The NAND flash memory chip is the core of the flash drive. Our information is stored on it. In order to appreciate its technology, we must understand a few concepts.
In layman terms, memory is an object that allows us to “selectively store or selectively retrieve … bits of information” (Cressler 142). There are essentially two basic forms of electronic memory (ibid):
- Hard disk drive memory
- Transistor-based semiconductor memories
The flash drive belongs to the second category of memories. How do we know this? Essentially, the hard disk drive contains movable parts. That’s why our computer’s hard disk drive makes sounds and we can feel it moving when we put our hands on the computer’s metal casing. ‘Transistors’ however operate electrically. This means that we don’t need moving parts and the flash drives becomes more durable and sturdy. Think about it: you can drop a flash drive without worrying that you’ll lose data as compared to dropping a laptop.
There are also two types of electronic memories:
- Volatile (temporary)
- Non-Volatile (semipermanent)
Volatile memory means that it is temporary. Once the power is cut off, the data stored disappears. Non-volatile means that the information saved on the chip will not disappear if there is no longer power running the flash drive. (Think of technologies like the television that requires a constant power source in order for it to operate. If you turn off the power source, we immediately lose the images and sounds.)
Essentially, the flash drive memory is transistor-based and non-volatile. The flash drive, therefore, has no moving parts and information stored on the NAND flash memory chip remains semi-permanently (until you delete/edit the file).
Why is the NAND flash memory called “NAND”?
Simply put, there are two forms of flash memory:
- NOR Flash (arranged in parallel)
- NAND Flash (arranged in serial)
The NAND flash memory derives its name from the way the memory cells are arranged. Because the memory cells are connected in series. As a result, NAND is least flexible as compared to the NOR flash’s parallel connections. Though it is less flexible, NAND is cheaper to manufacture. Thus, the majority of our flash drives utilize NAND flash memory technology.
Below is an image that shows the arrangement of NAND flash memory on the chip:
It looks complex but just pay attention to its connection pattern. As aforementioned, the NAND flash memory cells are connected in series. For our learning purposes, knowing that the flash memory cells’ arrangement is in series is sufficient. The more relevant portion of our understanding lies in how the NAND flash memory cell functions.
In order to understand what happens when we save a file on the flash drive, let’s zoom in on the NAND flash memory cell in the picture below:
Here’s another perspective of the flash memory cell:
In order to appreciate the flash memory technology, we must understand the function of the two transistors known as the “Control gate” and the “Floating gate”.
Flash memory is built on a poly-silicon structure floating-gate (MOSFET – Meta-oxide-Semiconductor Field-Effect Transistor). The MOSFET forms the basis of many memory devices, including the flash drive.
Floating Gate (where the flash drive’s data is stored)
The Floating gate is the place where all our data is recorded.
As its name implies, it floats above the “p-type silicon substrate.” The floating gate is isolated from the substrate by a thin oxide layer of about 10nm thick. This isolation is needed to enable the floating gate layer to store charges (or data). (Khurshudov 231)
The control gate as its name implies, controls the flow of charges between the “Source” and the “Drain”. It might be helpful to think in terms of direction – the charges flow from the Source to the Drain (as its name implies, ‘draining’ the charges).
How the two Gates work to save and delete data:
For simplification purposes, when I talk about “Charges” – think of it as “data” of our programs/files/pictures, etc. that we want to save on our flash drive.
The floating gate, as mentioned, is where all our data/charges are recorded. It is coated with non-conductive material (the thin oxide layer) that serves to protect the data stored on it.
Saving data on the flash drive [writing]:
When we want to save a file on our flash drive, a voltage is applied to the control gate which then sends electrons from the source to start flowing towards the drain. In the process of the flowing, the electrons gain energy to penetrate the oxide layer and gets stored in the floating gate. As a result, the floating gate forms a negative charge (that is, data is recorded on the gate)
Remember, the floating gate is coated with non-conductive material, right? This means that even when we plug out the flash drive, the data is safely stored on the floating gate. Also, remember we talked about the flash drive having non-volatile memory (or semipermanent memory)? Yes, because the floating gate is coated with non-conductive memory, it forms a protective cocoon around our data. As a result, the data will not be lost even when there is no power!
Deleting data on the flash drive [erasing]:
If the charged floating gate contains our data, then deleting the data on the flash drive is achieved by removing the charge from the floating gate. NAND flash memory technology achieves this: by applying high voltage to the source. The positive charge on the source makes the negative-charged electrons move from the negative-charged floating gate to the source. As a result, the charged floating gate which stores our data loses its charge (and data). Our data on the flash drive is, thus, erased/deleted.
Advantages of the NAND flash memory technology:
- Huge amounts of data can be stored on the floating gate.
- The data stored on the floating gate can last for years (some can last for a decade) [non-volatile]
- NAND flash can be erased and rewritten up to 1,000,000 cycles (which is more than what the average user needs) Why is this important? Think of the flash memory as a piece of paper. When you write something on it and you want to erase it, you have to use an eraser and remove the marks. This constant repetition of writing and erasing, eventually wears out the paper. So then, when you have 1,000,000 cycles, you can be ensured that your data is safe and secure!
Data stored on the USB flash drive is secured even without a power source, because of the floating gate’s oxide layer which acts as a safety cocoon. When we save data, we are sending energized electrons/data to the floating gate to be recorded. When we delete data, we are attracting the electrons/data away from the floating gate.
Cressler, J.D. Silicon Earth: Introduction to the Microelectronics and Nanotechnology Revolution. USA: Cambridge University Press. 2009. Print.
Khurshudov, A. The Essential Guide to Computer Data Storage. USA: Prentice-Hall PT. 2001. Print