Non-volatile semiconductor storage device

在非易失半导体存储装置中，设置有分别产生不同电压的3种字线电压发生电路(11a～11c)。设置有用于从3种不同电压中预先选择1种电压的电压选择电路13。在下层硅氧化膜5a的膜厚比上层硅氧化膜5c膜厚还薄的ONO5膜中，预先选择字线电压发生电路11a，进行写入动作时施加低于常规电压(9V)的7V电压，在上层硅氧化膜5c的膜厚比下层硅氧化膜5a膜厚还薄的ONO5膜中，预先选择字线电压发生电路11b并施加高于通常9V的11V电压。这样，便能够获得可以稳定保留作为信息的电荷的非易失半导体存储装置。 In a non-volatile semiconductor memory device, three word line voltage circuits generating different voltages are provided. A voltage selecting circuit pre-selecting one voltage from the three different voltages is provided. In an ONO film in which lower silicon oxide film is formed thinner than upper silicon oxide film, word line voltage generating circuit is pre-selected, and in a write operation a voltage of 7V lower than the normal voltage of 9V is applied. In ONO film in which upper silicon oxide film is formed thinner than lower silicon oxide film, word line voltage generating circuit is pre-selected, and in a write operation a voltage of 11V higher than the normal voltage of 9V is applied. Thus, the non-volatile semiconductor memory device capable of retaining charges as information stably is attained.