The Memristor - they've found it!
HP's Henry Williams and his group have accidentally stumbled upon the fourth fundamental element in electronics - the memory resistor, or memristor. Basically, it's a resistor that changes its resistance with elapsed current flow, or total amount of charge that has passed through it, and retains its resistance even after current ceases to flow through it.
From the article:
"The classic analogy for a resistor is a pipe through which water (electricity) runs. The width of the pipe is analogous to the resistance of the flow of currentÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ¢Ã¢ÂÂ¬ÃÂthe narrower the pipe, the greater the resistance. Normal resistors have an unchanging pipe size. A memristor, on the other hand, changes with the amount of water that gets pushed through. If you push water through the pipe in one direction, the pipe gets larger (less resistive). If you push the water in the other direction, the pipe gets smaller (more resistive). And the memristor remembers. When the water flow is turned off, the pipe size does not change.
Such a mechanism could technically be replicated using transistors and capacitors, but, Williams says, ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂit takes a lot of transistors and capacitors to do the job of a single memristor.ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂ
The memristor's memory has consequences: the reason computers have to be rebooted every time they are turned on is that their logic circuits are incapable of holding their bits after the power is shut off. But because a memristor can remember voltages, a memristor-driven computer would arguably never need a reboot. ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂYou could leave all your Word files and spreadsheets open, turn off your computer, and go get a cup of coffee or go on vacation for two weeks,ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂ says Williams. ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂWhen you come back, you turn on your computer and everything is instantly on the screen exactly the way you left it. "
Apparently, evidence of memristors has been around for quite a while, but it was only until now, during the age of nano-technological exploration, that we finally have labeled the "strange voltages" we've found in experimental circuits as a result of memristor behavior. The discovery was made while doping Titanium dioxide with some dopant that the article fails to cover. Anyhow, the resistance of TiO2 changes with the amount of dopant covering it. Since the dopant does not adhere perfectly to the TiO2 substrate, the flow of charges (electrons) can move the dopant, and cause it to cover more or less of the substrate, thus changing its resistance.
"Williams found an ideal memristor in titanium dioxideÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂ¢Ã¢ÂÂ¬ÃÂthe stuff of white paint and sunscreen. Like silicon, titanium dioxide (TiO2) is a semiconductor, and in its pure state it is highly resistive. However, it can be doped with other elements to make it very conductive. In TiO2, the dopants don't stay stationary in a high electric field; they tend to drift in the direction of the current. Such mobility is poison to a transistor, but it turns out that's exactly what makes a memristor work. Putting a bias voltage across a thin film of TiO2 semiconductor that has dopants only on one side causes them to move into the pure TiO2 on the other side and thus lowers the resistance. Running current in the other direction will then push the dopants back into place, increasing the TiO2's resistance.
HP Labs is now working out how to manufacture memristors from TiO2 and other materials and figuring out the physics behind them. They also have a circuit group working out how to integrate memristors and silicon circuits on the same chip. The HP group has a hybrid silicon CMOS memristor chip ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂsitting on a chip tester in our lab right now,ÃÂÃÂ¢ÃÂ¢ÃÂÃÂ¬ÃÂÃÂ says Williams."
I think this is pretty awesome, considering the current breakthroughs in nanotechnology and downsizing of transistors, memristors would enable a whole new field to be born, and circuit theory may have to be re-designed.
Like Leon Chua said, the man who first came up with the idea of a memristor, "now all the EE textbooks need to be changed."
These next few years in the field of EE should be very interesting =)
Image from Spectrum Online