Flash memory (floating gate)

Non-volatile memory that remembers without power by trapping electrons on an island of conductor buried inside a transistor — the 'floating gate'. Electrons are forced onto this insulated island during programming (by quantum tunnelling or hot-electron injection) and stay there for years because nothing is wired to drain them away. Their presence shifts the transistor's threshold voltage, so when you later try to read the cell, it conducts or doesn't — and that becomes your stored 1 or 0. It's like sealing a charge in a glass jar: no power needed to keep the lid shut.

Flash trades flexibility for density. You can't rewrite a single bit cheaply — cells are erased only in big blocks (apply a high reverse voltage to tunnel the electrons back out), and each erase slowly damages the insulating oxide, so a cell wears out after thousands to hundreds of thousands of erase cycles. NAND flash strings cells in series for maximum density (the storage in SSDs and USB drives), while NOR flash wires them in parallel for fast random read (used to hold boot firmware). Modern chips store multiple bits per cell (MLC/TLC/QLC) by sensing several distinct charge levels, and stack cells vertically in 3D layers — over 200 today.