Flash memory

09040612Flash memory or flash memory combines the advantages of semiconductor memory and hard drives. Like any other semiconductor memory is flash memory has no moving parts. And the data are transferred just as a hard drive even after switching off the power supply.
The flash memory has evolved from the EEPROM (Electrical Erasable and Programmable Read Only Memory). Depending on the literature, there are also the designations flash EPROM and flash ROM.

In the flash memory, the storage of data is functionally identical as in the EEPROM. However the data are in blocks in data blocks 64, 128, 256, 1,024, … bytes simultaneously read, written and erased.

Computers whose memory purely based on flash memory, are the dream of every software developer and user. The computer never takes several minutes to boot at startup, but would be ready within a few seconds immediately. Just as quickly, he would also be turned off. And the next time you start the same programs and files were loaded, as the datum.

Flash memory cell

The memory cell of a flash memory is very similar to the field effect transistor (FET). In gate, however, a charge trap containing the floating gate is called. It is an electrically insulated semiconductor layer. The floating gate stores charge such as a capacitor. It is insulated from the terminals drain, source and control gate with an oxide layer. The oxide layer prevents the flow of the charge. When disconnected, the charge for many years is maintained.

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When deleting the charge jumps in a flash (Flash) on the floating gate over. It is charging. The current flow between source and drain is cut off. The transistor is then in the zero state.

To read the memory cell voltage is applied to the transistor and the current flowing between drain and source is measured. If the floating gate is discharged, a current flows between source and drain. The state of the transistor is then first

Difference NAND and NOR Flash

The NAND and NOR architectures differ grossly seen in the storage density and access speed.

In NOR flash the memory cells are connected in parallel. Access to the memory cells is random and directly. Are correspondingly short access times. The parallel connection guarantees a lower resistance between the power source and evaluation. NOR flash is for the program memory of microcontrollers. So it comes as well as non-volatile memory for the BIOS in PCs used.

In NAND flash is possible due to the internal series connection to read and write only in blocks. Due to the small number of data lines needed NAND Flash less space. Since data are also read and written in blocks on hard drives, NAND flash is excellently suitable as a hard disk replacement, and thus as a storage for memory cards, USB flash drives and SSDs.

Compared to other non-volatile memory types NAND flash allows higher storage densities at low cost and operates with significantly faster write speed and low power consumption. The differences in the velocities at the controllers.

SLC, MLC and TLC flash

Typically, a memory cell to only two different states, which are expressed by charged and uncharged. If one is able to hold a plurality of states in a memory cell, then you can increase the storage density. In flash memory trying to map by different voltage levels, a plurality of states in a memory cell.

SLC flash (Single Level Cell)

SLC flash stores only one bit per memory cell. It has approximately 100,000 write cycles a reliable flash memory for SSDs and also very expensive.

MLC flash (Multi Level Cell)

MLC flash stores two bits per memory cell (x2 MLC). He therefore has a higher storage density compared to SLC flash, at the same silicon cost. Why MLC flash can be manufactured cheaper and is especially in products for the mass market. However, the MLC memory cells can be described not as fast as SLC memory cells.

In 2 bit memory cell has a plurality of voltage levels tolerated (00, 01, 10, 11). Occur frequently read errors. Especially, the more a cell has been described. Therefore MLCs need more error correction mechanisms. Accordingly, the reading takes longer. Also, the read operation has a physical stress on the cells result. Overall, MLCs are especially well suited for systems that have more disk reads than writes and overall make do with little access to the data store.

Initially there was still talk of 10,000 writes per cell MLC. Because finer semiconductor structures, the number has fallen to 5,000 and has now arrived at 3000 (October 2013). Because of the differences in charge for several bits, the memory cell with around 3,000 write cycles per cell (at 19-nm manufacturing process) defective vulnerable than SLC. Only by wear-leveling and spare memory cells can be the total service life of the flash memory increase, but which also increases the manufacturing cost.

MLC flash is used mainly because of the limited write cycles for flash drives and memory cards. MLC flash can also be found sometimes in cheap SSDs.

Durability and reliability of flash memory

For durability and reliability of flash memory has the following findings: The number of write and erase cycles can be no direct conclusions about the durability or reliability. Unlike traditional hard drives, there is no direct correlation between the memory cells and the sectors of the file system. Generally, the flash controller distributes write operations evenly across all the flash memory cells. The data in the cells, which are occupied by rare changed data, such as operating system and programs, now and redeployed to come so again less worn cells. At the same time, all memory cells are refreshed regularly, so that creeps over time no data loss due to loss of charge in the memory cells.
All procedures and mechanisms that fall prolong the life of the memory cells within the definition of “wear-leveling”.

Generally one can assume that keep SSDs in everyday desktop operating longer than specified by the manufacturers. You can read more about this over at Flash Storage vs SSD 3,000 to 100,000 memory or erase cycles are perfectly adequate for most applications. Especially when loaded with wear-leveling all memory cells evenly and so the service life is extended.