Flash Memory -
Page 2 of 2
<Back to Page 1>
A typical MLC consists of a
single transistor with direct electrical connections to its gate, source,
and drain that allow very precise control of the charging of the cell's
floating gate. For a multi-level cell to work, it must be able to
deposit charge with precision, sense charge with precision, and store
charge over time.
High-precision charging and
charge sensing are the key
to a MLC's ability to distinguish several charge levels. Table 1 illustrates how a
2-bit multi-level cell assigns digital codes to 4 different charge voltage levels.
Table 1.
2-Bit Intel MLC Digital Code Assignment
|
Charge Level |
Digital Code |
|
Level 3 |
00 |
|
Level 2 |
01 |
|
Level 1 |
10 |
|
Level 0 |
11 |
MLC programming
is accomplished by
charging
the floating gate through a precise process of
Channel Hot-Electron (CHE) injection. During programming, the source
of the MLC transistor is usually grounded. Column decoding of the MLC
provides direct bitline connection to the drain which is pulsed at a
constant voltage. Row decoding of the MLC, on the other hand,
provides direct wordline connection that causes the MLC transistor gate to
be connected to an internally generated supply voltage. This direct
and precise control of the drain and gate is critical to the correct
charging of the floating gate and, hence, correct storage of information.
Reading the
contents of multi-level cells involves highly precise sensing of the
amount of charge in the floating gate, measured in terms of
cell currents
that have an
inverse
relationship with the Vth. The sensed currents are compared to reference
currents, with the comparison results inputted to a logic circuit that
encodes them into the corresponding digital data.
Flash memory erasure is
achieved by 'discharging' the floating gate through a phenomenon known as
Fowler-Nordheim tunneling,
wherein electrons from the floating gate pass through the thin
dielectric layer and get dissipated at the source of the memory cell
transistor.
Flash memory is
used in a variety of
applications
such as:
personal and notebook computers, digital cell phones, digital cameras,
portable memory devices, LAN switches, embedded controllers,
etc.
<Back to Page 1>
Primary
Reference: www.intel.com
See Also:
What is a
Semiconductor?;
EPROMs; SRAMs; DRAMs
HOME
Copyright
©
2005
www.SiliconFarEast.com.
All Rights Reserved.
