Flash memory is a non-volatile, programmable, chip-based high-speed storage technology that retains data even when power is lost. There are two types of flash memory: NAND and NOR.

What is NAND?

Flash memory stores data in an array of memory cells defined by metal-oxide-semiconductor floating-gate transistors (FGTs), which store binary data 1 or 0. Every transistor has two gates, a control gate and a floating gate.

Unlike DRAM memory, NAND is capable of storing data even when power is turned off. When the flash memory is powered down, the metal oxide semiconductor of the floating gate transistor (FGT) supplies power to the memory cell, maintaining data integrity. An array of NAND cells stores 1 to 4 bits of data.

Common NAND Flash Types:

Common NAND types are SLC, MLC, TLC, and 3D NAND. 3D NAND with vertically stacked cells has higher performance and density.

SLC: Single-level cell, SLC NAND stores one bit of information per cell. SLC NAND has the highest endurance compared to similar products, and SLC NAND is the most expensive flash memory on the market today.

MLC: Multi-level cell, MLC NAND stores two bits of information per cell. Compared with SLC NAND, the amount of unit storage data is increased, thereby reducing the cost of unit data storage, but reducing the endurance rate.

TLC: Triple-level cell, TLC NAND stores three bits per cell, reducing cost and durability and increasing capacity. It has a low endurance rate, is the cheapest type of flash memory, and is mainly used in consumer electronics.

QLC: Quad Level Cell, QLC NAND stores four bits per cell, creating higher density and high capacity storage devices. QLC NAND has lower read latency and is more suitable for data read operations for applications such as machine learning, artificial intelligence (AI), big data, etc., but it is less durable and cheaper.

3D NAND: In order to increase the capacity of NAND devices, 3D NAND increases capacity and reduces costs by stacking multiple layers of storage cells vertically. 3D NAND devices achieve higher density and lower power consumption, faster read and write speeds, and more High durability.

What is NOR flash memory?

NOR Flash is the first available flash memory. The cells on the NOR flash memory chip are arranged in parallel to each other, so the reading efficiency is high, but the writing speed is slow. It is often used in the application scenario where the code is written once and read many times.

The difference between NAND and NOR:

Market share: The usage of NAND flash far exceeds that of NOR flash.
Read performance: The read speed of NOR flash is faster than that of NAND flash.
Writing and erasing performance: Contrary to reading performance, the writing and erasing speed of NAND chips is faster than that of NOR devices.
Endurance: Compared to NAND memory, NOR flash memory has higher endurance.
Storage density: NOR memory has a lower density than equivalent NAND flash memory chips.

Application scenario:

NOR Flash memory is usually used in consumer electronics, Internet of Things, automotive and industrial fields, while NAND is used in digital cameras, smartphones, tablet computers, memory cards, solid state drives and computers.

Although NAND flash memory is currently the most popular type of flash memory, NOR flash memory still has its own technical advantages. At present, SK hynix has announced the release of 321-layer 4D NAND samples, and it is expected to achieve mass production in the first half of 2025. With the continuous development of flash memory technology, we will be able to use flash memory products with better performance and more affordable prices in the future.

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