Number 18 (Story #4), January 23, 1991 by Phillip F. Schewe and Ben Stein MODULATION-DOPED FIELD EFFECT TRANSISTORS (MODFETS) are the fastest transistors available. In modulation doping, facing layers of gallium arsenide and aluminum gallium arsenide squeeze electrons into an essentially two-dimensional electron gas, or 2DEG. The electrons confined in this way scatter less because they are separated from their donor ions and can therefore move quickly. Consumer applications of the so-called low-dimensional devices which employ the effect, such as MODFETS or high electron mobility transistors (HEMT), have been limited so far; they are used in satellite television receivers, where the low-noise behavior of modulation-doped devices and the frequency range of GaAs come into play. (New Scientist, 12 Jan. 1991.)