Flash Memory
Flash Memory
is a semiconductor memory device that is electrically erasable and
programmable in sections of memory called
'blocks'.
In a flash memory, a whole block of
memory cells can be erased in a single action, or in a 'flash,'
which is how this device got its name. Flash memory
is
non-volatile,
i.e., it can retain its memory contents even if it is powered off.
A basic flash
memory cell consists of a MOSFET that was modified to include an isolated
inner gate between its external gate and the silicon (see Figure 1).
This inner gate is known as a
'floating gate',
which is the
data-storing
element of the memory cell. Flash memory is not the first memory device to
use a floating gate to store information. The uv-erasable
EPROM,
which preceded the Flash memory, is also a 'floating gate' memory device.
Figure 1. A Typical Flash
Memory Cell
Data is
stored in a flash memory cell in the form of
electrical
charge
accumulated
inside the floating gate. The amount of charge stored in the floating
gate depends on the voltage applied to the external gate of the memory
cell that controls the flow of charge into or out of the floating gate.
The data contained in the cell depends on whether the voltage of the
stored charge exceeds a specified threshold voltage Vth or not.
Intel has
developed flash memory technology wherein memory cells can hold
two or more bits of
data instead of just one each. The trick is to take advantage of the
analog nature of the
charge stored in the memory cell and allow it to charge to several
different voltage levels. Each voltage range to which the floating
gate can charge can then be assigned its own digital code. Thus, a
2-bit cell can distinguish 4 distinct voltage ranges, while a 3-bit one
can distinguish 8 of them. Intel
calls this technology
'Multi-Level Cell (MLC)"
technology.
<Proceed to Page 2>
See Also:
What is a
Semiconductor?;
EPROMs; SRAMs; DRAMs
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