SLC, MLC, TLC, QLC, and PLC: The Most Detailed Comparison

NAND Flash Memory is used everywhere including SSDs, flash drives, smartphones, digital cameras, tablets, memory cards, and various consumer electronics. Because of its high-density storage, fast read/write operations, high durability, and low power consumption, NAND Flash is preferred everywhere for permanent data storage.

This memory is the most popular type of memory in almost every permanent storage solution. The biggest reason is its scalability and the lower cost-per-gigabyte as compared to other types of Flash memories. However, this scalability comes at a cost of performance and reliability. But, that is a topic for another day.

Today, we are going to do a detailed comparison between SLC, MLC, TLC, QLC, and PLC NAND Flash. This topic really excites me whenever I write about it. So, I would make it interesting for you here as well.

Working of NAND Flash Memory

In the NAND flash memory cells, millions and billions of floating gate MOSFETs are combined in a circuit. Because the floating gate transistor can hold electric charge inside its floating gate even in the absence of power, it is the basic building block for the NAND Flash. SSDs with 3D NAND use the charge trap flash for data storage.

These cells are then combined into bigger groups i.e. strings, pages, and blocks. The smallest readable and writeable unit becomes the page and a block becomes the smallest erasable unit.

We just talked about the simplest type of NAND Flash i.e. the SLC NAND Flash. The manufacturers then customized it to store multiple bits per cell thus increasing the storage space in the same size as NAND chip and reducing the price.

SLC NAND flash remained a dream for normal consumers because data centers and enterprise customers remain the only candidates to afford this expensive type of Flash. To understand the workings of the NAND Flash memory, we must understand the workings of the floating gate transistors. But, before we discuss the floating gate transistor, we must know a little about the normal transistor.

Normal Transistor (As a switch)

Normal Transistor has two main applications in electronic circuits i.e. switching and amplifying. Imagine having a pipe that carries the water from one end to the other and this flow is controlled by a value. This is how the transistor’s working can be understood. This is the symbol of a normal NPN transistor. There is another type called the PNP transistor. The functioning of these both is the same but the difference is in the potential of the voltages i.e. negative and positive.

Working on a Transistor

The input current or voltage can be applied at the collector and some output can be received at the emitter if the gate is turned on with the gate threshold voltage. For the NPN, transistor, the gate voltage will be positive and negative for the PNP transistor. We would not get too much into the formation of transistors but just understand the basic working principle. The gate is acting as a switch. If you look at this simplified diagram, the gate voltage works as a switch to enable the flow of current and also adds to the main input voltage at the output.

The most important thing about this transistor is that it is switched On at a special level of voltage called its threshold voltage.

This is all about the normal transistor. It needs power at both the gate and source to get some output at the collector. Nothing extra.

Working of a Floating Gate transistor (NAND Flash cell)

A floating gate transistor is the same as the normal transistor but has some extra layers of floating gate and control gate. Also, there are two insulating layers around the floating gate which holds the charge.

Each cell is connected to three common lines word line (gate), bit line (drain), and source line (source). An empty cell without charge resembled the bit value (1) while a cell with charge inside its floating gate resembles (0).

Now, if we want to store this series of bits (1, 0, 1, 1, 0, 1), we have to program our cells in that way. SSDs don’t write data directly on the cells. The controller first clears a block of NAND flash (usually of 64KB or 2MB) and then it is prepared for writing.

Once, a block is cleaned, in the appropriate series of cells, this data will be written. When cleared, all the cells will be free of any charge and hence set to 1. So, the controller will keep the first cell as it is.

To program the second cell for “0”, it would select the common word line (gate) for that transistor. A very high voltage will be applied to the control gate. This high voltage will create a very strong electric field across the gate layer. Some electrons from this electric field would jump from the tunnel oxide and reach the floating gate. Now, because the floating gate is insulated, the trapped charge will stay there. After this programming, the controller also check the threshold voltage if the cell is properly programmed.

Interpretation of the Stored Bits inside the floating gate

When the controller has to read this cell, the same process will be applied but it will just check the threshold voltage to interpret the presence or absence of the charge i.e. 0 or 1.

Above, in the normal transistor, we discussed that the gate will have a threshold voltage after which the transistor is turned On. In this case, that threshold voltage will vary depending on whether there is a charge stored in the floating gate. For SLC, this presence and absence of voltage will be considered bit value 1 or 0. In the case of multiple bits per cell, this threshold can have multiple levels to indicate different bit values.

Data Storage in SLC (Single-Level Cell)

Everything we discussed about the floating gate transistors and their mechanism was for the single-level cells. SLC is the simplest type of NAND flash memory with simple circuitry and controller programming.

Take one cell from the array of millions of cells in your SLC SSD. If you get into it’s working and check its floating gate, it will either have a charge or not. That’t it. The condition of this single cell will determine whether the system will read it as 0 or 1.

If there is a charge near around 100%, it is considered 0 while a near empty cell will indicate a bit value of 1.

When we combine these cells together, we can easily program them and store binary values which can be then converted into HEXADECIMAL code and then other codes that the computer can understand. The same will go in reverse for storing the data. The codes will be converted into the bit values and stored in these cells one by one. The SSD or storage controller will keep track of all the data locations in the Flash transition layer.

Data Storage in MLC (Multi-Level Cell)

In MLC, a single cell is used to store two binary bits. Now, because there can be 4 different unique combinations for two binary bits, a multi-level cell is programmed to store 4 different levels of voltage levels.

