Unstable charging of Al/sub 2/O/sub 3/ spacers at high electrical stress

Partial discharge activity is measured at AC voltages close to breakdown of a 5 mm thick spacer. Patterns as a function of the phase angle of the applied voltage are observed which can be ascribed to charging processes on the insulator. Besides a stable regime of discharges, sudden changes in behaviour may also occur. These patterns often emerge as bursts (few seconds or minutes) after which they either quench or lead to a full breakdown.