Why can NAND flash memory cells only be directly written to when they are empty?

Question

I'm trying to understand why you have to erase cells before writing to them with respect to SSDs and how they slow down over time.

Answer 1

The Wikipedia article seems to at least hint at the answer. It appears that "tunnel injection" is used for writing and "tunnel release" for erasing. I'll leave it to the physicists to explain exactly what the implications of that are.

Answer 2

I'm trying to understand why you have to erase cells before writing to them

You don't have to erase a flash memory cell before writing to it. However, you can only write to one entire block of cells at a time. Typically these blocks of cells are at least 128KB in size.

So suppose you are writing a 4KB file to your SSD. Well, you have to write one 128KB block at a time. If there is already data in that 128KB block, the drive firmware has to read the 128KB block into its memory, modify the 4KB section you are writing to, and then write the entire 128KB block back out to the flash memory.