Generally, a cell full with almost 100% charge will indicate a bit value of “10” while an empty cell will indicate a bit value of “11”. Between them lies the “01” and “00” bit values. For storing these values, the controller just pushes the adequate amount of charge inside the cell. For reading that cell, it is just about checking the difference in threshold voltage.

These calculations and voltage/current sensors are built inside the controllers which will do their jobs as they are designed. But, the basic idea is to harness the floating gate transistor’s capability to hold different amounts of charge which then can be sensed back when required.

This all sounds good because we are now capable of storing 11 in a single cell instead of using two different cells and storing 1 and 1 separately. The storage density has been increased by 100%. But, this all comes with a price of reduced performance, less reliability, and high latency.

See, if we have to read and write different voltage levels in a single cell, the circuitry will surely become much more complex. The controller will take longer to read and write data because there is additional time required to check and create different voltage levels. Also, the data retention time is reduced because the difference between voltage levels decreases. It also results in higher error rates because of the same reason. The power consumption will also increase.

NAND Flash cells come with limited program/erase cycles which are amplified as we start to use single cells for multiple operations. You can read more about P/E cycles here.

Just like SLC, MLC drives have largely been phased out from the consumer market. Mostly, you will find the MLC drives in the enterprise drives and server environments. MLC drives are surely a little expensive but they have the best endurance and performance compared to the upcoming flash types.

Data Storage in TLC (Triple-Level Cell)

The TLC Flash will store 8 different voltage levels in a single cell. This increases the storage density by 50% from MLC and 300% from SLC. Now, a single cell can hold three bits from “000” to “111”. Because the possible unique values between these two numbers have increased to 8, there is a need for more fine voltage levels with less room for errors but higher chances of them.

Another difference is that, in TLC, an empty cell denotes 000 while a full cell indicates 111. This is to reduce the risks of overlapping, especially when the cell ages. Now, because the room for taking risks is less, the algorithms must work much more intensively.

So, the TLC becomes much more complex as compared to the MLC. The data retention capability and read/write performance will be reduced because the controller has to perform many more operations for both reading and writing. However, companies can easily put more storage capacities in the same size as NAND flash chips. This opens up room for higher profits but lower endurance and performance.

Now, if we combine multiple TLC cells and use them for storage, we might be able to fit more bits inside lesser cells but the time taken for calculations and ensuring the accuracy is surely going to be higher. This would result in slower performance, higher latency, more heat, and more chances for errors. On the other hand, we get cheaper storage drives and higher storage densities. The drives now have to rely more on the algorithms to prevent wear, and errors, and to prevent data loss.

TLC is the most common NAND flash in the consumer market. It offers a great balance of performance and price. So, whenever you have to choose between the TLC and QLC, it is better to go for the TLC one.

Data Storage in QLC (Quad Level Cell)

In QLC NAND Flash, a single cell is used to store 4 bits. This increases the density by 33.3% compared to the TLC NAND and 400% compared to the SLC. You can store a series of four bits so there are now 16 possible voltage levels. The chances of errors and complexity increase a lot more. However, the storage density has increased a lot. This allows the manufacturers to offer very cheap storage drives but with a compromise to performance and reliability.

Because a single cell is going to be used a lot more, it will have a lot fewer P/E cycles (will be discussed below). The risks of overlapping voltages increase as well along with the demands for much more complex circuitry. The algorithms will have to work with much more power to make sure the data is retained without errors.

For both reading and writing the data, much complex programming is required. Also, the heat generation will be very high even under normal loads.

The same cell that was storing just a bit of information in SLC is now storing 4 bits of data in QLC. With just two cells, we can store a byte of data. However, the data retention is reduced,d and the amount of wear is increased. The ECC has to work even harder to get things going without data corruption.

Data Storage in PLC (Penta Level Cell)

The Penta-Level cells are in development but they are going to reach the markets really soon. With this technology, the storage density is going to increase by 500% compared to SLC and 25% as compared to QLC. There will be 32 different voltage levels in a single cell and as a result a lot of errors. To work in the market, the manufacturers will have to employ advanced Error correction codes, wear leveling, temperate control, and many other things.

Detailed Comparison of SLC, MLC, TLC, QLC, and PLC NAND Flash Cells

The current situation in consumer storage market

TLC has got over most of the consumer-level SSD market. Even in smartphones and other portable storage devices, you will mostly find the TLC NAND flash. However, whenever you see a storage device with cheap storage, it is surely going to be the QLC.

Most of the time, you will find drives with TLC NAND flash and QLC NAND flash at around the same price points. In those cases, the TLC drives will surely be missing some important features, mostly DRAM and encryption. If it doesn’t have a DRAM, you may have to experience a little poor performance in productivity tasks and the snappiness of the system will be missing.

The QLC drives will generally have lesser TBW as compared to the TLC drives. You may also get more storage capacity in QLC for the same price point but still, if you want your drive to last longer, you should go for the TLC one. If your work includes heavy usage of drives, especially for important data, you should definitely choose the TLC drives.

Conclusion

SLC, MLC, TLC, QLC, and PLC, are just the methods to increase the storage density by using the same physical footprint in solid state storage devices. These devices could be smartphones, flash cards, SSDs, or pen drives.

The basics remain the same. There are billions of floating gate MOSFET employed inside chips and complex controllers are designed to access and write them. There is heavy software running in the background, controlling all the things from interacting with the operating system to keeping track of all the data.

However, these methods for using a single cell for storing multiple bits are now reaching their saturation point because the room for discrete voltage is going less and less. However, Penta-Level Cell NAND flash still excites me a lot. Let’s see how it goes.

Thanks for reading!