Flash memory is a type of long-term memory for computers in which the contents can be reprogrammed or deleted electrically. Compared to the Electrically Erasable Programmable Read Only Memory, actions on it can be performed in blocks that are located in different places. Flash memory costs a lot less than EEPROM, which is why it has become the dominant technology. Especially in situations where stable and long-term data storage is required. Its use is allowed in a wide variety of cases: in digital audio players, photo and video cameras, mobile phones and smartphones, where there are special android applications on the memory card. In addition, it is also used in USB flash drives, traditionally used to save information and transfer it between computers. She gained some fame in the world of gamers, where she is often involved in a slip to store data on the progress of the game.
general description
Flash memory is a type that can store information on its board for a long time without using power. In addition, one can note the highest speed of data access, as well as the best resistance to kinetic shock in comparison with hard drives. It is thanks to such characteristics that it has become so popular for devices powered by batteries and accumulators. Another indisputable advantage is that when the flash memory is compressed into a solid card, it is almost impossible to destroy it using some standard physical methods, so it can withstand boiling water and high pressure.
Low Level Data Access
The method of accessing data stored in flash memory is very different from that used for conventional types. Low-level access is provided through the driver. Regular RAM immediately responds to calls to read and write information, returning the results of such operations, and the flash memory device is such that it takes time to think.
Device and principle of operation
At the moment, flash memory is common, which is created on single-transistor elements having a "floating" shutter. Thanks to this, it is possible to provide a higher data storage density in comparison with dynamic RAM, which requires a pair of transistors and a capacitor element. At the moment, the market is replete with a variety of technologies for building the basic elements for this type of media, which are developed by leading manufacturers. They are distinguished by their number of layers, methods of recording and erasing information, as well as the organization of the structure, which is usually indicated in the name.
At the moment, there are a couple of types of chips that are most common: NOR and NAND. In both, the storage transistors are connected to the discharge buses - in parallel and in series, respectively. In the first type, the cell sizes are quite large, and there is an opportunity for quick random access, which allows you to run programs directly from memory. The second is characterized by smaller cell sizes, as well as fast sequential access, which is much more convenient if you need to build block-type devices where large amounts of information will be stored.
In most portable devices, an SSD uses the NOR memory type. However, devices with a USB interface are becoming increasingly popular. They use memory like NAND. Gradually she displaces the first.
The main problem is fragility
The first samples of mass-produced flash drives did not please users at high speeds. However, now the speed of writing and reading information is at such a level that you can view a full-length movie or run an operating system on a computer. A number of manufacturers have already demonstrated machines where the hard drive is replaced by flash memory. But this technology has a very significant drawback, which becomes an obstacle to replacing existing media with this media. Due to the characteristics of the flash memory device, it allows you to erase and record information in a limited number of cycles, which is achievable even for small and portable devices, not to mention how often this is done on computers. If you use this type of media as a solid state drive on a PC, then a critical situation will come very quickly.
This is due to the fact that such a drive is built on the property of field-effect transistors to store an electric charge in a "floating" gate , the absence or presence of which in the transistor is considered as a logical unit or zero in the binary system. Writing and erasing data in NAND-memory is carried out by means of tunneling electrons by the Fowler-Nordheim method with the participation of a dielectric. This does not require high voltage, which allows you to make cells of minimum size. But it is this process that leads to the physical deterioration of the cells, since the electric current in this case causes the electrons to penetrate the gate, overcoming the dielectric barrier. However, the guaranteed shelf life of such a memory is ten years. Microcircuit wear is not due to reading information, but because of operations to erase and write it, since reading does not require changing the structure of cells, but only passes electric current.
Naturally, memory manufacturers are actively working to increase the service life of this type of solid-state drives: they are aimed at ensuring uniform recording / erasing processes across the array cells, so that some do not wear out more than others. For uniform load distribution, software paths are mainly used. For example, to eliminate this phenomenon, the technology of “wear leveling” is used. At the same time, data that is often subjected to changes is moved to the address space of the flash memory, therefore, recording is performed at different physical addresses. Each controller is equipped with its own alignment algorithm, so it is very difficult to compare the effectiveness of certain models, since implementation details were not disclosed. As the volumes of flash drives become more and more every year, it is necessary to apply more and more efficient work algorithms to ensure the stability of the devices.
Troubleshooting
One of the most effective ways to deal with this phenomenon was the reservation of a certain amount of memory, which ensures load uniformity and error correction through special logical redirection algorithms to replace physical blocks that arise during intensive work with a flash drive. And to prevent loss of information, failed cells are blocked or replaced with backup ones. This software distribution of the blocks makes it possible to ensure uniformity of load, increasing the number of cycles by 3-5 times, but this is not enough.
A memory card and other types of such drives are characterized by the fact that a table with a file system is entered in their service area. It prevents the failure of reading information at a logical level, for example, with an incorrect shutdown or with a sudden interruption in the supply of electrical energy. And since the system does not provide caching when using removable devices, frequent overwriting has the most detrimental effect on the file allocation table and directory contents. And even special programs for memory cards are not able to help in this situation. For example, in a single call, the user overwrites a thousand files. And, it would seem, only once did I use the blocks for recording where they are located. But the service areas corresponded with each update of any file, that is, the allocation tables went through this procedure a thousand times. For this reason, the blocks occupied by this particular data will fail in the first place. The technology of "wear leveling" works with such blocks, but its effectiveness is very limited. And it doesn’t matter which computer you use, the flash drive will fail just when the creator provides it.
