Pack them in


By Paul Marks HARD drives could be history within a decade if a new memory chip developed by an Anglo-Japanese team meets expectations. The chip stores data using only a fraction of the number of electrons needed by a conventional RAM chip. This saves so much chip space, say the developers, that it could allow thousands of gigabits to be stored on a single chip. These high-capacity chips could replace disc drives in computers. They are faster than disc drives, and as they have no moving parts they are more reliable too. The developers predict that entire movies could be stored uncompressed on such devices, which would be a boon to people such as movie editors and special effects producers, who currently have to retrieve chunks of footage off slow disc servers. Called Phase-state Low Electron-number Drive Memory (PLEDM), the new technology is the brainchild of a team at the Hitachi Cambridge Laboratory-a joint research venture of Hitachi of Tokyo and Cambridge University. At the very least, PLEDM chips should have twice the data-storage capacity of conventional dynamic RAM chips-and as chip-making processes improve, they should be able to squeeze in even more, says David Williams, group leader at the laboratory. To record just one bit of digital information, today’s RAM chips use a transistor to pour hundreds of thousands of electrons into a neighbouring capacitor. Unfortunately, the number of electrons required to store a bit has not changed much over the years (see Graph), as the data signal still has to be readable above electrical noise. Whether the chip stores 1 megabit or 64 megabits of data, each capacitor must hold around 500 000 electrons, according to Hiroshi Mizuta, senior researcher at Hitachi. So the maximum data density of RAM chips is limited by the size of the capacitors, which have to be big enough to store all those electrons. The PLEDM technology does away with these large capacitors (Electronics Letters, vol 35, p 849). Instead, the Cambridge researchers have created what they call a “gain cell”, which uses a low-noise transistor to amplify a signal from just 1000 electrons stored securely in the output region of another transistor directly above it. The storage transistor uses a novel electronic shutter that closes when the transistor is switched off. This holds in place any electrons that have entered it and makes the memory non-volatile: it keeps its data when power is switched off, just like a disc drive. Because the amplifying transistor is buried beneath the storage transistor, the cell only occupies the area of one transistor, allowing much denser memories to be built. Haroon Ahmed, professor of microelectronics at Cambridge, and one of PLEDM’s co-developers,
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