Physics News Update, Number 357

Number 357, February 4, 1998 by Phillip F. Schewe and Ben Stein

A QUANTUM TUNNELING TRANSISTOR, an on-off switch that exploits an electron's ability to pass through normally impenetrable barriers, has been built by Sandia researchers (Jerry Simmons, 505-844-8402), opening possibilities for record-speed transistors that can be mass-produced with current nanotechnology. In their device, the researchers control the flow of electrons between two GaAs layers (each only 15 nm thick) separated by an AlGaAs barrier (12 nm). Although the electrons in GaAs ordinarily do not have enough energy to enter the AlGaAs barrier, the layers are so thin (comparable in size to the electron wavelength) that the electrons, considered as waves rather than particles, can spread into the barrier and, with an appropriate voltage applied, out the other side. In the process, the electron waves do not collide with impurity atoms, in contrast to a traditional transistor's particlelike electrons, which are slowed down by these collisions. Transistors that switch on and off a trillion times per second--roughly 5 times faster than current GaAs channel field effect transistors--are projected to be possible with this approach. Although quantum tunneling transistors were first built in the late 1980s, it was originally infeasible to mass-produce them. Previous researchers engraved at least some of the ultrathin features side-by-side on a GaAs surface, something hard to do reliably with present-day lithography. Therefore the Sandia researchers stacked the features vertically, by using readily available techniques such as molecular beam epitaxy which can deposit layers of material with single-atom thicknesses. Having made quantum-tunneling memory devices and digital logic gates operating at 77 K, the researchers expect room-temperature devices in the next year. (J.A. Simmons et al., reported at the 1997 IEEE International Electron Device Meeting in Washington, DC; figure at Physics News Graphics)

THE SPALLATION NEUTRON SOURCE (SNS), costing $1.3 billion, will be built at Oak Ridge National Lab if Congress approves a plan announced recently by Vice President Gore. The SNS would produce neutrons by spallation, a process in which a beam of protons smashes into a target (liquid mercury), releasing a flood of neutrons. Other than the high neutron flux, an important feature of the spallation approach is that the timing of the proton pulses used to make the neutrons permits researchers to determine neutron energies through time-of-flight measurements. Neutrons have no electrical charge and can easily penetrate into crystals for doing structural studies; moreover, neutrons' nonzero magnetic moments make them useful for exploring magnetic fields inside a sample. The SNS will be a sort of neutron microscope: neutron waves with just the right wavelength (a neutron energy of 20 meV corresponds to a wavelength of about 2 angstroms) will illuminate a variety of specimens---new materials, biological objects, superconductors---of great interest to physicists. (Science, 23 January 1998; and Physics World, December 1997.)

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