Flash memory

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. NOR flash 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.

A disassembled USB flash drive. The chip on the left is flash memory. The controller is on the right.

Flash memory, a type of floating-gate memory, was invented at Toshiba in 1980 and is based on EEPROM technology. Toshiba began marketing flash memory in 1987.[1] 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.

The NAND type is found mainly in memory cards, USB flash drives, solid-state drives (those produced since 2009), feature phones, smartphones, and similar products, for general storage and transfer of data. NAND or NOR flash memory is also often used to store configuration data in numerous digital products, a task previously made possible by EEPROM or battery-powered static RAM. A key disadvantage of flash memory is that it can endure only a relatively small number of write cycles in a specific block.[2]

Flash memory [3] is used in computers, PDAs, digital audio players, digital cameras, mobile phones, synthesizers, video games, scientific instrumentation, industrial robotics, and medical electronics. Flash memory has fast read access time, but it is not as fast as static RAM or ROM. In portable devices, it is preferred to hard disks because of its mechanical shock resistance.

Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over non-flash EEPROM when writing large amounts of data. As of 2019, flash memory costs much less than byte-programmable EEPROM and had become the dominant memory type wherever a system required a significant amount of non-volatile solid-state storage. EEPROMs, however, are still used in applications that require only small amounts of storage, as in serial presence detect.[4][5]

Flash memory packages can use die stacking with through-silicon vias and several dozen layers of 3D TLC NAND cells (per die) simultaneously to achieve capacities of up to 1 tebibyte per package using 16 stacked dies and an integrated flash controller as a separate die inside the package.[6][7][8][9]