The previous parts of the "Introduction to SSD" series told the reader about the history of the emergence of SSD drives, interfaces for interacting with them, and popular form factors. The fourth part will talk about storing data inside drives.
In previous articles in the series:
Storage of data in solid state drives can be divided into two logical parts: storing information in a single cell and organizing the storage of cells.
Each cell in the SSD stores one or more bits of information. Various information is used to store information. physical processes. When developing solid-state drives, the following physical quantities were worked out for encoding information:
- electric charges (including Flash-memory);
- magnetic moments (magnetoresistive memory);
- phase states (memory with a change in the phase state).
Memory based on electric charges
Encoding information using a negative charge underlies several solutions:
- UV erasable ROM (EPROM);
- electrically erasable ROM (EEPROM);
- Flash memory.
Each memory cell is floating gate MOSFET, which stores a negative charge. Its difference from a conventional MOS transistor is the presence of a floating gate - a conductor in a dielectric layer.
When creating a potential difference between the drain and the source and the presence of a positive potential on the gate, current will flow from the source to the drain. However, if there is a sufficiently large potential difference, some electrons “break through” the dielectric layer and end up in the floating gate. This phenomenon is called .
A negatively charged floating gate creates an electric field that interferes with the flow of current from the source to the drain. Moreover, the presence of electrons in the floating gate increases the threshold voltage at which the transistor turns on. With each “write” to the floating gate of the transistor, the dielectric layer is slightly damaged, which imposes a limit on the number of rewriting cycles for each cell.
Floating gate MOSFETs were developed by Dawon Kahng and Simon Min Sze of Bell Labs in 1967. Later, when examining defects in integrated circuits, it was noticed that due to the charge in the floating gate, the threshold voltage that opens the transistor changed. This discovery prompted Dov Frohman to start working on memory based on this phenomenon.
Changing the threshold voltage allows you to "program" the transistors. Transistors with a charge in the floating gate will not turn on when the gate voltage is greater than the threshold voltage for a transistor without electrons, but less than the threshold voltage for a transistor with electrons. Let's call this value reading voltage.
Erasable Programmable Read-Only Memory
In 1971, an Intel employee, Dov Frohman, created a rewritable transistorized memory called Erasable Programmable Read-Only Memory (EPROM). Recording in memory was carried out using a special device - a programmer. The programmer applies a higher voltage to the chip than is used in digital circuits, thereby "writing" electrons into the floating gates of transistors where necessary.
EPROM memory was not supposed to clean the floating gates of transistors electrically. Instead, it was proposed to expose the transistors to strong ultraviolet radiation, the photons of which energize the electrons with the energy needed to leave the floating gate. For ultraviolet access deep into the chip, quartz glass is added to the case.
Froman first presented his EPROM prototype in February 1971 at the Philadelphia Solid State Microcircuits Conference. Gordon Moore recalled the demonstration: “Dov demonstrated the bit pattern in EPROM memory cells. When the cells were exposed to ultraviolet light, the bits disappeared one by one until the unfamiliar Intel logo was completely erased. … The beats were disappearing, and when the last one disappeared, the whole audience burst into applause. Dov's article was recognized as the best at the conference." — Article translation
EPROM memory is more expensive than previously used "one-time" read-only memory (ROM), but the ability to reprogramme allows you to debug circuits faster and reduce the development time of new hardware.
Reprogramming ROMs with ultraviolet light was a significant breakthrough, however, the idea of electrical rewriting was already in the air.
Electrically Erasable Programmable Read-Only Memory
In 1972, three Japanese: Yasuo Tarui, Yutaka Hayashi and Kiyoko Nagai introduced the first Electrically Erasable Programmable Read-Only Memory (EEPROM or E2PROM). Later, their scientific research will become part of the patents for commercial implementations of EEPROM memory.
Each EEPROM cell consists of several transistors:
- floating gate transistor for bit storage;
- transistor to control the read-write mode.
This design greatly complicates the wiring of the electrical circuit, so EEPROM memory was used in cases where a small amount of memory was not critical. EPROM was still used to store large amounts of data.
Flash memory
Flash memory, which combines the best features of EPROM and EEPROM, was developed by Japanese professor Fujio Masuoka, an engineer at Toshiba, in 1980. The first development was called NOR Flash and, like its predecessors, is based on floating gate MOSFETs.
NOR flash memory is a two-dimensional array of transistors. The gates of the transistors are connected to the word line, and the drains are connected to the bit line. When voltage is applied to the word line, transistors containing electrons, that is, storing a “one”, will not open and current will not flow. By the presence or absence of current on the bit line, a conclusion is made about the value of the bit.
Seven years later, Fujio Masuoka developed NAND flash memory. This type of memory is distinguished by the number of transistors on a bit line. In NOR memory, each transistor is directly connected to a bit line, while in NAND memory, the transistors are connected in series.
