FLASH MEMORY
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Flash memory is a type of EEPROM that has a grid of columns and rows with a cell that has two transistors at each intersection. A thin oxide layer separates the two transistors from each other. One of the transistors is known as the floating gate and the other as the control gate.
The floating gate's only link to the row, or 'wordline', is through the control gate. As long as this link is in place, the cell has a value of '1'. This value can be changed to '0' using the Fowler-Nordheim tunneling process.
Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10-13 volts, is applied to the floating gate. The charge comes from the column, or bitline, which enters the floating gate and drains to ground.
This charge causes the floating gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate.
A special device called cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of '1'. When the charge passing through the gate drops below the 50 percent threshold, the value changes to '0'. A blank EPROM has all of the gates fully open, giving each cell a value of '1'.
The electrons in the cells of a flash memory chip can be returned to the normal '1' state by application of an electric field (a higher-voltage charge). Flash memory uses in-circuit wiring to apply the electric field to the entire chip or to the predetermined sections known as blocks. The targeted area of the chip is erased, which can be rewritten. Flash memory works much faster than traditional EEPROM's because instead of erasing one byte at a time, it erases a block or the entire chip and then rewrites it.
An ideal memory sub-system is one that has high density, can be read fast and preserve data in non-volatile condition, and is easy to program/reprogram and cost-effective. Different memory technologies meet one or more of these requirements very well, but have certain limitations that prevent the product from becoming a genuine solution, especially in newer applications.
Flash memory is a non-volatile memory combining the advantages of EPROM/EEPROM, ROM, and DRAM. The table in the next page lists the features of each of these memories. There may be differences between the specific technologies used by different manufacturers but their basic principle is same. For example, Intel Flash memory use NOR gates, While Samsung memory uses NAND gates.
The technology used by Intel is further classified based on the core memory cell. The first technology is the original single-bit/cell flash memory that allows a single bit of information to be stored in each cell (1=erased and 0=programmed).
The second and the most recent technology uses a multi-level cell structure, for example, Intel StrataFlash^TM memory. This technology allows two bits of information to be precisely stored in a single transistor. Programming a cell (charge placement) and reading (sensing) must be precisely controlled in order to have four states within a single transistor. According to Intel, the multi-level cell flash memory employs a reliable NOR-based architecture and is ideally suited for high-density applications.
The standard modes of operation for flash components are read, program, and erase. A few examples of flash memory applications are given below:
1. Replacement for OTPROM and EPROM in microcontrollers.
2. BIOS chip of the computer.
3. CompactFlash (most often found in digital cameras).
4. SmartMedia (most often found in digital cameras).
5. Memory Stick (most often found in digital cameras).
6. PCMCIA Type-I and Type II memory cards (used as solidstate disks in laptops).
