Solid-state media

Memory cards

Electrically Erasable Programmable Read-Only Memory (EEPROM or E2PROM) is made from floating gate Metal Oxide Semiconductor (MOS) transistors. The gate of regular MOS transistors controls the flow of current from the drain to the source regions of the transistor (refer to the class on Solid-state devices and Analog Circuits). The floating gate sits between the control gate and the conducting channel of the transistor.

A floating gate transistor

The floating gate is insulated from the rest of the transistor (floating) and retains a charge after the writing mechanism injects voltage. The charge on the floating gate can remain for up to 10 years.

Charge is removed from the floating gate by a mechanism called quantum tunneling. This process causes molecular damage to the transistors, and floating gate transistors can only be erased about 100,000 times. However, if a floating gate transistor is erased three times per hour, 365 days per year, it will last ten years. The expected life of Flash memory is about the same as a hard disk.

EEPROM technology is now usually called Flash Memory and sometimes non-volatile RAM (NVRAM). It is used where read-only memory (ROM) was traditionally used. This allows for the changing of firmware without replacing the physical memory chips.

Flash memory is also used as a replacement for hard disk drives. The circuitry of Flash drives (whether a memory card, USB flash drive, or a solid-state drive) presents the flash memory such that it looks like a hard disk to the computer. The technology is entirely different and handled differently, but it looks like a hard disk to the computer. They are formatted with a file system and can be made bootable for computers that can boot from such devices.

Most solid-state media uses a technology called NAND Flash. In NAND Flash, the floating gate transistors are arranged to resemble a NAND gate (refer to the class on digital circuits).

New NAND devices were developed in late 2012 that are "self-healing." Tiny heaters built into each memory cell melts the Silicon on a molecular level and return it to its new state. When (if ever) this technology hits the market, flash memory with an unlimited life will be available.

Some NAND memory uses multi-level logic. Instead of storing one bit per transistor as one voltage or another, data can be stored as one of several voltages. Four voltage levels are equivalent to two bits, and eight are. Therefore, multilevel logic can store up to three bits per transistor. Multilevel logic is slower and sometimes less reliable than single-level logic. The fastest solid-state media uses single-level logic.

Before removing flash media, use the "safely remove hardware" or “eject” option.

Some claim to have rigorously tested whether you can remove a memory card without using the above options. They report no damage to the card under worst-case scenarios. Nevertheless, using the “safely remove hardware” option is safer. At least give the computer a few seconds to flush the write-behind cache to the card first.