EMZETT.NAND flash
In short: NAND flash is the memory in SSDs, USB sticks and SD cards. It stores data as electrical charge in transistor cells, which is retained even without power.
In more detail: Each cell is a transistor with an insulated layer (“floating gate” or charge trap) in which electrons are trapped. The amount of charge determines whether the cell is read as 0 or 1. The name NAND comes from the series wiring of cells (like a logical NAND gate).
In Depth
How an SSD stores:
- Writing (programming): With high voltage, electrons are “tunneled” through the insulating layer into the floating gate.
- Reading: A small test voltage measures whether the cell conducts. The threshold voltage reveals the state.
- Erasing: Charge is pulled out again, only block-wise (block = several MB), while writing happens page-wise (page = 4 – 16 KB).
- No moving parts: Hence silent, shock-proof and with extremely short access times.
Cell kinds (bits per cell):
| Type | Bits | Write cycles | Density | Speed | Use |
|---|---|---|---|---|---|
| SLC | 1 | ~100,000 | low | very high | Industry, cache |
| MLC | 2 | ~10,000 | medium | high | Professional |
| TLC | 3 | ~1,000 – 3,000 | high | good | Standard SSDs |
| QLC | 4 | ~100 – 1,000 | very high | lower | Cheap large SSDs |
3D NAND: Cells are stacked on top of each other in 100 to 300 layers. Capacity rises without cells having to shrink further.
The controller: Every SSD has a controller with tasks:
- Wear leveling: Distributes writes evenly so no cell wears out early.
- Garbage collection: Tidies blocks that partly contain invalid data.
- TRIM: The operating system reports free areas that the controller erases in advance.
- Error correction (ECC) and spare cells (over-provisioning).
- SLC cache: A part first writes fast with 1 bit per cell, then is reorganized.
Limits:
- Finite write cycles. Measure TBW (terabytes written). Normal users rarely reach it.
- Deleted data is hard to overwrite (secure deletion only via secure erase or encryption).
- Charge can leak over years (data retention without power 1 – 10 years depending on temperature).
Comparison: NAND flash is the technology behind “SSD”, SATA/NVMe only the connection. The counterpart is NOR flash (faster reading, directly executable, smaller size, for example BIOS chips).
See also: SSDs, Wear leveling in SSDs, SATA, PCIe and NVMe compared