If I correctly understand, RAM and SSDs both use flash memory, though SSDs use NAND flash memory. RAM loses its data when unpowered. Why don't SSDs lose data the same way?


The short answer is that they aren't the same: RAM is volatile memory, whereas flash memory is non-volatile.

More specifically, the RAM in your PC is dynamic RAM, or DRAM. Each bit of DRAM consists of a transistor and a capacitor, which stores a bit of charge. The charge in a capacitor decays quickly, and so it must be continuously refreshed.

There also exists static RAM, SRAM. This uses latches to store data, so it doesn't have to be continuously refreshed, but power still needs to be applied. (Also, it is more complex and expensive, since it takes several transistors to make a latch for each bit of memory.)

Flash memory, on the other hand, uses floating-gate transistors for storage. NOR flash utilizes them somewhat differently than NAND, but the principle is the same. NAND flash is used for SSDs, because although it's slower, it's much more dense and cheaper to mass-produce.

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    So basically RAM has capacitors that save the data but lose their charge when there is no electric current causing the data to be lost while SSDs use floating gate transistors that save data without having to be constantly refreshed/have an electric current? – Tyler H Nov 13 '13 at 22:57
  • @user2417339 you might be better off asking about the specifics on a electronics site. I am sure they could go into a lot deeper into the details. – Zoredache Nov 13 '13 at 23:23
  • @user2417339 Correct. Although I think the stored charges in flash memory will decay over time too - it's just that the time is measured in years instead of minutes. I wouldn't use a SSD for long-term backups, unless I rewrote it at least once a year or so. – user55325 Nov 14 '13 at 0:36

On Why does RAM have to be volatile? , MSalters wrote :

DRAM uses small capacitors which leak. Bigger capacitors would leak less, be less volatile, but take longer to charge.

Flash uses electrons which are shot at high voltage into an isolator. The energy barrier is so high that you can't get them out in a controlled way; the only way is to clean out an entire block of bits. Full Answer

Read full thread there.It gives vast informations into how various RAM devices work.

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