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

1. A memory device, comprising:

an operation code generator configured to generate a program code and a verify code in response to a program control code and to output an operation code using the program code and the verify code;

a verify counter configured to store a count value acquired by counting a number of times a verify operation is performed depending on the verify code;

a verify determiner configured to compare the count value with a reference value depending on a result of the verify operation and to generate the program control code such that a step voltage for raising a program voltage is raised when the count value is greater than the reference value, decreased when the count value is less than the reference value, and maintained at a default voltage when the count value is equal to the reference value depending on a comparison result; and

a voltage generator configured to generate the program voltage and a verify voltage depending on the operation code.

2. The memory device according to claim 1 , wherein the operation code generator comprises:

a program operation controller configured to generate the program code in response to the program control code; and

a verify operation controller configured to generate the verify code in response to the program control code.

3. The memory device according to claim 2 , wherein:

the program code includes information about a level of the program voltage changed depending on the program control code, a level of the step voltage, and a time at which the program voltage is output from the voltage generator, and

the verify code includes information about a level of the verify voltage changed depending on the program control code and a time at which the verify voltage is output from the voltage generator.

4. The memory device according to claim 1 , wherein the verify counter comprises:

a counter configured to accumulate the count value depending on the verify code and to store the count value; and

a counter controller configured to control the counter in response to an output signal output from the verify determiner.

5. The memory device according to claim 4 , wherein the counter controller outputs one of a first count code for outputting the count value stored in the counter and a second count code for resetting the counter in response to the output signal.

6. The memory device according to claim 1 , wherein the verify determiner comprises:

a first determiner configured to output one of a first program-processing code for performing a next program loop and a second program-processing code for performing a program termination depending on the result of the verify operation, and to output an output signal for outputting the count value; and

a second determiner configured to compare the count value with the reference value and output a verify-processing code depending on the comparison result.

7. The memory device according to claim 6 , wherein the first determiner outputs the second program-processing code as a code for the program termination when the result of the verify operation is a pass, and outputs the first program-processing code as a code for performing the next program loop when the result of verify operation is a fail.

8. The memory device according to claim 6 , wherein the second determiner stores the reference value, compares the count value with the reference value when the count value is input, and outputs the verify-processing code depending on the comparison result.

9. The memory device according to claim 6 , wherein the verify determiner generates the program control code using the verify-processing code and one of the first program-processing code and the second program-processing code.

10. The memory device according to claim 6 , wherein the reference value is an average number of times a verify operation of normal memory cells passes, and the reference value is stored in the second determiner through a test operation of the memory device.

11. The memory device according to claim 1 , wherein the voltage generator generates a first program voltage to be used in a first program loop, generates a second program voltage to be used in a second program loop, and generates a voltage that is the step voltage higher than the first program voltage as the second program voltage.

12. 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.

13. The method according to claim 12 , wherein the count value is reset depending on a program loop, which includes raising the threshold voltage of the memory cells and performing the verify operation.

14. The method according to claim 13 , wherein the count value is reset whenever the program loop is changed, is reset whenever the program loop is performed a plurality of times, or is reset when a program operation of the memory cells is terminated.

15. The method according to claim 12 , further comprising:

stopping following steps and processing a memory block including the memory cells as a bad block when the count value is greater than the reference value.

16. The method according to claim 15 , wherein the memory block processed as the bad block is prevented from being accessed.

17. The method according to claim 12 , further comprising:

after comparing the count value with the reference value and before setting the step voltage, storing an address of a memory block including the memory cells when the count value is greater than the reference value.

18. The method according to claim 17 , wherein a number of times the memory block, the address of which is stored, is accessed decreases in a next program operation.