Researchers from Hitachi Europe Ltd’s Cambridge Laboratory, working with Cambridge University’s Microelectronics Research Centre claim to have fabricated at memory cell that uses a single electron to store one bit, vastly reducing the power that would be required by huge-capacity memory chips – a device the size of a silver dollar could store 1T-bits while drawing just 0.1W: in current technology it would be the size of a tennis court and dissipate 10KW. The one-bit device developed by the Cambridge scientists utilises the so-called Coulomb Blockade Effect where, if an isolated area of conductor is made suitably small, the change of stored energy resulting from the gain or loss of an electron prevents further electrons from entering, and existing ones from leaving. The memory node itself is a small conducting region connected to an external circuit by a mulitple tunnel junction and controlled by special gate capacitors. Electron transfer to or from the node is only through the tunnnel junction. Memory node voltage depends on the number of electrons at the node and the the voltage applied to the gate electrode. When operating within the Coulomb blockade, the number of electrons in the memory is kept constant but slight increases in node voltage changes the boundaries to enable electrons to be transferred. To discharge the node, added electrons must be removed one at a time, which needs a definite voltage change. This leads to hysteresis, with two possible electron states for the same applied voltage, enabling the memory states to be defined. The device is fabricated in Gallium Arsenide using electron beam lithography to define special side-gated channels. The researchers have found that it is possible to control the movement of electrons by adjusting side gate voltage though they say it is an area that needs further research. More work is also needed to reduce the memory structures to dimensions of below five thousandths of a micron and so enable it to operate at room temperature. At present the device will only operate at around minus 273.05oC! Indeed, the reserchers expect it to be another 10 years before devices using the technology can come to market.
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