THE WORLD'S FASTEST SEMICONDUCTOR has been devised by physicists at the Weizmann Institute in Israel. Using an ultra-clean vacuum system and highly purified materials, the researchers (Moty Heiblum, heiblum@hpl.hp.com) created a sandwich consisting chiefly of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) layers. Cooling the material to 0.1 K and subjecting it to a small electric field, electrons at the GaAs/AlGaAs interface drifted in the direction of the field at speeds 40% greater than those in the best previous GaAs-based materials. The electrons journeyed an average of 120 microns before colliding with anything. In general, electrons tend to scatter less in GaAs than in silicon; moreover, this material greatly outperformed its GaAs-based cousins because it had significantly fewer impurities. This is not the case at room temperature, however, where thermal vibrations of the semiconductor crystal diminish the importance of purity. Additionally, the researchers detected possible signs of large negative magnetoresistance, in which magnetizing the cold sample actually appeared to decrease its electrical resistance substantially ---something not seen before in similar AlGaAs/GaAs materials. (V. Umansky et al., Applied Physics Letters, 4 August.)