Vacuum tubes' new image: Too small to see 

 In the lilliputian realm of modern electronics, old-style vacuum tubes have all the charm of hulking Gullivers. However, researchers have recently been reducing these components to daintier proportions. They hope to exploit ways in which vacuum tubes outperform semiconductor devices.

 A team in England has now developed a vacuum tube whose size rivals that of transistors in today's microcircuits. The evacuated hollow in the so-called nanotriode occupies only about a billionth the volume of a grain of salt, says Haroon Ahmed, whose group describes the device in the Nov. 1 Applied Physics Letters.

 Capp Spindt of SRI International in Menlo Park, Calif., hails the article as "the first credible report of an operating vacuum diode or triode on this scale." Diodes act as one-way valves for current between two electrodes; triodes control current via a third electrode.

 Akintunde I. Akinwande of the Massachusetts Institute of Technology calls the results "really spectacular."

 Alexander A.G. Driskill-Smith, David G. Hasko, and Ahmed, all of the University of Cambridge, fabricated the prototype device from alternating layers of metals and insulators. The inventors expect the triode to operate under conditions of radiation or heat that would make standard semiconductor components fail.

"It looked pretty exciting from that point of view," says Ahmed.

 Old-fashioned vacuum tubes initiate a current by boiling electrons off heated electrodes. By contrast, in the microscopic vacuum tubes, devices called field emitters shoot electrons from the most prominent tip of an array of tiny, unheated posts or pyramids. The electrons are torn from the tip by an enormous voltage produced when an external electric field becomes concentrated there. Field-emission research has intensified in the past decade because emitters can be used in flat displays for computers and other items.

 Another research team in 1991 reported sealing minuscule emitters inside an evacuated cavity, thus creating a vacuum tube, but it's not as small as the Cambridge tube, Akinwande says.

 For tubes of the Cambridge type to play a role in digital circuits, their traits must improve, says Ivor Brodie of SRI. The maximum current is low-about 10 nanoamperes-and too irregular, he says. Akinwande estimates that the tube's roughly 10-volt operating voltage probably can be reduced to around 2 volts, a level at which some low-voltage semiconductor devices now function.

 Vacuum tubes handle high-frequency signals better than semiconductor components do. Unlike electrons in a semiconductor, which are slowed by collisions with crystal-lattice atoms, electrons in a tube fly unobstructed through the vacuum.

 Consequently, arrays of nanotriodes may find use as amplifiers and oscillators for high-frequency, high-power signals, such as those in cellular phone systems or military radar, Ahmed says.

 Other possible roles include pressure and acceleration sensors and satellite microthrusters, Akinwande adds.