Solid state (flash) memory: trapping electrons

A hard disk stores bits as magnetism on a spinning platter. Flash memory has no moving parts at all.

Instead, each bit is a tiny room where you can lock some electrons in, or let them out. Locked in, they stay put for years, power or no power.

• Solid state drives (SSDs) in laptops and PCs
• USB flash drives and SD cards
• the storage in phones, tablets and cameras

Most use NAND flash: chips holding a huge grid of memory cells, managed by a controller chip.

"Describe how data is stored in solid state (flash) memory." Usually 3 to 4 marks.

Often followed by "Give reasons why an SSD is used instead of a hard disk in [device]". Both are below.

1. Flash memory is a grid of cells; each cell is a floating gate transistor, with a control gate above a floating gate, separated by thin layers of insulator.
2. To write, a voltage is applied to the control gate.
3. Electrons pass (tunnel) through the thin insulating layer and become trapped on the floating gate.
4. Because the floating gate is surrounded by insulator, the electrons stay trapped with the power off: flash is non-volatile.
5. Whether a cell's floating gate holds trapped electrons or not represents a 0 or a 1. The controller reads it by testing whether the transistor conducts.
6. To erase, a voltage pulls the electrons back out. This is done a whole block of cells at a time.

Step 3 is the forgotten one. "Flash stores charge" isn't enough; say where the charge goes and why it stays.