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Flash Memory
Today Flash memory is everywhere in Pen drives, Digital camera, MP3
players and in almost all portable electronic devices. Flash memory is a Non
volatile Erasable memory device that can be reprogrammed. Dr.Fujio
Masuoka of Toshiba introduced the Flash memory in 1984 to the world.
Flash memory uses a special type of Electrically Erasable Programmable
Read Only Memory (EEPROM) technology to erase and reprogamme the
datas. Since flash memory is non-volatile, no power is needed to maintain
the information stored in the chip.
The FGMOSFET Technology
Flash memory uses the EEPROM Chip to store datas. This chip has a lattice
of columns called Bit lines and rows called word lines. Each point of
intersection is considered as a Cell. Billions of such intersecting points are
present in the chip. The Cells in the chip are made of MOSFET (Metal
Oxide Semiconductor Field Effect Transistor).The MOSFETs used in flash
memory chip are special kind with a Floating Gate (Floating Gate Metal
Oxide Semiconductor FET- FGMOSFET). The FGMOSFET has two gates
called Control Gate and Floating gate instead of the one in ordinary
MOSFET. The Floating gate is coated with an insulating Oxide layer made
up of Silicon Oxide. The Floating gate is interposed between the Control
gate and the MOSFET channel. Since the Floating gate is insulated, the
electrons trapped in it remain as such for many years. When the Floating
gate traps electrons, it partially cancels the electric field from the Control
gate.
MOSFET
FGMOSFET
This modifies the Threshold voltage of the cell. Depending on this
threshold voltage, the MOSFET channel conducts or remains non
conducting during data read out process. During the read out, an
intermediate voltage is applied to the Control gate to trigger the MOSFET
channel. The current thus flowing in the MOSFET channel is sensed to
create a Binary code. This reproduces the data.
Electronics in FGMOSFET
When a positive potential is applied to the control gate, the negatively
charged electrons pushes through the insulated layer between the p-type
substrates. Some of these electrons passes into the floating gate and get
trapped in the insulating layer. The movement of electrons into the floating
gate occurs through two mechanisms like Fowler – Nordhein Tunneling or
Hot carrier Injection mechanism. In Fowler – Nordhein Tunneling
mechanism, the electrons emitted from the semiconductor move into a
dielectric due to strong electric field. In Hot carrier injection, the electrons or
holes get kinetic energy to pass through the barrier.
Flash Memory Chip
Types of Flash memory
There are two kinds of Flash memory. These are NAND flash memory and
NOR flash memory
NAND (Not AND) flash memory
In Boolean logic operation, NAND means that if any one input is false, the
conclusion is true. The binary coding used in computers are 0s and 1s. So if
any one input is 0, then the output is 1.In NAND flash memory, the cells are
arranged in series like a NAND gate used in ICs. This helps to erase and
write datas very fast with less space. NAND flash memory technology was
introduced by Toshiba in 1989.The NAND memory has blocks having
many pages. Each page has 512 or 2.048 or 4,096 bytes size. Also these
pages have Error correcting codes (ECC) having a size of 1/32 of the data
size. Typical block size of the NAND memory includes 32 pages block size
of 16 KB, 64 pages of block size 128 KB, 64 pages of block size 256 KB,
and 128 pages block size 512 KB.
NOR (Not OR) Flash memory
In Boolean logic, NOR means, if any of the input is 1, the output is 0.In
NOR flash memory, the cells are connected to ground in parallel to the bit
lines .This helps to address each byte individually. But NOR gate writing
and erasing is slower but the readout is very fast. Typical block sizes are 64,
128, or 256 KB.
How to Erase the data
Erasing in NOR flash memory is the resetting of all the codes to 1s. For this
a large potential is applied between the control gate and source of the
MOSFET. The electrons are then pulled from the floating gate. All the cells
in a block are erased together.
How to programme?
Programming of NOR cells, however, can generally be performed one byte
or word at a time. In the erased state, all the cells are in 1 state. The setting
of a cell to 0 state is done in programming. During programming, a higher
voltage around 7 volts is applied to the drain (Bit line) and around 12 volts
to the Source (Word line) of MOSFET. Electrons then move from source to
drain. Some over excited electrons then jump into the floating gate through
Hot carrier injection mechanism. These electrons get trapped in the insulated
gate giving a negative charge to the floating gate. When the power is off,
these electrons remain inside the insulated layer. Thus each call block can be
programmed into millions of zeros and ones at very high speed.
How to read?
When a potential difference is applied between the control gate and drain of
MOSFET, current begins to flow between the two. This voltage is not
sufficient to remove electrons into the floating gate but detects the insulation
or conduction of the floating gate. The cells then sense the current flow and
identifies whether the cell is conducting or not. The conduction is then
identified as 1 and insulation as 0.
Size of Flash memory
To increase the capacity of the flash memory, multiple chips are often
arrayed in devices like Multimedia players and GPS systems. Common flash
drives have a size 2 GB to 8 GB or more.
D.Mohankumar