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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 asingle
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 cellconsists 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-storingelement of the memory cell. Flash memory is not the first
memory device to use a floating gate to store information. The uverasable 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 thefloating 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 thevoltage 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 oneeach. 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 floatinggate 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.
A typical MLC consists of a single transistor with direct electrical
connections to its gate, source, and drain that allow very precise control of

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