Double the normal number of bits are crammed into each memory cell.
SanDisk has announced a significant advance in flash-memory technology that enables 64 gigabits of data to be stored on a chip the size of a fingernail. The new, more spacious flash chips do this by holding four bits per memory cell, as opposed to the standard one or two bits per cell. SanDisk presented details of the advance at the International Solid State Circuits Conference in San Francisco on Tuesday.
In recent years engineers have found a way to increase the capacity of flash drives, without waiting for the transistors to shrink. They do this by storing more than one bit of data per transistor, within what are referred to as multilevel cells (MLCs). In a single-level cell, data is stored using two distinct states, defined by different voltage levels. In contrast, a four-bit MLC stores information in 16 states, which translates into four bits of data per cell, or four times the amount of information.
This trick is by no means easy. Ensuring that each memory cell maintains precisely the right voltage, without disturbing that of neighboring cells, is a major challenge. Another issue is reducing the time that it takes to write to these cells.