NAND Flash Memory: How Solid-State Storage Stores Data in Trapped Electrons

NAND flash is the non-volatile memory technology underlying SSDs: Solid-State Drives That Replaced Spinning Disks, USB drives, SD cards, and smartphone storage. Data is stored as electrical charge trapped in floating-gate (or charge-trap) transistors arranged in a NAND configuration — no charge represents a 1, trapped charge represents a 0. ## Cell Types Different NAND variants store more bits per cell, trading density for endurance: - **SLC** (Single-Level Cell): 1 bit/cell, ~100,000 write cycles — fastest, most durable, most expensive - **MLC** (Multi-Level Cell): 2 bits/cell, ~10,000 cycles - **TLC** (Triple-Level Cell): 3 bits/cell, ~1,000–3,000 cycles — dominant in consumer SSDs - **QLC** (Quad-Level Cell): 4 bits/cell, ~500–1,000 cycles — cheapest, used for read-heavy workloads ## 3D NAND Modern NAND stacks layers vertically (up to 200+ layers) rather than shrinking cell size further, increasing density without the reliability problems of smaller geometries. This 3D NAND approach has been the primary scaling mechanism since ~2015. ## Wear Management Flash controllers use wear leveling (distributing writes evenly across cells), over-provisioning (reserving spare capacity), and error correction codes to extend drive lifespan well beyond raw cell endurance ratings. **See also:** Flash Memory Data Retention: Why Unpowered Storage Loses Data · SATA: The Serial ATA Interface for Storage Devices