VACUUM TUBES ATTEMPT A COMEBACK. Vacuum tubes were the backbone of the electronics industry until the 1960s, when their large size, excessive power dissipation, and lack of integration allowed solid-state technology to win out. Now forests of 100-nm sized nanotriodes might bring vacuum designs back, at least for niche applications. Researchers at the University of Cambridge (Alexander Driskill-Smith, David Hasko, and Haroon Ahmed, aagd100@cus.cam.ac.uk, 011-44-1223-337556) have made an anode-gate-cathode device in which the cathode consists of multiple nanopillars which can be crowded together in a dense formation. This will eventually enable nanotriode densities of 10⁹ per cm² (including interconnects) to be reached, comparable with the best packing densities for metal-oxide-semiconductor (MOS) transistors, the electronics industry workhorse. Shooting electrons through vacuum rather than a semiconductor not only makes switching fast (the ballistic electrons always travel without scattering), but gives nanotriodes a few advantages over MOS technology: the nanotriodes are radiation resistant, operate well at high and low temperatures, and, because they are vertically-oriented, will permit integration in the third dimension, allowing even greater packing densities. Electrons (or, more accurately, the electron waves) issuing from the nanopillars are coherent and highly focused, and might be useful for doing holography or nanolithography. Remaining problems with this vacuum design include a relatively high operating voltage (10 V) for large scale integration applications and the reproducibility and longevity of the nanotriodes. (Applied Physics Letters, 1 November 1999: for journalists the text is available at Select Articles.)