The reliability of MANOS devices with an oxide buffer layer (MAONOS) in between SiN trapping layer and high-K Al2O3 top dielectric is extensively studied. We conclude that the primary function of high-K Al 2O3 is to suppress the gate electron injection during erase instead of increasing the P/E speed. As a result, inserting a buffer oxide only changes EOT but does not change the P/E mechanisms. On the other hand, the buffer oxide can greatly improve data retention by suppressing leakage through Al2O3. However, owing to the slow erase performances with a thick bottom oxide, both MANOS and MAONOS erase slowly and very high erase voltages must be used. Also, both MANOS and MAONOS devices show very fast endurance degradation below P/E<10, which is inherent due to electron de-trapping mechanism. Moreover, the large erase voltage also causes severe degradation of tunnel oxide after many P/E cycling. To get both speed and reliability performances, it is necessary to introduce bandgap engineered tunneling barrier (BE-MANOS) to solve the fundamental problems of MANOS.