Methods and apparatuses including an asymmetric assist device

1. An apparatus, comprising:

a string of memory cells;

a first select device coupled to the string of memory cells;

an assist device coupled between the first select device and a first source or drain connection; and

a controller configured to control the first select device and the assist device,

wherein the first select device comprises a symmetric device that shares an undoped junction with the string of memory cells and an undoped junction with the assist device,

wherein the assist device comprises an asymmetric device that shares an undoped junction with the first select device and a doped junction with the first source or drain connection, and

wherein the controller is configured to bias gates of the first select device and the assist device to a first voltage during an erase operation, and to bias the gates of the first select device and the assist device to different second and third voltages during a program operation.

2. The apparatus of claim 1 ,

wherein the first source or drain connection comprises a source connection, and

wherein the first select device comprises a source select gate transistor and the assist device comprises a source assist device coupled between the source connection and the source select gate transistor.

3. The apparatus of claim 1 ,

wherein the first select device comprises a drain select gate transistor,

wherein the first source or drain connection comprises a drain connection, and

wherein the assist device comprises a drain assist device coupled between the drain connection and the drain select gate transistor.

4. The apparatus of claim 3 ,

wherein the first select device is coupled to a first end of the string of memory cells, the apparatus further comprising:

a source select gate transistor coupled to a second end of the string of memory cells; and

a source assist device coupled between the source select gate transistor and a source connection.

5. The apparatus of claim 1 , comprising:

a semiconductor pillar, wherein the assist device is separated from the first select device by a virtual junction,

wherein a portion of each memory cell of the string of memory cells at least partially surrounds a semiconductor pillar,

wherein the assist device comprises an asymmetric vertical transistor having one doped junction,

wherein a portion of the assist device at least partially surrounds both the semiconductor pillar and the first source or drain connection, and

wherein a portion of the first select device at least partially surrounds the semiconductor pillar.

6. The apparatus of claim 1 ,

wherein the first select device comprises a plurality of select devices coupled between the plurality of memory cells and the assist device, and

wherein the assist device comprises a plurality of assist devices, the plurality of assist devices comprising at least one asymmetric transistor.

7. The apparatus of claim 1 , wherein the first voltage is higher than the second and third voltages, and the third voltage is higher than the second voltage.

8. The apparatus of claim 7 , wherein the controller is configured to bias the gate of the first select device to the second voltage and the gate of the assist device to the third voltage during a program operation to reduce junction leakage in contrast to the junction leakage during the erase operation.

9. An apparatus, comprising:

a semiconductor pillar having a drain connection at one end and a source connection at an opposite end;

a string of memory cells, wherein a portion of each memory cell of the string of memory cells at least partially surrounds the pillar;

a symmetric select device coupled to an end of the string of memory cells, a portion of the select device at least partially surrounding the pillar;

an asymmetric assist device coupled between the select device and a first source or drain connection; and

a controller configured to control the symmetric select device and the asymmetric assist device;

wherein the symmetric select device shares an undoped junction with the string of memory cells and an undoped junction with the asymmetric assist device,

wherein the asymmetric assist device shares an undoped junction with the select device and a doped junction with the first source or drain connection, and

wherein the first source or drain connection is one of the drain connection or the source connection, and

wherein the controller is configured to bias gates of the symmetric select device and the asymmetric assist device to a first voltage during an erase operation, and to bias the gates of the symmetric select device and the asymmetric assist device to different second and third voltages during a program operation.

10. The apparatus of claim 9 ,

wherein the semiconductor pillar extends from a substrate.

11. The apparatus of claim 9 ,

wherein the semiconductor pillar is configured to operate as a channel for the string of memory cells, the select device, and the asymmetric assist device.

12. The apparatus of claim 9 ,

wherein the first source or drain connection comprises a source connection, and

wherein the symmetric select device comprises a source select gate transistor and the asymmetric assist device comprises a source assist device coupled between the source connection and the source select gate transistor.

13. The apparatus of claim 9 ,

wherein the first source or drain connection comprises a drain connection.

14. The apparatus of claim 9 , wherein the first voltage is higher than the second and third voltages, and the third voltage is higher than the second voltage, and

wherein the controller is configured to bias the gate of the first select device to the second voltage and the gate of the assist device to the third voltage during a program operation to reduce junction leakage in Contrast toile junction leakage during the erase operation.

15. A method, comprising:

biasing gates of a symmetric select device and an asymmetric assist device coupled between the symmetric select device and a first source or drain connection coupled to a string of memory cells to a first voltage during an erase operation; and

biasing the gates of the symmetric select device and the asymmetric assist device to different second and third voltages during a program operation,

wherein the symmetric select device shares an undoped junction with the string of memory cells and an undoped junction with the asymmetric assist device, and

wherein the asymmetric assist device shares an undoped junction with the select device and a doped junction with the first source or drain connection.

16. The method of claim 15 ,

wherein biasing the gates of the symmetric select device and the asymmetric assist device to the first voltage shifts a voltage drop from a virtual junction to a junction between the asymmetric assist device and the first source or drain connection such that the virtual junction has a smaller voltage drop than the junction between the asymmetric assist device and the first source or drain connection.

17. The method of claim 15 ,

wherein the first source or drain connection comprises a source connection,

wherein the select device comprises a source select gate transistor, and

wherein the asymmetric assist device comprises a source assist device coupled between the source connection and the source select gate transistor.

18. The method of claim 15 ,

wherein the first source or drain connection comprises a drain connection.

19. The method of claim 15 ,

wherein the asymmetric assist device comprises a first doped junction and a second undoped junction, and

wherein the select device comprises first and second undoped junctions.

20. The method of claim 15 , wherein the first voltage is higher than the second and third voltages, and the third voltage is higher than the second voltage, and

wherein biasing the gates of the symmetric select device and the asymmetric assist device to different second and third voltages during a program operation comprises biasing the gate of the first select device to the second voltage and the gate of the assist device to the third voltage during a program operation to reduce junction leakage in contrast to the junction leakage during the erase operation.