Tikalon Blog by Dev Gualtieri

Vacuum Tube Redux

August 9, 2011

The transistor and I were born at about the same time, at the end of 1947. Useful and inexpensive transistors weren't available until many years thereafter, so it's no surprise that the first electronic circuits I built used vacuum tubes . I was aided in my electronics education by two neighbors, one of whom was a civilian electronics technician for the US Air Force . He repaired television sets in his home as a side job, and he gave me many old television sets that I harvested for tubes and components.

The other neighbor was an electrical engineer who worked at a military electronics division of General Electric . He was also an amateur radio operator , and "hams," as they are called, are always good at proselytizing their craft. Living in between these two, I got useful hardware from the North side of my house, and useful knowledge from the South side.

It took some time for transistors to displace vacuum tubes, especially in certain applications. Early transistors had very low gain , and they were suitable just for low power applications. They were temperature sensitive to the point that they wouldn't work at all at what would be considered modest temperatures today. The first transistors were also current amplifiers , not transconductance amplifiers like vacuum tubes, which led to other application problems.

It was therefore no surprise, at least to those engineers who worked with both vacuum tubes and transistors, that the Mikoyan-Gurevich MiG-25 , a very successful Soviet fighter aircraft that saw first active duty in 1970, was found to have most of its avionics implemented with vacuum tubes. It's powerful radar system had a transmitter capable of sending radar pulses of about half a megawatt ! The MIG-25 was produced through 1984.

A Soviet MiG-25 interceptor aircraft armed with four AA-6 Acrid air-to-air missiles .

(Via Wikimedia Commons).

Eventually, transistors were developed with better properties, and they were sold at a lower price. The field effect transistor , which had a high impedance control electrode , just like a vacuum tube, was developed. Soon, transistors replaced nearly all vacuum tubes. There were however, some environments in which transistors could not function. These were generally at high temperatures, above about 125°C.

It's for reasons such as these that scientists were still interested in vacuum tubes, but they realized that the thermionic emission principle on which they were based needed to be updated. Thermionic electron emitters need high temperatures, supplied by a resistance heater , for electron emission. For vacuum tubes of usual dimension, a high voltage was required to accelerate electrons across the gap from cathode to anode .

Charles Spindt of the Stanford Research Institute (Menlo Park, California ) combined the advancements made in lithography developed for the manufacture of integrated circuits with a principle that was known since Benjamin Franklin's research on lightning rods . Franklin had noticed that pointed conductors would discharge over a greater gap than others. As electromagnetic theory developed, it was proven that electric fields are indeed concentrated at sharp points.

(Via Google Patents).

Spindt's patented field emitter ,[1-2] called a Spindt cathode , will emit electrons without being heated by a process called field emission . Not only that, but they would emit under a hundred volts of applied potential in the small dimension structures fabricated by Spindt and his colleagues. One intended application for these electron emitters was for displays as an updated version of the cathode ray tube , but there was a fundamental problem.

The problem there was that translating cathodoluminescence from the high voltages of cathode ray tubes to the low voltages of field emission devices meant that you needed a much higher current to get the same power. High current is just another way of saying a greater number of electrons per second, and all these electrons deposit their kinetic energy in the phosphor material. What you get is the same degradation of phosphor screens that was responsible for burnt-in images in cathode ray tubes, the effect that screensavers were originally designed to prevent.

A paper in the August 4, 2011, issue of Electronics Letters by Jimmy L. Davidson , Emeritus Professor of Electrical Engineering, Materials Science and Engineering Management, and colleagues at Vanderbilt University , describes a planar triode electron device made from diamond . The diamond layers in the device are made by reaction of hydrogen and methane , a common process for diamond formation.

Diamond is an ideal material for such a device, since it has an extremely low work function .[4] Because of this low work function, diamond will produce field emission electron beams at low voltage. The device, as shown in the electron micrograph , is formed by a layer of diamond on a silicon dioxide insulator . The silicon dioxide substrate is formed on a silicon wafer . The only unusual requirement in fabrication of such devices is the need to maintain the diamond triodes in a high vacuum .

Left, scanning electron microscope image of a nanodiamond transistor. Right, detail of the device, showing the nanodiamond field emitter cantilevered above the insulating silicon dioxide surface. (Images, Davidson Lab, Vanderbilt University). [3]

The diamond triodes can be used at high temperatures, possibly to about 500°C, and they should function at near liquid nitrogen temperatures. It takes a little trickery to make transistors radiation resistant, but the diamond triodes are inherently radiation resistant . Said lead author Davidson,

"When I read about the problems at the Fukushima power plant after the Japanese tsunami, I realized that nanodiamond circuits would be perfect for failsafe circuitry in nuclear reactors... It wouldn't be affected by high radiation levels or the high temperatures created by the explosions."[3]

Davidson's diamond triode research was supported by the U.S. Army .[3]

Vacuum Tube Redux

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