A Metal-high-K Bandgap-Engineered SONOS (MA BE-SONOS) with an additional SiO2 buffer layer is proposed. The thin SiO2 (5-6 nm) layer between the high-K (Al2O3) and nitride serves to prevent shallow trap generation. Contrary to the previously proposed MANOS or MA BE-SONOS devices using a simple high-k top dielectric, this composite structure eliminates the unstable high-K/nitride interface. Experimental results show that this new device can well suppress the erase saturation, just like MANOS. On the other hand, the data retention is greatly improved, owing to the much more stable interface between the nitride-trapping layer and top oxide. Very large memory window (> 7V) with excellent cycling endurance, read disturb immunity, and data retention has been successfully demonstrated. Theoretical model is also proposed to explain the principle of this device.