Reading from the memory of this configuration is more complicated: the necessary word line is supplied with the voltage necessary for reading, and all other word lines are supplied with a voltage that turns on the transistor, regardless of the level of charge in it. Since all other transistors are guaranteed to be open, the presence of voltage on the bit line depends on only one transistor, to which the read voltage is applied.
The invention of NAND-type flash memory makes it possible to significantly compact the circuit, accommodating more memory for the same size. Until 2007, the amount of memory was increased by reducing the manufacturing process of the chip.
In 2007, Toshiba introduced a new version of NAND memory: Vertical NAND (V-NAND), also known as 3D NAND. This technology emphasizes the placement of transistors in several layers, which again allows you to compact the circuit and increase the amount of memory. However, schema compaction cannot be repeated indefinitely, so other methods have been explored to increase the storage capacity.
Initially, each transistor stored two levels of charge: a logic zero and a logic one. This approach is called Single-Level Cell (SLC). Drives with this technology are highly reliable and have a maximum number of rewrite cycles.
Over time, it was decided to increase the volume of drives at the cost of wear resistance. So the number of charge levels in a cell is up to four, and the technology was called Multi Level Cell (MLC). Then came Triple Level Cell (TLC) и Quad Level Cell (QLC). In the future, a new level will appear - Penta Level Cell (PLC) with five bits in one cell. The more bits fit in one cell, the larger the volume of the drive at the same cost, but less wear resistance.
Compacting the circuit by reducing the process technology and increasing the number of bits in one transistor negatively affects the stored data. Despite the fact that EPROM and EEPROM use the same transistors, EPROM and EEPROM can store data without power for ten years, while modern Flash memory can "forget" everything in a year.
The use of Flash memory in the space industry is difficult, since radiation adversely affects the electrons in the floating gates.
These problems prevent Flash-memory from becoming the undisputed leader in the field of information storage. Despite the fact that Flash-based storage devices are widely used, research is underway on other types of memory that do not have these shortcomings, including the storage of information in magnetic moments and phase states.
magnetoresistive memory
Encoding of information by magnetic moments appeared in 1955 in the form of memory on magnetic cores. Until the mid-1970s, ferrite memory was the main type of memory. Reading a bit from this type of memory led to demagnetization of the ring and loss of information. Thus, after reading a bit, it had to be written back.
In modern developments of magnetoresistive memory, two layers of a ferromagnet separated by a dielectric are used instead of rings. One layer is a permanent magnet, and the second layer changes the direction of magnetization. Reading a bit from such a cell is reduced to measuring the resistance when current is passed: if the layers are magnetized in opposite directions, then the resistance is greater and this is equivalent to the value "1".
Ferrite memory does not require a constant power supply to maintain the recorded information, however, the magnetic field of the cell can affect the "neighbor", which imposes a limitation on circuit densification.
According to Unpowered Flash SSDs should retain information for at least three months at an ambient temperature of 40°C. Designed by Intel promises to keep data for ten years at 200°C.
Despite the complexity of development, magnetoresistive memory does not degrade during use and has the best performance among other types of memory, which does not allow writing off this type of memory.
Phase change memory
The third promising type of memory is memory based on a phase transition. This type of memory uses the properties of chalcogenides to switch between crystalline and amorphous states when heated.
Chalcogenides - binary compounds of metals with the 16th group (6th group of the main subgroup) of the periodic table of Mendeleev. For example, CD-RW, DVD-RW, DVD-RAM and Blu-ray discs use germanium telluride (GeTe) and antimony (III) telluride (Sb2Te3).
Research on the use of phase transition for information storage was carried out in 1960s year Stanford Ovshinsky (Stanford Ovshinsky), but then it did not come to commercial implementation. In the 2000s, interest in the technology arose again, Samsung patented a technology that allows bit switching in 5 ns, and Intel and STMicroelectronics increased the number of states to four, thereby doubling the amount possible.
When heated above the melting point, the chalcogenide loses its crystalline structure and, on cooling, turns into an amorphous form, characterized by high electrical resistance. In turn, when heated to a temperature above the crystallization point, but below the melting point, the chalcogenide returns to a crystalline state with a low level of resistance.
Phase change memory does not need to be "recharged" over time, and is also not susceptible to radiation, unlike memory on electric charges. This type of memory can retain information for 300 years at a temperature of 85°C.
It is believed that the development of Intel, technology 3D Crosspoint (3D XPoint) uses phase transitions to store information. 3D XPoint is used in Intel® Optane™ Memory drives, which are claimed to be more durable.
Conclusion
The physical design of solid state drives has undergone many changes over more than half a century of history, however, each of the solutions has its drawbacks. Despite the undeniable popularity of Flash-memory, several companies, including Samsung and Intel, are exploring the possibility of creating memory based on magnetic moments.
Reducing cell wear, compacting cells, and increasing overall drive capacity are areas that are currently promising for the further development of solid state drives.
You can test the coolest NAND drives and 3D XPoint today in our .
In your opinion, will the technology of storing information on electric charges be replaced by others, for example, quartz disks or optical memory based on salt nanocrystals?
Source: habr.com