICS

A theory of space-charge build-up in dielectrics during continuous irradiation with electrons of range smaller than the thickness of the dielectric is developed. Open-circuit conditions are analysed with one electrode (either rear or front) grounded. In the irradiated region of the sample the following processes are taken into consideration: spatially uniform generation of mobile electrons and immobile holes, drift of electrons controlled by a field-independent trapping time followed by capture in deep traps, and recombination of electrons and holes. Within the non-irradiated region, the same conditions for electron drift are assumed. Analytical solutions for the spatial distribution of the carrier densities and the time-dependent electric field are obtained. It is shown that if the rear electrode is grounded (mode A) the space-charge build-up takes place mostly in the non-irradiated region, while for a grounded front electrode (mode B) there is only weak charging in the irradiated region. Consequently a dielectric is much less charged for mode B than for mode A.