A microSD card measures 15 by 11 by 1 millimetres, weighs roughly a quarter of a gram, and can hold a terabyte. That density comes from a transistor deliberately engineered to leak. Every bit is stored as trapped charge. A flash cell is a MOSFET with a second gate buried inside the oxide, electrically isolated on every side. To write, the controller drives the control gate to 15 or 20 volts and electrons cross a barrier roughly 7 nanometres thick by Fowler-Nordheim tunnelling, a quantum effect rather than conduction. Once through, they have nowhere to go. That trapped charge shifts the threshold voltage of the transistor, and reading a cell means measuring where the threshold now sits. - SLC stores one bit per cell across two charge states - TLC stores three bits across eight states - QLC stores four bits, sixteen distinguishable levels in the same well Modern cells hold only a few hundred electrons. Losing a few dozen can flip a bit, which is why every card runs LDPC error correcti...