We investigated the effects of charge trapping on the asymmetrical increase in the memory window of metal-ferroelectric-insulator-semiconductor (MFIS) devices. We suggest that defect centers located at the ferroelectric-insulator interface play important roles in generating the asymmetrical increase in the memory window: Electron trapping at/near the SBN (or SBT)-Y2O3 interface via avalanche electron injection from the Si substrate results in the preferential domain switching, causing the asymmetrical increase in the memory window. (C) 2004 Elsevier B.V. All rights reserved.