Method of measuring threshold voltage for a NAND flash memory device

A method of measuring threshold voltages in a NAND flash memory device including a plurality of cell strings each having a plurality of memory cells connected in series, a plurality of bitlines connected to common drain nodes of the cell strings, a common source line connected to common source nodes of the cell strings, wordlines intersecting the bitlines so as to select the memory cells, and a well of a semiconductor substrate in which the memory cell is formed, the method comprising the steps of:

• applying an operation voltage to the well;
• applying a voltage, which is obtained by summing up a precharge voltage and the operation voltage, to the bitline, applying a test voltage to a selected wordline, applying a voltage, which is obtained by summing up a pass voltage and the operation voltage, to deselected wordlines, and applying the operation voltage to the common source line;
• and
• identifying, after detecting a voltage variation of the bitline during a read operation, a threshold voltage of a selected memory cell as a value obtained by subtracting the operation voltage from the test voltage when there is no voltage variation on the bitline, while increasing the test voltage by a predetermined level when the voltage of the bitline decreases.