Flash memory is the most up-to-date and common storage medium. This article will tell how many types of flash memory there are and what the differences are.
What is flash memory for?
Flash memory is used for writing and reading information. The absence of mechanical parts contributes to the high speed of data exchange, compact dimensions and weight, and also the resistance to overload and vibration. For this reason, flash memory is used in all areas.
How many types of flash memory exist?
There are 5 types of flash memory, differing in the number of bits stored in each cell:
- SLC – 1 bit.
- MLC – 2 bits.
- TLC – 3 bits.
- QLC – 4 bits.
- PLC – 5 bits.
All 5 types of memory work on the same principle. The chip consists of an array of cells representing floating-gate transistors. In an empty cell, the gate is open, and in an occupied cell, the gate is closed and stores the charge in the form of an electron. This process is regulated by the supply of energy to the cell, and also by the set threshold voltage. If the voltage in the cell is higher than the threshold, the control controller determines the cell is occupied, the voltage below the threshold – the cell is free.
Also, the more bits fit in a cell, the higher the power consumption of the memory and the device as a whole.
Memory with a single-level cell is capable of storing only 1 bit of data per cell. Thanks to this arrangement, the highest write and read speed among all types of flash memory is achieved, due to the two threshold voltage values. Also, the memory has a high durability when rewriting – 50,000 cycles.
At the same time, low data storage density negatively affects the production cost of memory chips. Therefore, SLC memory drives are used primarily in servers and data centers, where high throughput and durability are required.
SLC memory has become more widely used in SSD drives, as a cache. Approximately 5-20% of the total number of cells work in SLC mode, and the rest in TLC or QLC. This solution allows you to copy data to the drive at a high speed until the SLC buffer is full, after which the speed drops several times due to the use of a different type of memory.
Memory with a multi-level cell that allows you to store 2 bits of data in a single cell. For this purpose, 4 threshold voltage values are used. As a result, the speed of memory operation decreased by 1.5-2 times when compared with SLC, since the control controller requires more time to determine the recorded bit. The resource of the cells has decreased to 3000 cycles, because to write data, you need to overwrite all 4 values of the threshold voltage in the cell.
At the same time, the data storage density increased by 2 times. A reduction in the price of memory production has contributed to the spread of MLC drives in the consumer market.
Memory with a three-level cell that allows you to fit 3 bits of data in one cell. For the implementation, it was necessary to enter 8 values of the threshold voltage, which reduced the memory speed by about 1.5 times relative to the MLC type. Simultaneously with the drop in the speed of operation, the resource of cells decreased to 1000-1500 cycles. At the same time, the density of the chips increased by 3 times, which had a positive impact on the cost of production.
Memory with a four-level cell. To accommodate 4 bits in one cell, 16 threshold voltage values are used. As a result, the memory speed is further reduced, as the control controller takes longer to determine the combination of recorded bits. The resource of QLC memory cells has decreased by a factor of two relative to MLC.
The positive side of the QLC type is the highest density of stored data, as well as an additional reduction in production costs.
Memory with a five-level cell. Currently, the PLC chips are not available for sale. At the same time, taking into account the development of previous types of memory, to write 5 bits to a cell, you will need to enter 32 threshold voltage values. As a result, the memory speed and resource are expected to decrease relative to QLC.
Do SD cards use flash memory?
All SD cards are made using flash memory. In cards with an increased resource and service life, MLC memory is used. Most conventional drives use TLC memory with an optimal price, resource, and performance ratio. QLC memory is used in inexpensive cards or fakes.
This article describes in detail how many types of flash memory there are. Currently, MLC, TLC, and QLC types are actively used. Due to the high cost and low density of stored data, the SLC type is only available for the corporate sector and is also used as a cache with a combination of other memory types. And sales of PLC memory are expected in the foreseeable future.
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