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RAM cells need constant refreshing, however NAND FLASH cells can retain data for years together. What makes this possible ?

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Not all RAM cells need constant refreshing. –  Ignacio Vazquez-Abrams Apr 28 at 9:49

3 Answers 3

What makes NAND memories non-volatile is that the logic state is retained in a physical state that doesn't depend on the supply voltage.

For EEPROM memories in general (including Flash), this is a charge trapped in a floating gate.

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As clabacchio pointed out, NAND flash memories can stores information in an array of memory cells made from Floating-gate MOSFET. Also some newer flash memory(included NAND memories) has a new technology. it's Multi-level cell.

I think these links will be very helpful and useful:

How Does Flash Memory Work?

HOW DOES FLASH DRIVE WORK?

How Flash Memory Works

Note: I know that they have spoken about Flash memory in these links but the technology that they have spoken about it is NAND type.

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Also note that data retention is usually guaranteed for a period, e.g. 10 years. Also note that bit flipping can occur. SD cards for example usually have ECC built in to correct this, to some extent.

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While true this doesn't really answer the question of how they work, as opposed to the implications of using FLASH memory. –  PeterJ Jun 4 at 1:39

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