where and how is data actually stored on a Micro SD or other form of small memory device?
In: Technology
It’s Flash memory. And flash memory is made of MOSFETs with a floating gate. Explaining what exactly that is isn’t very ELI5 friendly, because it’s a deeply technical matter. But it goes something like this:
A MOSFET is a type of transistor.
A transistor is an electrically controllable switch with 3 terminals: the “drain”, which is where it takes power from, the “source”, where power is emitted if it’s in the “on” setting, and the “gate”, which determines whether it’s on or off. In the water analogy, it’s a [faucet](https://i2.wp.com/randomnerdtutorials.com/wp-content/uploads/2016/03/water_analogy-1.png?resize=508%2C273&ssl=1).
In a normal MOSFET for it to be on, the gate must be constantly powered. Think of it like one of those [public bathroom faucets](https://images-na.ssl-images-amazon.com/images/I/51VYcdSbneL._AC_SL1000_.jpg) that turns off by itself. If you stop pressing it, you stop getting water.
In a floating gate MOSFET you can imagine that there’s a can on top of the button. If the can is full, then it’s always pressing on the button, and if it’s off, then it never is. For technical reasons, your best way of filling or emptying this can is to stab it with a syringe, and so every time you do that it gets a new hole in it, and gradually wears out and becomes useless. Which is why Flash has an endurance limit of how many times it can be written before it fails.
In SLC flash (which is rarely seen anymore) there’s one bit per MOSFET: either water flows or doesn’t. In MLC flash there are several levels of water possible in the can, which produces different possible flow intensities. This stores more data, but it’s trickier to work with.
But this is a very, very surface level explanation.
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