NAND flash memory is the technology used to store data in all flash memory products, such as SSDs. Many modern NAND flash products are advertised as 3D NAND flash or V-NAND. This type of memory stacks the memory cells vertically in the flash chip, but what does that mean and why is it better?
Tip: 3D NAND flash is a different concept from MLC, TLC, and QLC. 3D NAND refers to the structure of physically stacking memory cells vertically and horizontally. MLC, or Multi-Level cell, along with Triple- and Quad-Level cells refer to the number of bits of data that a single cell can store which increases the number of distinct energy levels.
What is NAND flash?
NAND flash is a type of flash memory based on the logical NAND gate. A NAND gate is only false, if all of its inputs are true, with NAND standing for “Not AND”.
Flash memory is built on a relatively simple principle. There are two power cables, a source, and a drain. Between them is a floating gate and a control gate, all placed on a substrate of silicon. NAND flash links a number of cells together one after the other in series but follows the same principle. To set the NAND cell to a binary 1, an electric current is applied to the floating gate where it is trapped by the silicon-oxide insulation. To discharge the cell, more change is applied, until it reaches a threshold where it can jump to the drain.
A NAND cell is read by applying an electric charge to the control gate. The presence of an electric charge in the floating gate increases the amount of voltage that needs to be applied to the control gate to make it conduct. If only a small amount of voltage is required to conduct electricity through the control gate, then the memory value is 0, if more voltage is required, the memory value is 1.
Increasing memory capacity
Historically the capacity of flash memory has been increased by developing new ways to shrink the size of components and place them closer together. This essentially allows you to pack flash cells closer together. Unfortunately, there is a limit to how small these memory cells can be made before the electrical charge used to operate them is able to jump from one cell to another and rendering the whole thing useless.
To get around this, the shape of the silicon substrate that the memory cells are placed on was changed. By making the substrate into cylindrical shapes, each of which can have multiple memory cells, and then placing these cylinders vertically next to each other, the surface area for memory cells can be massively increased. With an increased surface area, more memory cells can be placed in the same volume allowing for significantly increased memory capacity for a NAND flash chip the same size.
Why is 3D NAND better?
3D NAND not only has a higher memory density, but the production method required to create the structure is actually easier to create than with the traditional NAND layout. This means that 3D NAND has both a larger capacity and a reduced cost.
On top of this 3D NAND memory is also twice as fast at both read and write speeds than traditional NAND flash. It also has up to ten times the longevity and consumes roughly half power.