NAND Flash — The Memory in Every SSD, iPhone, and USB Drive

Fujio Masuoka's 1987 Toshiba patent describes NAND flash memory — the non-volatile storage technology in every smartphone, SSD, and USB drive, invented over a weekend and presented at a conference Toshiba tried to block.

What it covers

This patent describes a NAND flash memory cell — a floating-gate transistor where charge can be stored on an electrically isolated 'floating gate' between the control gate and the semiconductor channel. Trapped charge on the floating gate shifts the transistor's threshold voltage, which can be sensed and decoded as a 0 or 1. To write data, high voltage is applied to inject electrons onto the floating gate (Fowler-Nordheim tunneling). To erase, high voltage of opposite polarity removes the stored charge. Cells are arranged in series strings (NAND configuration) — multiple cells share a single bit line, making the structure extremely dense. Unlike DRAM, the floating gate retains charge without power — NAND flash is non-volatile.

What it doesn't cover

• —NOR flash — a different architecture where cells are arranged in parallel, allowing random bit-level access but with lower density than NAND
• —3D NAND (V-NAND) — stacking NAND cell layers vertically; the fundamental cell is the same but manufacturing is different
• —Solid-state drive controllers and wear-leveling algorithms — the patent covers the cell structure, not the system to manage it
• —EEPROM (electrically erasable programmable ROM) — a predecessor technology that erases byte-by-byte; NAND erases in blocks

The clever bit

Masuoka invented NAND flash in 1980 while working nights and weekends at Toshiba without approval or budget. He presented the concept publicly at the 1984 IEEE International Electron Devices Meeting — Toshiba management was furious because he had not cleared the presentation through their IP process. Intel had developed NOR flash earlier, which was random-access but expensive per bit. Masuoka's NAND arrangement connected cells in series rather than parallel — sacrificing random access for dramatically higher density and lower cost per bit. The 'flash' name came from Masuoka's colleague Shoji Ariizumi, who thought the erase process resembled the flash of a camera. Masuoka received a bonus of approximately $19,000 from Toshiba; he later sued the company and received a settlement of $758,000 — a fraction of the trillions NAND flash has generated.

Why it matters

NAND flash made the smartphone era possible. Mechanical hard drives — even the smallest 1.8-inch laptop drives — were too large, too fragile, and too power-hungry to put in a pocket device. NAND flash has no moving parts, survives drops, uses minimal power, and can be made in any shape. The cost per gigabyte of NAND flash has fallen by a factor of over 50,000 since commercialization in the late 1980s, following a version of Moore's Law specific to storage. Today NAND flash is manufactured by Samsung, SK Hynix, Micron, and Kioxia (Toshiba's spinoff) — the company that didn't appreciate what Masuoka invented.

Real-world examples

• 1.The iPhone uses NAND flash for all storage — in 2007, Steve Jobs called it 'flash memory' and it was the key technology that let Apple eliminate the moving hard drive and build a thin phone
• 2.Modern SSDs (solid-state drives) replaced mechanical hard disks using NAND flash — read speeds increased from ~100MB/s to ~7,000MB/s and drives became shock-resistant and silent
• 3.USB drives, SD cards, and CF cards are all NAND flash in different form factors — the same cell Masuoka patented in 1987 stores your photos, music, and files

Glossary