It is worth noting that an increase in the capacitance of microcircuits of such devices led only to the fact that the total number of recording cycles was reduced, as the cells became smaller, therefore, less and less voltage was required to dissipate the oxide walls that isolate the “floating gate”. And here the situation is such that with an increase in the capacity of the devices used, the problem of their reliability began to worsen more and more, and the class of the memory card now depends on many factors. The reliability of such a solution is determined by its technical features, as well as the current market situation. Due to fierce competition, manufacturers are forced to reduce production costs in any way. Including due to the simplification of the design, the use of components from a cheaper set, the weakening of control over manufacturing and other methods. For example, a Samsung memory card will cost more than lesser-known counterparts, but its reliability raises far fewer questions. But here it’s difficult to talk about the complete absence of problems, and from devices of completely unknown manufacturers it is difficult to expect anything more.
Development prospects
If there are obvious advantages, there are a number of disadvantages that characterize the SD-memory card, preventing the further expansion of its scope. That is why there is a constant search for alternative solutions in this area. Of course, first of all, they try to improve the existing types of flash memory, which will not lead to any fundamental changes in the existing production process. Therefore, there is no doubt in only one thing: companies engaged in the manufacture of these types of drives will try to use their full potential before switching to another type, while continuing to improve traditional technology. For example, a Sony memory card is currently being released in a wide range of volumes, so it is assumed that it will continue to be actively sold.
However, today, on the threshold of industrial implementation, there is a whole range of alternative data storage technologies, some of which can be implemented immediately when a favorable market situation occurs.
Ferroelectric RAM (FRAM)
The technology of the ferroelectric principle of information storage (Ferroelectric RAM, FRAM) is proposed with the aim of increasing the potential of non-volatile memory. It is generally accepted that the mechanism of operation of existing technologies, which consists in overwriting data during readings with all modifications of the basic components, leads to some containment of the speed potential of the devices. And FRAM is a memory characterized by simplicity, high reliability and speed in operation. These properties are now characteristic of DRAM - non-volatile random access memory existing at the moment. But here will also be added the possibility of long-term data storage, which is characterized by an SD memory card. Among the advantages of this technology, it is possible to distinguish resistance to different types of penetrating radiation, which may be in demand in special devices that are used to work in conditions of increased radioactivity or in space exploration. The information storage mechanism is implemented here through the application of the ferroelectric effect. It implies that the material is able to maintain polarization in the absence of an external electric field. Each FRAM memory cell is formed by placing an ultrathin film of ferroelectric material in the form of crystals between a pair of flat metal electrodes forming a capacitor. Data in this case is stored inside the crystal structure. And this prevents the effect of leakage of charge, which becomes the reason for the loss of information. The data in the FRAM-memory is stored even when the power supply is disconnected.
Magnetic RAM (MRAM)
Another type of memory, which today is considered very promising, is MRAM. It is characterized by fairly high speed performance and non-volatility. The unit cell in this case is a thin magnetic film placed on a silicon substrate. MRAM is a static memory. It does not need periodic dubbing, and information will not be lost when the power is turned off. At the moment, most experts agree that this type of memory can be called next-generation technology, since the existing prototype demonstrates fairly high speed performance. Another advantage of this solution is the low cost of the chips. Flash memory is manufactured in accordance with a specialized CMOS process. A MRAM chip can be produced according to a standard process. Moreover, materials can be those that are used in conventional magnetic media. To produce large batches of such microcircuits is much cheaper than everyone else. An important property of MRAM is the ability to instantly turn it on. And this is especially valuable for mobile devices. Indeed, in this type the value of the cell is determined by the magnetic charge, and not by the electric one, as in a traditional flash memory.
Ovonic Unified Memory (OUM)
Another type of memory that many companies are actively working on is a solid state drive based on amorphous semiconductors. It is based on the phase transition technology, which is similar to the principle of recording on ordinary discs. Here, the phase state of a substance in an electric field changes from crystalline to amorphous. And this change persists even in the absence of stress. Such devices differ from traditional optical disks in that the heating occurs due to the action of an electric current, and not a laser. Reading in this case is due to the difference in the reflectivity of the substance in different states, which is sensed by the drive sensor. Theoretically, such a solution has a high data storage density and maximum reliability, as well as increased speed. Here, the indicator of the maximum number of rewriting cycles is high, for which a computer is used, the flash drive in this case is several orders of magnitude behind.
Chalcogenide RAM (CRAM) and Phase Change Memory (PRAM)
This technology is also based on phase transitions, when in one phase the substance used in the carrier acts as a non-conductive amorphous material, and in the second it serves as a crystalline conductor. The transition of the storage cell from one state to another is due to electric fields and heating. Such chips are characterized by resistance to ionizing radiation.
Information-Multilayered Imprinted CArd (Info-MICA)
The devices based on this technology work on the principle of thin-film holography. Information is written as follows: first, a two-dimensional image is formed, transmitted to the hologram using CGH technology. Data is read by fixing the laser beam at the edge of one of the recorded layers that serve as optical waveguides. The light propagates along an axis that is parallel to the plane of the layer, forming at the output an image corresponding to the information recorded previously. Initial data can be obtained at any time thanks to the reverse coding algorithm.
This type of memory compares favorably with semiconductor due to the fact that it provides high recording density, low power consumption, as well as low carrier cost, environmental safety and protection against unauthorized use. But such a memory card does not allow rewriting of information, therefore it can only serve as a long-term storage, replacement of paper media or an alternative to optical discs for distributing multimedia content.