Conduction electrons in KBr at fields close to breakdown

With a pulse technique method electron lifetimes, drift velocities, and microscopic mobilities were measured in uncoloured KBr single crystals. The lifetimes are field-dependent, what is interpreted as a heating up effect. The drift velocity for fields less than 2.5 × 105 V/cm is a linear function of the field, as Ohm's law requires. At higher fields the drift velocity departs from the linear behaviour and turns to a constant value. From the temperature dependence of the drift mobility the microscopic mobility is derived and found in satisfactory agreement with hall mobility data. In coloured KBr the electron life-time is so short, that the electron drift mobility cannot be determined. It is only possible to get some information about the electron lifetime via the current pulse half width: The lifetime rises with the field F. The measurement of the photoinduced pulse current in dependence on F shows, that Ohm's law is valid for F < 1.5 × 105 V/cm. For higher fields the current rises more strongly than linear because of the increasing lifetime. Somewhat below the breakdown strength the photocurrent no longer increases with the field. This indicates that impact ionisation does not occur. The search for impact ionisation in uncoloured KBr also remained without success. So one can say, that the dielectric breakdown is not an avalanche breakdown.