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June 22, 1987


By CBR Staff Writer

Faster transistors, more sensitive photomultipliers and semiconductors with properties made to order are emerging from the new discipline of band gap engineering, reports Microbytes Daily. Ultimately band gap engineering will mean faster microprocessors and more powerful integrated circuits of all kinds, according to scientists at AT&T Bell Laboratories, a leader in the new technology. The band gap is the property of semiconductors that makes them semi state to a less stable, conducting state. Metals, which have effectively no band gap, always conduct when voltage is applied to the material. Silicon, the most common material for semiconductors today, has a bandgap which is basically fixed. Compounds, such as Gallium Arsenide and Aluminium Gallium Arsenide can be made with different band gaps by varying the percentage of each element in the compound and the structure of the semiconductor. By using techniques which can lay down an atomic layer at a time, scientists at Bell Labs are able to build devices with custom bandgaps. Techniques include sandwiching a very thin layer of a low-bandgap material between two layers of a high bandgap semiconductor. The thickness and depth of the layers controls the energy needed for an electron to become conducting. Among the devices already built at Bell Labs are ultra sensitive photomultiplier devices – avalanche photomultipliers – which amplify light, and high-speed transistors. The transistors switch nearly as quickly as so-called ballistic transistors (which are so constructed that conducting electrons do not collide with atoms in their journey across the junction). Unlike ballistic transistors, bandgap-engineered transistors (which work by quantum tunnelling of electrodes) can operate efficently at room temperature. According to the researchers, bandgaps can now tailored almost at will to produce devices for specific applications.

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