What Is Flash Memory?
Flash memory is a storage device used to read and write data.
Familiar devices include SD memory cards, SSDs, and USB memory devices. SSD stands for “solid state drive” and is often used for internal storage in notebook computers.
Flash memory can be divided into two main types: NAND and NOR. Each has different read and write speeds and has different uses.
Uses of Flash Memory
The main products in which flash memory is used include USB flash drives, SSDs, and SD cards. SSDs are characterized by faster processing speeds, lower power consumption, and higher resistance to vibration and shock than HDDs. In recent years, SSDs have become a popular choice for internal storage in notebook computers.
USB memory is an external storage device with a USB port for reading and writing data and is convenient for carrying around, while SD cards are a memory card-type storage medium used in a variety of electronic devices such as smartphones, digital cameras, and game consoles.
Principle of Flash Memory
Flash memory can be divided into two main types: NAND and NOR.
The characteristics of the NOR type and NAND type differ due to the differences in wiring.
1. NOR Type
The NOR type has a lower level of integration, but can be accessed in byte units. It is characterized by a slow write speed but high random access speed.
2. NAND Type
In contrast, the NAND type is suitable for high integration and has relatively high write speed. Access is block-based, and random access is slow.
Although NAND flash memory has the advantage of high-density memory cell placement, it has the disadvantage of bit errors. Therefore, an error-correcting code (ECC) circuit must be installed.
An error-correction code is a value assigned when data is read or written. The read data is checked by matching the error-correction code to see if it is really the value written to the flash memory.
Types of Flash Memory
NAND flash memory is classified into the following types based on the characteristics of the cells, which are the units used to store information.
1. SLC NAND
NAND flash memory that can store one bit of information per cell is called SLC (single-level cell) NAND. While it is characterized by high durability, it has the disadvantage of low capacity.
2 MLC NAND
NAND flash memory that can store two bits of information per cell is called MLC (multi-level cell) NAND. It is inexpensive, but has the disadvantage of lower read/write speed and endurance compared to SLC NAND.
Other Information on Flash Memory
Data Storage Method in NAND Type
When storing data in flash memory, the smallest unit of storage is called a cell, which consists of a tunneling insulator and a floating gate on a semiconductor.
Data is stored by storing and releasing electrons at the floating gate. When data is written or erased, electrons penetrate and move through the tunnel oxide film, causing the tunnel oxide film to degrade.
As this tunnel oxide film deteriorates, the cell is unable to store data correctly and reaches the end of its life. The estimated life of an individual cell is 1,000 to 10,000 write and erase cycles.