Flash memory controller in the context of NAND flash

Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use the same cell design, consisting of floating-gate MOSFETs. They differ at the circuit level, depending on whether the state of the bit line or word lines is pulled high or low; in NAND flash, the relationship between the bit line and the word lines resembles a NAND gate; in NOR flash, it resembles a NOR gate.

Flash memory, a type of floating-gate memory, was invented by Fujio Masuoka at Toshiba in 1980 and is based on EEPROM technology. Toshiba began marketing flash memory in 1987. EPROMs had to be erased completely before they could be rewritten. NAND flash memory, however, may be erased, written, and read in blocks (or pages), which generally are much smaller than the entire device. NOR flash memory allows a single machine word to be written – to an erased location – or read independently. A flash memory device typically consists of one or more flash memory chips (each holding many flash memory cells), along with a separate flash memory controller chip.

A USB flash drive (UFD) (also thumb drive, memory stick, and pen drive) is a data storage device that includes flash memory with an integrated USB interface. A typical USB drive is removable, rewritable, and smaller than an optical disc, and usually weighs less than 30 g (1 oz). Since first offered for sale in late 2000, the storage capacities of USB drives have ranged from 8 megabytes to 1 terabyte (TB). As of 2024, 4 TB flash drives were the largest currently in production. Some allow up to 100,000 write/erase cycles, depending on the exact type of memory chip used, and are thought to physically last between 10 and 100 years under normal circumstances (shelf storage time).

Common uses of USB flash drives are for storage, supplementary back-ups, and transferring of computer files. Compared with floppy disks or CDs, they are smaller, faster, have significantly more capacity, and are more durable due to a lack of moving parts. Additionally, they are less vulnerable to electromagnetic interference than floppy disks, and are unharmed by surface scratches (unlike CDs). However, as with any flash storage, data loss from bit leaking due to prolonged lack of electrical power and the possibility of spontaneous controller failure due to poor manufacturing could make it unsuitable for long-term archiving of data. The ability to retain data is affected by the controller's firmware, internal data redundancy, and error correction algorithms.