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What Is 3D NAND and How Does It Work?

What Is 3D NAND and How Does It Work?

What Is 3D NAND?

3D NAND is also known as vertical NAND (V-NAND). It’s a type of non-volatile flash memory in which the flash memory cells in a transistor die are stacked vertically to increase storage density. The more layers of cells you can stack on a single transistor die without significantly compromising data integrity, the greater your storage density will be.

How Does 3D NAND Work?

A typical flash memory chip works by storing data as voltage states within an electrical circuit. To understand how 3D NAND works, it’s important to first understand the inner workings of a NAND cell.

What does NAND stand for?

NAND stands for “NOT AND,” which refers to the Boolean operator or logic gate that governs the internal circuit of a NAND cell. The NAND operator produces a FALSE value only if both inputs are TRUE.

Basics of flash memory operation

The most basic NAND cell is a transistor composed of two gates:

  • A control gate, which is electrically connected to the rest of the circuit, allowing the cell to be programmed.
  • A floating gate, which is electrically isolated from the circuit, allowing it to store charge without power.

The floating gate is sandwiched between two isolation layers, with the control gate on top and the channel linking source and drain below.

To program a NAND cell, a voltage needs to be applied to the control gate, which allows electrons in the channel to overcome the threshold voltage of the first isolation layer and tunnel into the floating gate. When the floating gate is charged, data is effectively stored, and the cell’s binary value is set to zero.

To erase the cell, a high voltage needs to be applied across the source and drain to induce a negative voltage at the control gate. This allows the electrons trapped in the floating gate to tunnel back across the bottom isolation layer into the channel, effectively erasing the cell and setting its binary value to 1.

Why stack NAND cells vertically?

As cell size decreases so too does the distance between cells on a single transistor plane. This can be especially problematic when you consider that flash memory works by storing voltage states within a single transistor cell. Electrons leaking out of the cells wouldn’t be very good for persistent memory storage. Stacking NAND cells vertically into layers offers several advantages, most notably the ability to space out transistor cells to avoid interference from adjacent cells. This improves the stability and longevity of the cell.

Where does 3D NAND fit in the evolution of flash?

3D NAND technology has played a major role in helping the industry keep pace with Moore’s Law despite the physical limitations of making cells smaller. Here’s a brief overview of how the technology has evolved over time:

  • Single-level cell (SLC) flash: One bit per cell, two possible voltage states
  • Multi-level cell (MLC) flash: Two bits per cell, four possible voltage states
  • Triple-level cell (TLC) flash: Three bits per cell, eight possible voltage states
  • Quad-level cell (QLC) flash: Four bits per cell, 16 possible voltage states

With each subsequent generation, you increase the number of bits per cell by doubling the number of possible voltage states. As you might imagine, the complexity of dealing with multiple voltage states on a single cell requires greater electrical precision, which can translate into a reduction in the performance and longevity of the NAND device.

As a general rule, SLCs are the fastest and most stable, while QLCs can give you the greatest capacity. That said, it’s important to note that this trade-off between performance and endurance is relative. Flash is still orders of magnitude more performant than HDDs, and as technology improves, so too does the stability of higher-level cells. Fears of endurance limitations of TLC NAND were addressed over time, allowing today’s data centers to enjoy the high performance, reliability, and speed of modern TLC NAND.

What Are the Benefits of 3D NAND?

Now that we’ve covered the basics of how 3D NAND cells work, let’s look at the advantages 3D NAND offers. 3D NAND allows you to:

  • Fit more flash memory cells on a single chip for greater capacity.
  • Take advantage of the dimensional freedom to optimally place cells to avoid interference and electron leaks for greater cell reliability.
  • Store more voltage states, and therefore bits, per individual cell for even greater capacity.

What Are the Disadvantages of 3D NAND?

As we mentioned earlier, managing all those voltage states isn’t easy. The more bits you can cram per cell, the greater the electrical precision required to perform read/write operations reliably.

This reality manifests itself as the following disadvantages:

  • Higher manufacturing costs
  • Trade-off between capacity and the reliability and longevity of the cell

How Reliable Is 3D NAND SSD?

In a one-to-one comparison with 2D NAND, 3D NAND boasts better performance, speed, power consumption, endurance, and cost efficiency. On the other hand, it should be noted that architectures exist that employ stacking layers of 2D NAND in MLC configuration. Such systems can compete with 3D NAND architectures. This is why it’s important to look at the reliability of a total storage system on a case-by-case basis rather than make blanket assumptions based on components alone.

How Pure Storage Leverages 3D NAND to Deliver Custom Solutions for Your Needs

As a pioneer in all-flash storage solutions, Pure Storage® has relied on 3D NAND technology to develop powerful all-flash storage arrays that can compete with the costs of traditional spinning disk drives for a number of applications. These include:

  • FlashArray//X: An all-flash storage area network that leverages TLC NAND to provide high-performance block storage for Tier 0 and Tier 1 applications.
  • FlashArray//C: A capacity-optimized all-flash storage solution that leverages QLC flash to deliver all-flash performance at a cost per capacity comparable to hybrid and HDD storage arrays.
  • FlashBlade®: A scale-out all-flash storage solution that delivers unified fast file and object (UFFO) storage.

Ready to accelerate your data storage with the massive parallelism and speed of 3D NAND storage? See how Pure Storage products and solutions can help you with your data center needs.

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