Memory device and method of operating the memory device including program verify operation with program voltage adjustment

A method of operating a memory device, comprising:

• raising a threshold voltage of memory cells by applying a first program voltage to a selected word line;
• performing a verify operation for determining whether the threshold voltage of the memory cells is equal to or greater than a target voltage;
• storing a count value by counting a number of times the verify operation is performed;
• comparing the count value with a reference value when the verify operation fails;
• setting a step voltage depending on a result of comparing the count value with the reference value;
• and
• generating a second program voltage, which is higher than the first program voltage by the step voltage, and programming the memory cells using the second program voltage,
• wherein setting the step voltage is configured such that:
• the step voltage is maintained at the default voltage when the count value is equal to the reference value,
• the step voltage is set higher than the default voltage when the count value is greater than the reference value, and
• the step voltage is set lower than the default voltage when the count value is less than the reference value.