Memory device and method of operating the same
A memory device with a plurality of memory block may include a plurality of memory cells constituting a memory blocks, and two or more select transistors coupled to the plurality of memory cells constituting the memory block and configured to select the memory block, the two or more select transistors having different threshold voltages from one another.
1. A memory device, comprising:
a first cell string coupled to or isolated from a first bit line by first select transistors having different threshold voltages and coupled to or isolated from a first common source line by second select transistors having different threshold voltages;
a second cell string coupled to or isolated from a second bit line by third select transistors having different threshold voltages and coupled to or isolated from the first common source line by fourth select transistors having different threshold voltages;
a third cell string stacked on the first cell string, the third cell string being coupled to or isolated from the first bit line by fifth select transistors having different threshold voltages and coupled to or isolated from a second common source line by sixth select transistors having different threshold voltages; and
a fourth cell string stacked on the second cell string, the fourth cell string being coupled to or isolated from the second bit line by seventh select transistors having different threshold voltages and coupled to or isolated from the second common source line by eighth select transistors having different threshold voltages,
wherein different voltages are applied to the first common source line and the second common source line, respectively.
2. The memory device according to claim 1 , wherein the first to eighth select transistors have different threshold voltages.
3. The memory device according to claim 1 , wherein:
the first and second cell strings are included in a first memory block, and
the third and fourth cell strings are included in a second memory block.
4. The memory device according to claim 1 , wherein:
the first select transistors comprise first and second drain select transistors,
the second select transistors comprise first and second source select transistors,
the third select transistors comprise third and fourth drain select transistors,
the fourth select transistors comprise third and fourth source select transistors,
the fifth select transistors comprise fifth and sixth drain select transistors,
the sixth select transistors comprise fifth and sixth source select transistors,
the seventh select transistors comprise seventh and eighth drain select transistors, and
the eighth select transistors comprise seventh and eighth source select transistors.
5. The memory device according to claim 4 , wherein:
gates of the first, third, fifth, and seventh drain select transistors are coupled to a first drain select line,
gates of the second, fourth, sixth, and eighth drain select transistors are coupled to a second drain select line,
gates of the first, third, fifth, and seventh source select transistors are coupled to a first source select line, and
gates of the second, fourth, sixth, and eighth source select transistors are coupled to a second source select line.
6. The memory device according to claim 5 , wherein:
the second source select transistors have a first threshold voltage,
the sixth source select transistors have a second threshold voltage higher than the first threshold voltage,
the fifth source select transistors have a third threshold voltage higher than the second threshold voltage, and
the first source select transistors have a fourth threshold voltage higher than the third threshold voltage.
7. The memory device according to claim 5 , further comprising a voltage generation circuit configured to generate a program voltage and a pass voltage to be applied to the first or second drain select line or to the first or second source select line so that the first to eighth drain select transistors and the first to eighth source select transistors have different threshold voltages.
8. The memory device according to claim 7 , wherein the voltage generation circuit generates the different voltages to be applied to the first or second common source line.
9. A method of operating a memory device, comprising:
programming first and second select transistors so that the first and second select transistors have different threshold voltages, wherein the first select transistors are included in a first memory block and the second select transistors are included in a second memory block stacked on the first memory block; and
selecting the first memory block by turning on all of the first select transistors and by turning off part of the second select transistors, or selecting the second memory block by turning on all of the second select transistors and by turning off part of the first select transistors,
wherein the first memory block and the second memory block are coupled to different common source lines to which different voltages are applied, respectively.
10. The method according to claim 9 , wherein the first or second memory block is selected by adjusting a voltage to be applied to a select line coupled in common to gates of the first and second select transistors.