Implementing MLC NAND Flash for
Floating
Program Gate (Inject electrons)
Source
Select Gate
e-
Substrate
Oxide
Erase
(Remove electrons)
Drain
Figure 1: A Basic Flash Cell
Binary and MLC Technologies
In flash devices that implement Binary flash technology, there are two possible ranges for VTh. MLC technology can have several valid ranges for VTh, instead of just two. The first implementation of MLC uses four voltage levels (see Figure 2). Each state is mapped to one of four combinations of two bits. Therefore, the cell can store two bits of data.
Figure 2 also shows some of the complexity caused by the migration from Binary flash to MLC. The programming and erase processes become more complicated since the circuits must maintain tighter VTh tolerances. This translates into longer program and erase times, and a more complicated read process.
Figure 2: Voltage Level Comparison between Binary and Flash Technologies
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