Sometimes good ideas in technology languish because of serious implementation hurdles. The MIM diode (Metal-Insulator-Metal) was one of those technologies. Note the past tense.
Essentially, a diode conducts an electrical current in only one direction. Like a check valve with water, it won’t allow back flow. However, more sophisticated diodes do more than act like a one-way valve, they also can modify the current, such as converting alternating current to direct current or changing the modulation (phase) of radio signals. In short, diodes are versatile and are one of the keystone components of the modern electronics industry. Most diodes these days are made of silicon, like pretty much everything else electronic. Silicon is relatively inexpensive and relatively easy to use in manufacturing, but it has limits. It’s these limits, especially in performance, that the electronics industry is now pushing.
Diodes made from something other than silicon could be one of the prime pushers. The top candidate, the MIM diode, is constructed like a sandwich with two very thin metal strips separated by an insulating material in the middle. Electric current (in the form of electrons) on one metal strip move through the insulator to the other metal strip not by conduction, which is dependent on the conductivity of the material in the middle (and is slow) but on an effect called quantum tunneling. In this case, electrons from one metal strip should not be able to cross the insulator to the other metal strip because, by the numbers, it should take more energy to cross the insulator barrier than the electrons have to begin with. However, cross it they do. As a gross over-simplification, a certain percentage of electrons derive additional energy within the barrier and manage to ‘tunnel through’ to the other side. This happens very fast, much faster than electrons travel through a semiconductor such as silicon. So, in theory, a MIM diode can be much faster than say a silicon transistor.
However, in practice there are problems; problems decades of research have been unable to solve. Now a team of researchers at Oregon State University (Corvallis, USA) has developed an effective solution, published online October 25, 2010 in Wiley Advanced Materials [Advancing MIM Electronics: Amorphous Metal Electrodes]:
As unfamiliar (non-intuitive) as quantum effects are, they are still affected by physical things that we can understand. What the researchers discovered is that an uneven surface of molecules on the metal strips of a MIM diode makes the quantum tunneling highly unpredictable (a.k.a. uncontrollable). As small as a diode might be (a centimeter or two, for example) at the molecular level the metal strips might as well be football fields. Over such a vast area, it’s not unusual to have irregularities – bumps and distortions – in the molecular surface. The material previously used for the metal strips of the MIM diode was aluminum, which is known to have a comparatively ‘rough’ surface. After much experimentation with different metals and combinations, the team settled on an amalgam of Zirconium, Copper, Aluminum, and Nickel (ZrCuAlNi), which they refer to as an ‘amorphous metal contact,’ the amorphous part meaning shapeless as in smooth.
First and foremost, this quantum tunneling diode is much faster than current silicon-based electronics. It is also practical: Not only does the amorphous (smooth) surface of the metal contacts make the tunneling controllable, but these MIM diodes are also relatively easy to manufacture.
The predictions for success of the MIM diode have an expansive quality:
“Researchers have been trying to do this for decades, until now without success,” said Douglas Keszler, a distinguished professor of chemistry at OSU and one of the nation’s leading material science researchers. “Diodes made previously with other approaches always had poor yield and performance.”
“For a long time, everyone has wanted something that takes us beyond silicon,” Keszler said. “This could be a way to simply print electronics on a huge size scale even less expensively than we can now. And when the products begin to emerge the increase in speed of operation could be enormous.”
[Source: Nanotechnology Today]
In short, a world-beater in electronics. Not tomorrow though. It will take some time for the development of manufacturing techniques. MIM diodes may have many potential uses, but they will have to meet stringent demands for reliability and predictability. They will have to be competitive with the costs and applications already in place for conventional silicon semiconductors. There may be limitations, as yet unknown, in what can be done with this kind of diode. Nevertheless, the MIM diode is well worth tracking.