How do solid state drives (SSD) store and retrieve data?
In: Technology
The basic element inside a solid state storage device like an SSD is called a floating gate transistor. More on that in a moment. Each of these devices used to store a single yes or no called a bit: 1 or 0. Now each of these devices stores a few yes or nos. Typically 3: 000, 001, 010, 011, 100, 101, 110, or 111. This increase in questions that can be asked to a single device is why you can have really big SD cards and SSD drives.
That and scaling. The size of one of these devices is on the order of 40 nm. Your hair is about 200 um or 200,000 nm across.
How do these devices work?
If you have ever rubbed your hair on a balloon and attracted your hair to it, you have seen something called static electricity. On that surface you have trapped some electrons. That is possible because the rubber is an insulator. On your hair of you have left the absence of electrons. This allows there to be a force between the absence and the extra electrons. It is this force that makes you hair stick up opposing gravity. The force is described by an electric field.
Floating gates on transistors trap charge on them too just like the balloon, only it stays a lot longer because the layer of insulator the charge is trapped in is very isolated.
This charge can attract mobile charges into a region of the transistor called the “channel”. The channel is physically directly under the gate. Think of the charges as a liquid, say water. If the channel is full of water then it conducts, if it is empty of water is does not. It conducts better if there is more water. So depending on how much charge you store on the floating gate you will get more or less conduction.
If we measure how well it conducts we can measure some levels and assign those levels values: 000, to 111.
How do we get the charge on the floating gate? Do we have little hairs we rub on the gate? No. What we do is we use another gate. This one is attached to a chunk of metal which allows us to flood it with electrons. If we put enough electrons on it, it will create such a strong electric field to the channel that some of the very crowded metal will jump to the floating gate, this is called “tunneling” it is governed by a branch of physics called quantum mechanics. We make the other gate crowded by putting a high voltage on the metal gate.
The reason flash memory or SSDs wear out is that sometimes the electrons jumping to the floating gate hit things instead of jumping between the atoms they smack into them and break chemical bonds. When they do this they change the properties of the insulator that lets the floating gate store charge for a long time so some of the electrons escape just like they normally do from the balloon into the air (due to humidity). This makes the floating gate transistor “forget” the information that was stored on it over time as the high level of 111 becomes 110 which is the wrong answer, not what you stored. If the memory is not reliable to remember then its bad/dead.
Typical flash memory can only be written a few 1000 times now before it starts to forget too quickly because everything is so small and fragile.
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