02 July 2012

Nanotechnology And Vacuum Tube Technology For Faster And More Efficient Electronics

A vacuum tube is simply a sealed glass tube or container. The inside of the tube is a space that is empty of any matter or gas; a vacuum. The vacuum tube is a device that controls the current through the vacuum. Vacuum tubes are used for rectification, amplification, switching, or creation of electrical signals.

Thomas Edison discovered this when experimenting with light bulbs. He discovered that when an electric contacts are introduced to both ends, the current jumps from the hot filament of the bulb to a metal plate at the bottom. This is called the Edison Effect.

The Edison Effect or Thermal Electron Emission basically says that electric current can travel through a gas or a vacuum without the need of an electric wire to move through.

The vacuum tube found its use in 1904 when John A. Fleming invented the Diode. A diode is the most simple vacuum tube. It is basically a light bulb with an electrode inside it. The diode works when the bulb's filament is heated white hot and electrons are boiled off its surface and into the vacuum inside the bulb. If the extra electrode is made more positive than the hot filament, a direct current flows through the vacuum to the anode.

The diode acted like a valve (and it was known as such) because the current in the tube travels exclusively in one direction. This turned out critical for radio sets then since it needed to turn alternating current into direct current, which the diode does.

Lee De Forest followed with his invention of the Audion. This type of vacuum tube performs like a diode but additionally also increases the current along the way. This is done by placing a metal grid in the middle of the vacuum and using a small input current to change the voltage on the grid. An audion can control the flow of the second more powerful current through the tube. The strength of the two currents need not to be related; a weak current can be applied to the tube's grid while a much stronger one can come out the main electrodes of the tube.

The audion soon led to the Triode. A triode is an three electrode version of the Audion. The triode is basically the first electronic amplifier, It served as an electronic amplifier in radio communications. This revolutionized the production of radio transmitters and receivers. It also led the great improvements to the telephone system in the US.

The invention of the triode ushered in the Electronic Revolution of the 20th century.

Smaller, cheaper, efficient and reliable solid state semiconductor devices such as transistors and solid state diodes have replaced vacuum tubes in modern devices. But some applications still need the use of vacuum tubes such as High Power Radio Frequency Transmitters and Microwave Ovens.

In the music industry, professional musicians still prefer vacuum tube based sound amplifiers because of the quality of sound it produces.

Pitt researchers propose new spin on old method to develop more efficient electronics

With the advent of semiconductor transistors—invented in 1947 as a replacement for bulky and inefficient vacuum tubes—has come the consistent demand for faster, more energy-efficient technologies. To fill this need, researchers at the University of Pittsburgh are proposing a new spin on an old method: a switch from the use of silicon electronics back to vacuums as a medium for electron transport—exhibiting a significant paradigm shift in electronics. Their findings were published online in Nature Nanotechnology July 1.

For the past 40 years, the number of transistors placed on integrated circuit boards in devices like computers and smartphones has doubled every two years, producing faster and more efficient machines. This doubling effect, commonly known as "Moore's Law," occurred by scientists' ability to continually shrink the transistor size, thus producing computer chips with all-around better performance. However, as transistor sizes have approached lower nanometer scales, it's become increasingly difficult and expensive to extend Moore's Law further.

Video: 1943 Electronics Video About Vacuum Tubes

"Physical barriers are blocking scientists from achieving more efficient electronics," said Hong Koo Kim, principal investigator on the project and Bell of Pennsylvania/Bell Atlantic Professor in the University of Pittsburgh's Swanson School of Engineering. "We worked toward solving that road block by investigating transistors and its predecessor—the vacuum."

The ultimate limit of transistor speed, says Kim, is determined by the "electron transit time," or the time it takes an electron to travel from one device to the other. Electrons traveling inside a semiconductor device frequently experience collisions or scattering in the solid-state medium. Kim likens this to driving a vehicle on a bumpy road—cars cannot speed up very much. Likewise, the electron energy needed to produce faster electronics is hindered.

"The best way to avoid this scattering—or traffic jam—would be to use no medium at all, like vacuum or the air in a nanometer scale space," said Kim. "Think of it as an airplane in the sky creating an unobstructed journey to its destination."

However, says Kim, conventional vacuum electronic devices require high voltage, and they aren't compatible with many applications. Therefore, his team decided to redesign the structure of the vacuum electronic device altogether. With the assistance of Siwapon Srisonphan, a Pitt PhD candidate, and Yun Suk Jung, a Pitt postdoctoral fellow in electrical and computer engineering, Kim and his team discovered that electrons trapped inside a semiconductor at the interface with an oxide or metal layer can be easily extracted out into the air. The electrons harbored at the interface form a sheet of charges, called two-dimensional electron gas. Kim found that the Coulombic repulsion—the interaction between electrically charged particles—in the electron layer enables the easy emission of electrons out of silicon. The team extracted electrons from the silicon structure efficiently by applying a negligible amount of voltage and then placed them in the air, allowing them to travel ballistically in a nanometer-scale channel without any collisions or scattering.

"The emission of this electron system into vacuum channels could enable a new class of low-power, high-speed transistors, and it's also compatible with current silicon electronics, complementing those electronics by adding new functions that are faster and more energy efficient due to the low voltage," said Kim.

With this finding, he says, there is the potential for the vacuum transistor concept to come back, but in a fundamentally different and improved way.


University of Pittsburgh
Nature Nanotechnology
National Science Foundation
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