Samsung MZ-V6P512 User Manual download pdf (Page 12)

Although it may all look the same, all NAND is not created equal: SLC, 2-bit MLC, 3-bit MLC (also called TLC), synchronous,

asynchronous, ONFI 1.0, ONFI 2.0, Toggle 1.0, Toggle 2.0. To the uninitiated, all of this looks like nonsense. As the

SSD market gets more complex, however, so does your buying decision. This paper will help to lay down some of the

differences between various types of NAND technology, with the ultimate goal of making you a savvier SSD shopper.

A Brief Introduction to NAND Flash

NAND Flash memory stores data in an array of memory cells made

from floating-gate transistors. Insulated by an oxide layer are two

gates, the Control Gate (CG, top) and the Floating Gate (FG, bottom).

Electrons flow freely between the CG and the Channel (see diagram

to the right) when a voltage is applied to either entity, attracted

in the direction to which the voltage is applied. To program a cell,

a voltage is applied at the CG, attracting electrons upwards. The

floating gate, which is electrically isolated by an insulating layer,

traps electrons as they pass through on their way to the CG. They

can remain there for up to years at a time under normal operating

conditions. To erase a cell, a voltage is applied at the opposite side

(the Channel) while the CG is grounded, attracting electrons away from the floating gate and into the Channel.

To check the status of a cell, a high voltage is applied to the CG. If the floating gate holds a charge (electrons are trapped

there), the threshold voltage of the cell is altered, affecting the signal emanating from the CG as it travels through to

the Channel. The precise amount of current required to complete the circuit determines the state of the cell. All of this

electrical activity effectively wears out the physical structure of the cell over time. Thus, each cell has a finite lifetime,

measured in terms of Program/Erase (P/E) cycles and affected by both process geometry (manufacturing technique)

and the number of bits stored in each cell. The complexity of NAND storage necessitates some extra management

processes, including bad block management, wear leveling, garbage collection (GC), and Error Correcting Code (ECC), all

of which is managed by the device firmware through the SSD controller.

SLC vs. 2-bit MLC vs. 3-bit MLC NAND

NAND technology has been naturally progressing with the needs and expertise of the industry. In the simplest terms, the

data stored in NAND flash is represented by electrical charges that are stored in each NAND cell. The difference between

Single-Level Cell (SLC) and Multi-Level Cell (MLC) NAND is in how many bits each NAND cell can store at one time. SLC

NAND stores only 1 bit of data per cell. As their names imply, 2-bit MLC NAND stores 2 bits of data per cell and 3-bit MLC

NAND stores 3 bits of data per cell.

Advantages of MLC NAND

The more bits a cell stores at one time, the more capacity that fits in one place, thus reducing manufacturing costs

and increasing NAND manufacturing capacity – a phenomenon called “bit growth.” This phenomenon has allowed

NAND technology to penetrate a continually greater number of usage applications at increasingly higher capacities

over the years.

NAND technology’s first home was in external storage devices (e.g. USB memory devices) at very modest capacities. As

the technology matured, NAND found applications in a host of digital devices, including digital cameras, MP3 players, and

mobile phones. Having proven itself a strong and durable performer, the technology made its way into consumer and

finally enterprise solid state storage devices (SSDs). NAND’s rise in popularity and usefulness was directly a result of the

NAND Basics

Understanding the Technology Behind Your SSD

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Samsung MZ-V6P512 User Manual Page 12