This might seem like an unusual topic for nanotechnology. The vacuum tube was the centerpiece of electronic advancement prior to solid state devices. Their application was incorporated into most electronics through the mid 1960s. Even today, audiophiles will tell you that the sound produced by equipment that uses vacuum tubes is superior to anything else that is currently available. Very high power devices also incorporate vacuum tubes. The fundamental concept of a vacuum tube consists of a cathode, a grid(s), and an anode in a glass tube, which had the internal gases exhausted to a hard vacuum level (less than 10-6 Torr). One Torr is equal to 0.00132 standard atmospherics (One atmosphere of pressure is that experienced at sea level).
The principle of operation for the vacuum tube is to heat the cathode material to generate a stream of electrons that are captured by the anode. This current flow can be amplified by applying voltages to the grid(s). What started with one grid evolved into multi-grid tubes. There are issues with the vacuum tubes. The heat generated by the vacuum tubes impacts the lifetime of the cathode’s emission of electrons. There is also a wait time until the tubes in a circuit have warmed up in order to function properly. Average lifetime of the tubes was roughly 1,500 hours. The impact of this short lifetime was a continual need to replace vacuum tubes. (More information can be found in Ref. 1)
The advantage of semiconductors were much more than extended lifetimes. Semiconductor are much smaller, don’t require a vacuum, which makes them more rugged. There is a consistency and reliability of semiconductors that vacuum tubes were unable to match. One negative of semiconductors is that the state changes are either on or off (digital signals). Vacuum tubes can provide an analogue change in signal. The semiconductor industry worked around this issue by creating digital to analogue and analogue to digital converters.
There has been research on the development of “Vacuum transistors”. [Ref. 2] By reducing the dimensions of the solid state device to the nano realm, the distance from the cathode to the anode becomes shorter than the mean free path of a gas molecules. The result is that the device can be operated in atmosphere and does not need a vacuum. The low current levels also required for the device to operate, works in preventing damage to the cathode that would typically occur at atmospheric pressure. There are more details available from NASA [Ref. 3] and other sources [Ref. 4].
Where does this effort stand today? NASA Ames has demonstrated it in the lab. Some parallel work is being done at CalTech. The demonstration of a functioning circuit that is manufactured using mass production techniques has not been shown. However, this is a “nano” technology that is promising. Keep watching for future developments.
References:
- http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
- http://spectrum.ieee.org/semiconductors/devices/introducing-the-vacuum-transistor-a-device-made-of-nothing
- https://www.nasa.gov/ames-partnerships/technology/technology-opportunity-nanostructure-based-vacuum-channel-transistor
- http://www.extremetech.com/extreme/185027-the-vacuum-tube-strikes-back-nasas-tiny-460ghz-vacuum-transistor-that-could-one-day-replace-silicon-fets
