How a few trapped electrons changed the world

Rahul Jairaj | May 2024


In my left hand is a Neolithic stone tool from around 7000 years ago made by hunter-gatherers in what is now Arkansas to help process bison meat and in my right hand is the latest Micron 25001 SSD, built using the world’s first and most advanced 232-layer monolithic 1TB QLC NAND chip capable of storing up to 2 terabytes (TB) of data developed in Boise, Idaho & Singapore. Each represents the pinnacle of monolithic technology of their ages, from the Stone Age to our present Silicon Age – a mere 7 millennia separate these two marvels of human engineering and, to me, serve as a true testament to our species’ unending pursuit of creation and innovation. In this post, I hope to outline how Micron is leading the way in developing some of the world’s most advanced QLC-based solid state drives.

What is QLC NAND and why does it matter?

To truly appreciate the technological leaps in storage technology it helps to get a handle on the basics. NAND flash is a type of non-volatile storage medium that holds onto bits of binary data even when no power is supplied to it. It’s used widely in thumb drives, iPods and almost all modern PCs to store your personal data, the operating system and any applications you might have on it. It does this by modifying the humble MOSFET transistor to trap and hold electrons that represent a logical binary state of either 0 or 1. If each modified transistor or cell held onto one binary bit of data it would be called a single level cell or SLC NAND. So, to hold a trillion bits (1Tb) of data you would need a trillion cells. However, if you could somehow hold 2 bits of data on a single transistor that would be called a multi-level cell or MLC. Similarly, 3 bits of data per cell is called a tri-level cell or TLC, and finally, 4 bits of data is called a quad-level cell – QLC.

The common analogy I use to explain this is by trying to imagine buckets of water. An empty bucket would represent a binary 1 and a bucket filled with water would be a binary 0. This would be an analog for an SLC NAND cell. Now, if you could accurately tell if the bucket was empty, quarter full, half full, three-quarters full, or completely full, you can use it to store more than one logical state of data – MLC! Scale this to tell 16 individual states apart and you get to QLC. Now replace the water in buckets with a few dozen electrons trapped in a nanometer-sized layer of exotic oxides across billions of transistors all fitting in a thumbnail-sized piece of silicon 232 layers tall and capable of storing vast amounts of all the accumulated human knowledge – it’s easy to see how these trapped electrons have truly changed our world. 

There is an excellent online resource from Branch Education that explains this technology in far greater detail – link.

My love affair with NAND flash

My first real job out of college was with Micron working on NAND cell characterization as a product engineer. The first product I worked on was a 64Gb (8GB) MLC planar NAND in the early 2010s. One of my prized possessions in college was a pricy 1GB thumb drive, and the prospect of working on a single chip that was 8 times denser was mind-boggling to me. Over the years, I have had the privilege of working on Micron’s first 3D NAND chip, where multiple layers of these cells are arranged much like a skyscraper, massively increasing the storage density per chip, which also ushered in the transition to TLC technology. In time I had the opportunity to work on Micron’s very first QLC NAND die, and while developing the product datasheet, I was hooked on the technology. The Micron 2500 SSD I hold in my hand is based on Micron’s 4th generation of QLC NAND and has a storage capacity over 2000 times greater and orders of magnitude faster than the thumb drive I owned back in college.

Defining the fastest QLC SSD in the world

My team of product managers defined what the Micron 2500 SSD had to be, and we decided it had to be the fastest and most user-experience-focused SSD in the world, rivaling any TLC-based drives out there.

Broadly speaking there are four knobs to make this happen. 

  1. Monolithic NAND die size
  2. Plane count of the NAND die
  3. Interface speed (ONFi Rate) of the NAND chip and controller
  4. NAND timings

Each of these has its own set of challenges to overcome ranging from cost to engineering complexity.

While the wider industry was hesitant and even skeptical about the technology, the team persevered and was convinced we could make this a reality. Being highly vertically integrated at Micron, the broader teams defined and refined every aspect of this drive from the very start — from the NAND design, SSD architecture, media management and final firmware.

Working closely with our customers and with one of the best engineering teams in the industry, we were able to deliver what previously seemed impossible and is arguably the world’s most advanced QLC SSD.

In a recent blog post, Prasad Alluri outlined how the Micron 2500 ranks against other drives out there How to make a better user experience available to more people | Micron Technology Inc.

Looking into the Future

With the proliferation of AI and the advent of the AI PC (AI in PC: Why not? | Micron Technology Inc.) the need for storage in PCs is only growing. QLC technology allows for a rapid scaling of this capacity demand and in very small form factors all with uncompromising user experience. When I look at the next generation of QLC products in development I can confidently say the best is yet to come. 

Perhaps in 7000 years' time, mankind’s descendants might hold hitherto undreamt of technologies in their right hand, but they could very well hold one of our current-day engineering marvels, the Micron 2500 SSD, in their left.


Micron 2500 NVMe SSD | Micron Technology Inc.


Rahul Jairaj

Rahul Mitchell Jairaj is the Director of Technical Product Managment for Micron's Client SSD Business Unit. He has spent his career working on NAND flash storage at Micron from components engineering to SSD product management. He holds a Masters degree in Semiconductor Device Physics from Clemson University and a bachelor's in electrical engineering. Outside work, Rahul is passionate about collecting fossils and amateur microscopy.