Flash memory
Since the mid-1960s, when the great potential of metal-oxide semiconductors (MOS) technology to lead to high-density and high-performance became known, chip makers were thinking hard of how to solve the main problem associated with the MOS memory concept - its volatility.
System (main) memory is called RAM (Random Access Memory) and is most responsible for handling operating and application needs. Unfortunately, RAM loses all data when the system is powered down. Thus, RAM belongs to the so-called Volatile Memory.
Another type of memory is called ROM, or Read Only Memory. ROM has data permanently stored even when the system is off. ROM represents Nonvolatile Semiconductor Memory (NVSM) technology.
The first solutions for the volatility problem came in 1967 and in form of the floating gate concept and the metal-nitride-oxide-semiconductor (MNOS) memory device. A UV-erasable Programmable ROM (PROM / EPROM) of 1 Kb became widely available in 1971. In 1983, 16 KB EEPROM were introduced.
There are three types of ROM: PROM (Programmable ROM), EPROM (Erasable PROM), and EEPROM (Electrically Erasable PROM).
PROM can not be changed after being recorded once. EPROM allows manufacturer to remove one set of instructions in exchange for another set. EEPROM is upgraded by 'flashing' the chip and is called Flash ROM. This is done with the help of a special software program that stores new data on the chip. The term Flash refers to the fact that the entire content of the memory chip is erased in one step.
Basic principles
The basic operating principle of non-volatile semiconductor memory devices is the storage of electric charges in the gate insulator of a MOSFET.
The EPROM was the first NVM that could be electrically programmed by the user and erased later. EPROM devices work on the floating gate cell concept and use (at present) hot-electron injection. EPROM devices are not electrically erasable and require UV light to erase the memory. Both operations - programming and data erasing can be done by the user equipped with EPROM programmer and UV light source.
Flash memory is called so because the entire sections of the microchip are erased at once or (flashed). The erasure is caused by Fowler-Nordheim tunneling.
Flash memory is used in the digital cameras and camcorders (still image), digital cellular phones, PC cards for laptop PCs, audio digital recorders, PDAs, GPS, answering machine sets.
Flash memory cards lose power when they are disconnected (removed) from the PC, yet the data stored in it is retained for indefinitely long time or until it is rewritten.
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