| 摘要 |
<p>PROBLEM TO BE SOLVED: To obtain a nonvolatile semiconductor memory device which can suppress an increase in the number of times of a verifying operation by a method wherein an erasure operation and the verifying operation are performed to a memory cell string until the threshold voltage of all memory cells reaches a prescribed reference value, the verifying operation is performed to a next memory cell string and the erasure operation and the verifying operation are repeated until the threshold voltage of all the memory cells reaches the reference value. SOLUTION: A voltage of 0 V is applied to a control gate and a word line in a memory cell which is to be erased in an erasure operation, and a positive high voltage is applied to a substrate or a well. The threshold voltage of the memory cell is lowered, and a verifying operation is performed to confirm an erased result. Whether its threshold value reaches a prescribed reference value or not is detected. The erasure operation progresses according to the result of the verifying operation sequentially after every erasure operation. Since the erasure operation and the verifying operation are repeated to two memories which are connected to an identical bit line BT, the number of times of the verifying operation can be suppressed to a minimum, and the high speed of the erasure operation can be achieved.</p> |
