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After being completely ousted following the advent of transistors, vacuum tubes are again finding a lot of uses in electronic gadgets

Published: Wednesday 31 July 1996

anyone familiar with the radios of the previous generation will remember vacuum tubes: these glass tubes which functioned as amplifiers formed the heart of essentially all electronic gadgets from primitive computers to radios. The essential component of the vacuum tube was a cathode which was heated to give electrons. The vacuum tubes were bulky, unreliable and cumbersome to manufacture. With the advent of transistors, these tubes became obsolete except in certain specialised applications. But recently they have made a comeback, albeit in a modified form.

In the last few years, a lot of work has gone into the development of cold cathodes -- miniaturised vacuum diodes which are finding applications in microelectronics. The essential difference in the vacuum tubes of the olden times and these objects is that the electron emission from the cathode in cold cathodes is not obtained by heating the cathode. Instead, the material of the cathode is so chosen that it spontaneously gives out electrons.

The material of choice is diamond because it is one of the few of its kind which has this property called negative electron affinity. But pure diamond is not suitable and so it has to be doped with impurities. Several dopants have been tried like boron, phosphorous and nitrogen. Out of these, nitrogen gives the best results. Unfortunately, it has been exceedingly hard to get high concentration nitrogen in diamond films.

Now comes a report from Ken Okano and colleagues from the Tokai University, Japan and the universities of Surrey and Liverpool, uk, which could change this scenario. The technique used by Okano and his group is that of growing diamond films by chemical vapour deposition (cvd). They incorporated nitrogen into the diamond films using urea (mixed with methanol and acetone) as the source of gaseous nitrogen. With this, they are able to achieve a high (around 0.2 per cent) nitrogen content in the films. The diamond films made in such a manner have a tremendous advantage because their threshold voltage (the lowest voltage at which the tube operates) is very low and hence the devices can be powered by a standard 1.5 volt battery (Nature , Vol 381, 1996).

The applications of cold cathodes are enormous. Since they can be made at the scale of microelectronics, they could be incorporated in integrated circuits, where they have an advantage over conventional devices in speed and efficiency. They could also be used as electron sources in display devices, the advantage is that their making requires only the lithographic process which is commonplace in the electronics industry.

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