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Distributions of both electron traps and hole traps in metal/oxide/nitride/oxide/silicon (MONOS) structures were studied. These trap distributions were analyzed by using a combination of an avalanche-injection method and capacitance-voltage measurement. The thicknesses of the nitride layers in the MONOS capacitors were varied. It was found that electron traps are mainly located at both the top and bottom oxide/nitride interfaces, whereas hole traps are located at the interfaces as well as in the nitride bulk. This analysis clarified that electron-trap capability decreases anomalously in the range of nitride thickness of less than 4 nm. Moreover, the effective activation energy of electron emission also decreases at nitride thickness less than around 5 nm. These decreases are considered to be accounted for by detrapping due to the Coulomb repulsion between electrons at both interfaces and electron trapping in the interfacial transition layers.