Bring Back the Valve!
Valves were great weren't they? Chunky vacuum tubes have a nice warm glow, allow for high-quality sound in audio equipment, and really add a retro touch to electronics.
For those who may not realize, valves are still in production for audio and radio equipment. This is due to the fact that valves actually perform much better than transistors in certain applications. For example, valves can operate at gigahertz frequencies while amplifying hundreds of watts of power whereas semiconductor devices (which can also operate at high frequencies), cannot conduct as much.
Valves are also considered by audiophiles to sound better in audio equipment including guitar pedals and audio amplifiers. Valves also have other advantages which make them ideal (especially in analog circuits), including their linearity, high voltage tolerance, large heat dissipation capabilities, tolerant of overloads, and typically can be operated well below the maximum capabilities of the valve.
Valves in all their glory! Image courtesy of Stefan Riepl [CC BY-SA 2.0]
Valves, however, do have some large drawbacks which is why they've mostly been replaced by solid-state devices. Valves frequently break for many reasons, including cathode poisoning, heat damage, breakage, and internal short-circuits. Valves also have problems with driving capabilities, require the use of heaters, need high voltages, and are incredibly large.
Imagine if a device could be invented that takes the good from both vacuum tubes and semiconductors. You would have a device that could be incredibly linear, have good overload tolerances while being very small and requiring low amounts of power. This is exactly what researchers at UC San Diego have done.
Teeny Tiny Free-Electron Mushrooms
Researchers at UC San Diego have created an electrical device that does not rely on the conduction of a semiconductor to control the flow of current. The device is controlled with only a low voltage and a low-powered infrared laser. But what makes the device even more impressive is its conductivity change of 1000% when activated, which gives the possibility of high-gain, high-powered amplifiers.
The idea of the device is rather simple: to have free electrons in space where the concentration of electrons can be controlled externally (similar to a vacuum tube). The only problem with this is that getting electrons into free-space is the requirement of high temperatures and high voltages, which would destroy any microelectronic device.
However, the team (led by Professor Dan Sievenpiper), came up with a solution which can free electrons from a surface without the need for either a high temperature or large voltage. The solution starts with a “metamaterial” that consists of many parallel gold conductors with mushroom-shaped gold nanostructures in a large array. This structure sits on top of a layer of silicon dioxide which is grown on top of a layer of silicon.
The gold mushroom array. Image courtesy of UC San Diego Applied Electromagnetics Group
When the array is exposed to a low-powered infrared laser and a low voltage (less than 10VDC) is applied, small yet intense electrical fields form in so-called “hot-spots”. These hot-spots have enough energy to pull electrons off the metal and into space, providing a conductivity increase of up to 1000%.
While this design is only a proof-of-concept, the results suggest that these tiny devices could very well be the future for high-power, high-frequency switching. The team leader, Professor Dan Sievenpiper, said:
“This certainly won’t replace all semiconductor devices, but it may be the best approach for certain specialty applications, such as very high frequencies or high power devices."
The Need for New Nanoscale Tech
With Moore’s Law seemingly always teetering on the edge of its final days, it won’t be long before new technologies are needed to replace silicon. Semiconductor technology, as it currently stands, can only do so much, which is why such tiny vacuum tube like devices will be imperative.
These devices can potentially operate faster (especially when controlled by low-powered infrared lasers) while able to conduct more power. This gives them real potential in applications involving radio transmission, high-power switching, and maybe even optical-based computing (which could use these devices to convert the processed light into output electrical signals that do not require large control voltages or consume large amounts of power).
Valves were great in their day and still have real potential. While these devices are not the same as valves (but operate on a similar principle), there is no doubt that these small gold mushrooms could enlighten the electronics industry on new forms of power and frequency control. The idea of a device having the advantages of valves without their distinct disadvantages is something that could really change the industry forever.