Flash memory

Flash memory or flash drives also known as SSD (solid state drives as opposed to HDD which have a physical spinning disk) are a kind of non-volatile computer memory. 

How the information is stored

Flash memory stores information in an array of memory cells made from floating-gate transistors. In traditional single-level cell (SLC) devices, each cell stores only one bit of information.

 Some newer flash memory, known as multi-level cell (MLC) devices, including triple-level cell (TLC) devices, can store more than one bit per cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells.

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1 USB Standard, Male A-plug, 2 USB mass storage controller device, 3 Test point, 4 Flash memory chip, 5 Crystal oscillator, 6 LED (Optional), 7 Write-protect switch (Optional), 8 Space for second flash memory chip. image: wikipedia


How data is written and erased

NAND (In digital electronics, a NAND gate (Negated AND or NOT AND) is a logic gate )  flash uses tunnel injection for writing and tunnel release for erasing. This is also known as a quantum mechanical effect called field electron emission known as Fowler–Nordheim tunnelling. 

NAND flash memory forms the core of the removable USB storage devices known as USB flash drives, as well as most memory card formats and solid-state drives available today

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A chip containing 4 NAND logic gates. image: wikipedia


source and more informationhttp://en.wikipedia.org/wiki/Flash_memory#NAND_flash