Hall effect and resistivity measurements have been made in the temperature range 4.2–360 °K on several samples of n-type GaAs grown under oxygen atmosphere and without any other intentional dopings. The principal shallow donor in this material is considered to be Si. All samples exhibited impurity-band conduction at low temperature. Electron concentrations in the conduction band were calculated, using a two-band model, and then fitted to the usual equation expressing charge neutrality. A value of 2.3 × 10−3 eV was obtained for the ionization energy of the donors, for donor concentration ranging from 5 × 1015 cm−3 to 2 × 1016 cm−3. The conduction in the impurity band was of the hopping type for these concentrations. A value of 3.5 × 1016 cm−3 was obtained for the critical transition concentration of the impurity-band conduction to the metallic type.
The behavior of an oilpaper interface in the presence of carbon particle contamination was investigated under AC voltage. M u e n d by an electric field perpendicular to the interface, the carbon particles are deposited on and firmly fixed to the paper surface. This constitutes an excellent means of reproducing particle contamination at the oil-paper interface, since it duplicates the particledeposition mechanism in power equipment, Field deposition of carbon contamination was found to be reproducible and easy to obtain. On the other hand, a field parallel to the oil-paper interface tends to remove the contamination from the pressboard. The cleaning effect is sufficient to rapidly restore the dielectric performance of a carboncontaminated oil-paper interface.
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