Analysis and improvement of retention time of memorized state of metal-ferroelectric-insulator-semiconductor structure for ferroelectric gate FET memory

Retention characteristics of metal-ferroelectric-insulator-semiconductor (M FIS) structures have been studied theoretically by considering effects of c harge injections derived from the difference between leakage current densit ies in the ferroelectric and insulator layers. The calculated curves for ti me-dependent capacitance have shown good agreements with experimental resul ts. The numerical results for the MFIS structure have indicated that excess current over a certain value through the ferroelectric and the insulator l ayers causes the retention time to rapidly degrade. An idea of inserting an insulator film between the metal and the ferroelectric layers in an MFIS h as also been examined in order to cut down the currents through the ferroel ectric layer. The calculations based on our model have found this metal-ins ulator-ferroelectric-insulator-semiconductor (M-I-FIS) structure to exhibit much longer retention time than the conventional MFIS.