Different Types of NAND Flash

Welcome back to Semiconductors 101, semiconductor science in plain and easy English. Last time on ‘Semiconductors 101 Lesson 1, The Difference Between HDDs and SSDs’, we examined the difference between HDDs and SSDs. This time, we’ll be looking into ‘NAND flash memory’, a type of non-volatile memory.

NAND flash plays a critical role in data storage in SSDs, so it might be helpful to brush up on our last episode before we begin. Let’s see what types of NAND flash there are, and examine the characteristics of each through analogy. The cell: the studio were data lives

To understand the types of NAND flash and their characteristics, it’s important to first understand the data we store and the space this data is stored, called a cell. Put simply, data is ‘a person’, and a cell is ‘a small room within a semiconductor.’ In this case, storing data is like tiny people moving into these tiny rooms we call cells. How does reading stored data work, then? Reading data would be like doing a headcount of how many people are living in this small room inside a semiconductor. The more residents there are in the small room, the longer this headcount would take. Likewise, the more data there is stored in the cells, the longer the process of reading the data will take. Understanding the Different Types of NAND Flash with an Analogy Now, we will examine the different types of NAND flash. NAND flash is categorized into four different types, depending on the data storage method: single-level cells (SLCs), multi-level cells (MLCs), triple-level cells (TLCs), and quad-level cells (QLCs).

In the first, SLC (Single Level Cell), a single data is stored in a single cell, as the name suggests, in the form of a ‘1’ or a ‘0’. This data uses the whole room for itself, and therefore the rent (or price) is high. That said, in SLC NAND flash the ‘head count’ process takes the least amount of time. In an MLC (Multi Level Cell), a partition is placed in a room that was originally inhabited by just one person. Now the room houses two. Two pieces of data are stored in a single cell, in the form of ‘00, 01, 10, 11’ and so on. The rooms here are smaller, making MLC cheaper than SLC. But data processing speed is slow compared to SLC.

Three pieces of data are stored in a single cell in TLC (Triple Level Cell), while four pieces of data are stored in a QLC (Quadruple Level Cell). TLC and QLC are capable of storing large data volumes, but with more data per room, their data processing speeds are relatively slow compared to SLC and MLC. Now, here is a question: Does this mean that the fastest SLC is the NAND flash with the best performance? Simply put, no. This is because different users have different purposes for using SSD. SLCs are fast but it is difficult and expensive to increase data storage capacity for each cell. In theory, QLCs are four times cheaper than SLCs because they divide one cell (which stores one bit of data) into four cells. So if you’re looking for an ultra high-performance SSD, an SLC or MLC NAND flash SSD is the way to go. But if it’s economy you’re after, your best options will be TLC or QLC NAND flash SSDs. V-NAND: high-rise accommodation to reduce interference Samsung began mass production of a new type of NAND flash in 2013: ‘3D V-NAND.’ What is V-NAND, and why was its development necessary? As with other types of NAND flash memory, think of the data in 3D V-NAND as ‘people’, and the cells as the ‘rooms’ these people live in.

When trying to accommodate as many people as possible in a limited plot of land, homes need to be built as small and as close together as possible. This inevitably causes trouble among the neighbors, including noise complaints. NAND flash has a two-dimensional structure, and the need to improve performance and increase capacity has resulted in smaller cells arranged closer together. This began to cause data interference. Switching the AC on next door would cause the AC in your room to turn off, and when the neighbor switched channels on his TV, your own TV would do the same. To reduce this kind of interference, Samsung devised a way to stack cells vertically as in a high-rise apartment building, instead of cramming as many single-story units as possible into a limited plot of land. This gave rise to ‘V-NAND’, the world’s first three-dimensional cell structure NAND flash memory. The first generation of Samsung V-NAND started with 24 stories in 2013, but by the 6th generation in 2019, we reached 1xx stories. While high-rise apartments can accommodate large numbers of residents, it sometimes takes forever to reach the higher stories in a normal elevator. Likewise, V-NAND initially suffered from slower read times as the number of stories increased. In response, Samsung came up with ‘channel holes’ linking the vertically stacked stories like high-speed elevators. The solution boosted both NAND flash capacity and speed. Today we’ve looked into the types of NAND flash. Just remember: data is ‘people’, and cells are the ‘rooms’ they live in. See you in the next lesson for more on semiconductor tech!