Electronic – Why must flash memory be written/erased in pages/blocks

flashmemorynon-volatile-memorytransistors

Title says it all.

I'm trying to understand the workings of flash memory technologies, at the transistor level. After quite some research, I got good intuitions about floating-gate transistors, and how one injects electrons or remove them from the cell. I'm from a CS background, so my understanding of physical phenomena like tunneling or hot electron injection are probably quite shaky, but still I'm comfortable with it. I also got myself an idea about how one reads from either NOR or NAND memory layouts.

But I read everywhere that flash memory can only be erased in blocks units, and can only be written to in page units. However, I found no justification for this limitation, and I'm trying to get an intuition about why it is so.

Best Answer

The best answer I've found to your question is covered at How Flash Memory Works where it says:

The electrons in the cells of a flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip, and then rewrites it.

I don't understand why the "in-circuit wiring" allow for bit level programming (switching from 1 to 0) but it might be related to the different way the transitions 1 to 0 is performed (programming via hot injection) compared to 0 to 1 transition (erasing via Fowler-Nordheim tunnelling).

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