A type of memory chip that is programmed, erased, and reprogrammed in memory segments called 'blocks.' Compared to standard memory, which is reprogrammed byte-by-byte, erasure and reprogramming in flash blocks takes considerably less time. ...
Flash memory stores information in an array of memory cells made from floating-gate transistors. In traditional single-level cell (SLC) devices, each cell stores only one bit of information. Some newer flash memory, known as multi-level cell (MLC) devices, can store more than one bit per cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells.
A flash memory cell.
A flash memory cell.
[edit] NOR flash
Programming a NOR memory cell (setting it to logical 0), via hot-electron injection.
Programming a NOR memory cell (setting it to logical 0), via hot-electron injection.
Erasing a NOR memory cell (setting it to logical 1), via quantum tunneling.
Erasing a NOR memory cell (setting it to logical 1), via quantum tunneling.
In NOR gate flash, each cell resembles a standard MOSFET, except the transistor has two gates instead of one. On top is the control gate (CG), as in other MOS transistors, but below this there is a floating gate (FG) insulated all around by an oxide layer. The FG is interposed between the CG and the MOSFET channel. Because the FG is electrically isolated by its insulating layer, any electrons placed on it are trapped there and, under normal conditions, will not discharge for many years. When the FG holds a charge, it screens (partially cancels) the electric field from the CG, which modifies the threshold voltage (VT) of the cell. During read-out, a voltage is applied to the CG, and the MOSFET channel will become conducting or remain insulating, depending on the VT of the cell, which is in turn controlled by charge on the FG. The current flow through the MOSFET channel is sensed and forms a binary code, reproducing the stored data. In a multi-level cell device, which stores more than one bit per cell, the amount of current flow is sensed (rather than simply its presence or absence), in order to determine more precisely the level of charge on the FG.
A single-level NOR flash cell in its default state is logically equivalent to a binary "1" value, because current will flow through the channel under application of an appropriate voltage to the control gate. A NOR flash cell can be programmed, or set to a binary "0" value, by the following procedure:
* an elevated on-voltage (typically >5 V) is applied to the CG
* the channel is now turned on, so electrons can flow from the source to the drain (assuming an NMOS transistor)
* the source-drain current is sufficiently high to cause some high energy electrons to jump through the insulating layer onto the FG, via a process called hot-electron injection
To erase a NOR flash cell (resetting it to the "1" state), a large voltage of the opposite polarity is applied between the CG and drain, pulling the electrons off the FG through quantum tunneling. Modern NOR flash memory chips are divided into erase segments (often called blocks or sectors). The erase operation can only be performed on a block-wise basis; all the cells in an erase segment must be erased together. Programming of NOR cells, however, can generally be performed one byte or word at a time.
Despite the need for high programming and erasing voltages, virtually all flash chips today require only a single supply voltage, and produce the high voltages via on-chip charge pumps.
NOR flash memory wiring and structure on silicon
NOR flash memory wiring and structure on silicon
[edit] NAND flash
NAND gate flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB storage devices known as USB flash drives, as well as most memory card formats available today.
NAND flash memory wiring and structure on silicon
NAND flash memory wiring and structure on silicon
[edit] Industry
One source states that, in 2008, the flash memory industry includes about US$9.1 billion in production and sales. Apple Inc. is the third largest purchaser of flash memory, consuming about 13% of production by itself.[4] Other sources put the flash memory market at a size of more than US$20 billion dollars in 2006, accounting for more than eight percent of the overall semiconductor market and more than 34 percent of the total semiconductor memory market.[5]
[edit] Limitations
[edit] Block erasure
One limitation of flash memory is that although it can be read or programmed a byte or a word at a time in a random access fashion, it must be erased a "block" at a time. This generally sets all bits in the block to 1. Starting with a freshly erased block, any location within that block can be programmed. However, once a bit has been set to 0, only by erasing the entire block can it be changed back to 1. In other words, flash memory (specifically NOR flash) offers random-access read and programming operations, but cannot offer arbitrary random-access rewrite or erase operations. A location can, however, be rewritten as long as the new value's 0 bits are a superset of the over-written value's. For example, a nibble value may be erased to 1111, then written as 1110. Successive writes to that nibble can change it to 1010, then 0010, and finally 0000. In practice few algorithms can take advantage of this successive write capability and in general the entire block is erased and rewritten at once.
Although data structures in flash memory cannot be updated in completely general ways, this allows members to be "removed" by marking them as invalid. This technique must be modified somewhat for multi-level devices, where one memory cell holds more than one bit.
[edit] Memory wear
Another limitation is that flash memory has a finite number of erase-write cycles (most commercially available flash products are guaranteed to withstand 100,000 write-erase-cycles for block 0, and no guarantees for other blocks).[6] This effect is partially offset by some chip firmware or file system drivers by counting the writes and dynamically remapping the blocks in order to spread the write operations between the sectors; this technique is called wear levelling. Another approach is to perform write verification and remapping to spare sectors in case of write failure, a technique called bad block management (BBM). For portable consumer devices, these wearout management techniques typically extend the life of the flash memory beyond the life of the device itself, and some data loss may be acceptable in these applications. For high reliability data storage, however, it is not advisable to use flash memory that has been through a large number of programming cycles. This limitation does not apply to 'read-only' applications such as thin clients and routers, which are only programmed once or at most a few times during their lifetime.