The Flash Memory Cell

A flash memory cell is essentially a MOSFET with an additional, electrically isolated gate electrode – the floating gate. It sits between the regular control gate and the channel, fully enclosed by silicon dioxide: separated from the channel below by the thin tunnel oxide (a few nanometers), and isolated from the control gate above by a thicker interpoly oxide.

Because the floating gate has no external electrical contact, charge once placed on it remains stored for years – this is the basic principle of non-volatile storage.

To program a cell (placing charge on the floating gate), either hot-electron injection or Fowler-Nordheim tunneling is used, depending on the cell type: with hot-electron injection, a high drain-source field accelerates electrons strongly enough that some gain sufficient energy to overcome the oxide barrier and become trapped on the floating gate.

Fowler-Nordheim tunneling instead uses a very high electric field across the tunnel oxide, allowing electrons to tunnel through the barrier quantum-mechanically – this mechanism is typically used for erasing, where charge is removed from the floating gate again. The stored charge measurably shifts the cell's threshold voltage, allowing "0" and "1" to be distinguished during readout.