Nand Flash: SLC, MLC and TLC, how much do you know?

Part 1 - Principle of Nand Flash Storage

                                       (Basic architecture of a Cell)

As we know that all the data store as “0” and “1” in computers, so in theory if the storage unit can provide two or more recognized status, it can be used for data recording.

1. “Write” Data

When writing data, we need to apply high voltage to the Control Gate, and allowing electrons flows into the N-Channel which is between the Source and Drain, when the current is strong enough, electrons gain sufficient energy and then will cross the SiO2 layer (which is under the Floating Gate), finally being captured by the Cell, we call the whole process as “Tunnel Effect”.

2. Data Stabilization

Once the electrons enter into the Floating Gate, they can’t get away from the SiO2 layer if there is not enough energy, even turn off the power supply, then the statue of the electron capture will continue and could be last for years, of course it depends on the wear condition of SiO2 layer.

3. “Read” Data

When reading data, we also need to apply voltage to Control Gate in order to absorb the electrons in the Floating Gate, but it’s not necessary to apply voltage as high as its Tunnel Injection process. Meanwhile, let the current cross the N-Channel to detect the amount of electron capture in Floating Gate, convert to binary “0” &“1” based on the induction intensity, finally output the data.

4. “Erase” Data

If need to erase data, we won’t apply voltage to the Control Gate but to the P-well Current between the Source and Drain flows through the N-Channel and will react on electrons, then the electrons in the Floating Gate will cross the SiO2 layer again and be attracted in the opposite direction, that’s what we called “Tunnel Release”.

Part 2: Brief Introduction to SLC, MLC and TLC

1. Definition of SLC, MLC, TLC

SLC (Single-Level Cell), means every single Cell only stores 1bit message, data output is based on the Yes or No status of electrons capture (even though it’s in the status of “0”, there are some electrons in the Floating Gate), which is what we call “0” and “1”.

MLC (Multi-Level Cell), means every single Cell can store 2bit message, the number of electrons in Floating Gate is divided into 4 conditions: High, Medium, Low and None, converted to binary and becomes 00, 01, 10 and 11.

TLC (Triple-Level Cell), the status of electrons capture in the Floating Gate is further divided into eight types then converted to binary and becomes 000, 001, 010, 011, 100, 101, 110 and 111, that’s what we call “3bit”.

2. SLC vs MLC

Five aspects (Lifespan, Cost, Power consumption and Error rate) to illustrate the main difference between SLC and MLC: 

2.1 Lifespan

l  Principles

Data storage in the SSDs depends on the amount of electrons captured by the Floating Gate in the Cell. No matter entering or leaving the Floating Gate, the electrons need to cross the SiO2 layer through Tunnel Effect behavior. Actually the thickness of SiO2 layer is only about 10nm, so the atomic bond of SiO2 will be destroyed little by little when doing Tunnel Injection or Tunnel Release operation. Therefore, the speed of data erasure will be slower, that’s because the electrons will occupy the SiO2 layer, and counteract the voltage which applied on the Control Gate, resulting in needing higher voltage to finish its task. By the time when the SiO2 layer is throughout by electrons, the Cell unit is officially dead.

l  Conclusion

As know that the status of electrons capture of SLC only has two types, while MLC has four, in order to achieve at four kinds of status, the electrons in the MLC’s Cell unit need to frequently enter and leave the SiO2 layer, which leads to accelerate the depletion of the Cell, that’s also why SLC has 100,000 P/E cycles while MLC only has 10,000 P/E cycles.

Apperantly, TLC has the least P/E cycles, average 500 to 1000, which is usually used in USB stick.

2.2 Comparison in Speed, Power consumption, Reliability, Error rate and Cost

It’s very easy to , so when under the same controller chip and computational logic, the speed of SLC is the fastest, the lowest Power Consumption, the most stable performance, and the negligible error rate with the help of today’s technology.

A Cell unit of SLC only stores 1bit data, but MLC’s can store 2bit or more, the volume of MLC chip , that’s to say, MLC’s capacity gets higher than SLC (though they are the same chip), so MLC’s price is cheaper.

Since MLC and TLC highly rely on ECC (Error correction Coding), data will dramatically loss once error happens, but good news is that ECC technologies are quite mature till 2012.

2.3 Difference in storage capacity

For single Nnad flash, SLC max. up to 512GB, MLC up to 4TB, and TLC can up to 6TB (end by Aug-01-2017)

Part 3 – Nand Flash Classification

Wafer (see the above picture), each of the above squares can be made into a memory card. Not all the Die on the factory wafer is in superior quality, just like the Nand Flash who also has factory Bad Block. Take Intel and Micron for example (IM Flash Technology – IMFT is jointly owned by Intel and Micron)

(1) Some good Die can be used for high grade Synchronous Nand.

(2) Some will be used as inferior Synchronous Nand after repairing and verifying.

(3) And the low-grade asynchronous Nand might be used for memory cards.

So, do not be blinded by the brand Nand Flash, even Intel, Micron (no offence here), but you can judge the quality from their model number.

Part 4 - Synchronous and Asynchronous

The main difference between synchronous and synchronous Nand flash is their Pin definition, when in synchronous mode, Nand flash doesn’t need to tell the controller that “I’m ready for reading/writing”, but always be “Ready” status and time-saving.

e.g. Intel 25nm Nand flash Classification:

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