Micron is the latest NAND flash memory maker to announce plans for quadruple-level cell (QLC) flash memory, following similar announcements from Toshiba and Western Digital. It's a very technical story with a very real impact.

NAND flash memory stores data in one bit per cell, with billions of cells in the flash memory chips. For flash drives to gain capacity, there are two solutions: increase the number of chips in the drive, which has physical limitations, and increase the density per cell, which is limited by the laws of physics.

The first single-level cell, with one bit per cell, first emerged in the late 1980s when flash drives first appeared for mainframes. In the late 1990s came multi-level cell (MLC) drives capable of storing two bits per cell. Triple-level cell (TLC) didn't come out until 2013 when Samsung introduced its 840 series of SSDs. So, these advances take a long time, although they are being sped up by a massive increase in R&D dollars in recent years.

Multi-bit flash memory chips store data by managing the number of electronic charges in each individual cell. With each new cell, the number of voltage states doubles. SLC NAND tracks only two voltage states, while MLC has four voltage states, TLC has eight voltage states, and QLC has 16 voltage states.

This translates to much lower tolerance for voltage fluctuations. As density goes up, the computer housing the SSD must be rock-stable electrically because without it, you risk damaging cells. This means supporting electronics around the SSD to protect it from fluctuations.

Bottom line: This stuff is really hard to do and not cheap.

It's a catch-22 for the SSD industry. SLC drives have the greatest endurance and best voltage tolerance, but they have much lower capacity. Given the increasing use of flash, users want capacity first and foremost. I can't remember the last SLC drive I saw for sale.

By: DocMemory
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