What is NAND Flash?

What is NAND Flash?

NAND flash is the technology that makes solid-state drives possible. Unlike a hard drive, which stores data on spinning magnetic platters, NAND flash stores data in millions of tiny memory cells. Each cell is a transistor that can hold an electrical charge. The presence or absence of charge represents a 1 or a 0, the binary language that computers understand.

The transistor in a NAND cell has a special layer called a floating gate, which is surrounded by an insulating material. When you apply a voltage, electrons tunnel through the insulator and get trapped in the floating gate. This trapped charge changes the transistor's threshold voltage, which can be measured to read the stored data. The insulation is good, but not perfect, which is why NAND cells eventually wear out.

NAND is organized into pages, blocks, and planes. A page is the smallest unit that can be read or written, typically 16 KB. A block contains many pages, typically 256 or more. The important thing is that you cannot overwrite a page directly. You have to erase the entire block first, then write the new data. This is why SSDs need a controller that manages garbage collection and wear leveling.

The type of NAND determines how many bits each cell can store. SLC stores one bit per cell, MLC stores two, TLC stores three, and QLC stores four. More bits per cell means lower cost and higher capacity, but also slower speeds and lower endurance. The controller in the SSD has to work harder to distinguish between the different charge levels in multi-bit cells.

Modern SSDs use 3D NAND, which stacks cells vertically in layers instead of placing them side by side. This increases density without shrinking the cells, which would make them less reliable. Current 3D NAND has over 200 layers, and manufacturers are working toward 500 layers or more. More layers mean cheaper, faster, and more reliable SSDs.