Flash is programmed by increasing the probability that electrons will tunnel onto the floating gate and erased by increasing the probability they will tunnel back off. Those probabilities are never zero. Multiply that by time and the number of cells, and the probability you don’t end up with bit errors gets quite low.
The difference between slc and mlc is just that mlc has four different program voltages instead of two, so reading back the data you have to distinguish between charge levels that are closer together. Same basic cell design. Honestly I can’t quite believe mlc works at all, let alone qlc. I do wonder why there’s no way to operate qlc as if it were mlc, other than the manufacturer not wanting to allow it.
The difference between slc and mlc is just that mlc has four different program voltages instead of two, so reading back the data you have to distinguish between charge levels that are closer together. Same basic cell design. Honestly I can’t quite believe mlc works at all, let alone qlc. I do wonder why there’s no way to operate qlc as if it were mlc, other than the manufacturer not wanting to allow it.