Why is erasing of a single block in NAND flash usually much faster than erasing a single block in NOR flash?

For my current understanding: If all cells of the block share the same substrate and if all Wordlines and Bitlines are powered simultaneously, the time should be the same. Maybe, there could be a slight difference because of the fact that NOR cells are usually physically larger than NAND cells. However, when comparing the information in the two documents Link1 (NOR), Link2 (NAND) there are mentioned around 800ms for erasing a 64kB NOR block, while only 2ms for 128kB NAND block. Where does this huge difference in erase speed come from?

  • 1
    \$\begingroup\$ Much has already been written. No need to repeat a common question. \$\endgroup\$
    – Kartman
    Commented Oct 5, 2021 at 12:55
  • \$\begingroup\$ I already searched extensively for an answer to this question without success. So - if there has already been written a lot on exactly this question - please give a hint to one of those sources. \$\endgroup\$
    – gott18
    Commented Oct 5, 2021 at 12:59
  • \$\begingroup\$ Took me 10 seconds to find this article \$\endgroup\$
    – Finbarr
    Commented Oct 5, 2021 at 13:01
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    \$\begingroup\$ I alread knew this article which only says "Erase operations in NAND Flash are straightforward while in NOR Flash, each byte needs to be written with ‘0’ before it can be erased. This makes the erase operation for NOR Flash much slower than for NAND Flash." without any details on why the cells need to be written to "0" before erasing. \$\endgroup\$
    – gott18
    Commented Oct 5, 2021 at 13:04
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    \$\begingroup\$ Then ask the question you actually want an answer to, not the one you already know the answer to, \$\endgroup\$
    – Finbarr
    Commented Oct 5, 2021 at 13:56

1 Answer 1


Even that the responses here were not really helpful, I was able to find the answer on my own: After further search I found the document https://www.dialog-semiconductor.com/sites/default/files/an500.pdf

It describes why NOR flash (different to NAND flash) needs a so-called "pre-program" before the erase to avoid "over-erased" cells. Over-erased cells would negatively influence the reading of NOR (because of the architecture of how the cells are interconnected) The pre-program seems to be the most time consuming part at a NOR flash erase.


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