Implementing MLC NAND Flash for Cost-Effective, High-Capacity Memory
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of NOR flash and achieving barely adequate reliability, but it has serious limitations: its performance
is far slower than standard NOR flash.
NAND flash appeared to be the ideal media for data storage, due to its high-speed erase and write,
high density (thus high capacity) and small size, as compared with NOR and AND devices. Based on
these promising characteristics, Toshiba chose NAND flash as the basis on which to implement MLC
technology. Toshiba’s first MLC NAND product, just introduced in December 2002, offers up to a
50 percent decrease in die size compared to standard NAND, and about a 70 percent decrease in size,
compared with competing NOR MLC products.
However, NAND flash itself is not a perfect media. It contains a large number of randomly scattered
bad blocks, requires on-the-fly error correction, and uses a non-standard I/O interface, making it
difficult to integrate. These limitations are dramatically worsened in MLC NAND, along with a
slower programming time (compared to standard NAND) and a different software interface. The
combination of these characteristics makes MLC NAND all but unusable as a stand-alone local data
storage solution.
M-Systems’ x2 technology, selected by Toshiba to enable their MLC NAND technology, implements
reliability, performance and media management enhancements to perfect MLC NAND - without the
need for a full scale controller (e.g., ATA or SCSI). The combination of MLC NAND and x2
technology in Mobile DiskOnChip G3 brings smartphones, STBs and other embedded systems the
most cost-effective flash disk.
Comparing Binary and MLC Flash Technologies
Basic Flash Technology
Figure 1 shows the basic structure of a flash memory cell, which is similar to a standard MOS
transistor. However, unlike a standard transistor, a flash cell must be able to retain charge after
power removal in order to permanently store data. To accomplish this, a layer called the floating
gate is added between the substrate and the select gate. The floating gate is isolated from the
substrate and the select gate by layers of oxide.
A transistor can be biased (voltage can be applied to the source, drain, gate and substrate) to
optionally conduct a current between its source and drain. The voltage level at which the transistor
conducts is called its threshold voltage (V
Th
). The transistor conducts only if the voltage between the
select gate and source (V
GS
) is larger than V
Th
. Adding/Removing charge to/from the floating gate
modifies the V
Th
. To determine if the floating gate is charged, two conditions must be met: a specific
V
GS
must be applied to the cell and the circuit must be capable of sensing if the transistor is
conducting. These are the basic elements needed to implement flash data storage.
