MIM & MIIM Diodes: How Do They Work?

What drains power in today’s electronics, what causes high temperatures and costs in today’s electronic equipment, could be behind extreme electronics of tomorrow. Our world is gearing towards a better tomorrow, where the supercomputers will be millions of times faster than today’s. Processing will be remarkably faster, and high level applications will run in nanoseconds.

What will help us achieve this for our future? The technology known as MIM diodes and its successor, MIIM diodes (Metal-Insulator-Insulator-Metal diodes). Let’s check them out in detail.

What Is MIM & MIIM?

diodesA diode is one of the most basic electronic components of any circuit. It is used to conduct electrons in one direction, like a valve. Advanced diodes are capable of modifying the current as well, such as sine wave to square wave, AC to DC, etc.

Diode is such an important component in today’s electronics that without them nothing would be possible—no cellphones, tablets, PCs, display technologies, speakers, storage systems, servers, clients, automobiles, or anything.

These electronic components—diodes, transistors, ICs, etc.—are developed with some kind of semiconductor, and the most widely used of them is Silicon. Other semiconductors are also there, such as Germanium, Gallium Arsenide, and certain organic semiconductors. However, Silicon is the one major semiconducting material we use in all kinds of electronic equipment.

Today’s technologies, however, are pushing the limits of what Silicon-based diodes and ICs can achieve in terms of speed and performance. In a world like that, a better technology that transforms electronics as we know it today is the prime solution. A research to find such a solution resulted in further developing an already existing technology, MIM diodes.

MIM stands for Metal-Insulator-Metal diodes. These are sort of like capacitors in which thin metal plates are separated by an insulator. Here’s the basic structure of a MIM diode. See how the insulator separates metal strips.

MIM diode structure

Oregon State University (OSU) has been working on MIM diodes for such a long time. In 2011, there was a paper published by researchers in this particular area.


Douglas KeszlerIn 2010, OSU finally managed to create the first ever MIM diode. Douglas Keszler, chemist and one of the leading materials science researchers from OSU, said then:

Researchers have been trying to do this for decades, until now without success. Diodes made previously with other approaches always had poor yield and performance. It’s a basic way to eliminate the current speed limitations of electrons that have to move through materials.


Here’s the first ever MIM Diode:

first MIM diode

Now, OSU has gone further and created the next level of MIM, MIIM diodes. Here, instead of a layer of insulator, there are two.

Here’s an image. The material has four layers—ZCAN (Amorphous Zirconium), Hafnium Oxide (HfO2), Aluminum Oxide (Al2O3), and Aluminum (Al).

MIIM diode

As I mentioned earlier, the technology is not new. The MIIM diodes are a type of ‘tunnel diodes’ that were in existence since the 60’s, known for the technology behind it, ‘quantum tunneling’. It has never been researched further and developed for mass production.

SanDisk 3D LLC, a subsidiary of the flash storage memory maker, SanDisk, has a patent on MIIM diodes with a trench structure. The patent dates back to 2008, although no work has been done by the company based on the technology.

What Is Quantum Tunneling?

Quantum Tunneling is the technology behind MIM diodes. As we mentioned, the tunnel diodes are called so due to this reason.

Normally, we know an electron beam is unable to go through an insulator. However, when the insulating material is extremely thin at less than 3 nm, a special phenomenon known as quantum tunneling can happen. This phenomenon can only be explained with quantum mechanics, not classical mechanics. This is the reason why it is known as ‘quantum tunneling’. This concept has a more extensive reach than MIM diodes. For instance, the energy production in the Sun actually has a lot to do with quantum tunneling.

Imagine a ball rolling up a hill. It will not be able to cross the hill if the energy it already has is not enough. However, if you are thinking of a ball in the quantum scale (which is infinitesimal), the ball can no longer be regarded as an object, but as a wave. In such a state, it can be expected that the ball crosses to the other side of the said hill if the other side has a slope where the ball can stay without expending much energy.

The same concept applies to electrons in MIM diodes. When the electron reaches the barrier, the insulator in between the metal strips, it absorbs energy from the surroundings in order to pass through the insulator. In doing that, a part of the electron beam passes through the barrier, but another part reflects back. However, due to the energy absorption, the reflected electron beam has far more energy than it originally has.

quantum tunneling with electrons

The electron beam that traverses the barrier does not follow the normal laws of conduction as it is going through an insulator, which does not help in conducting electrons. Due to this reason, the energy the beam absorbs makes it travel at ultra-high speeds. The speed is much higher than that on a normal conductor or a semiconductor like Silicon.

Also, when we visualize electron beam as a wave, quantum tunneling can be seen in action below. Quantum tunneling happens in the form of a ripple effect you see at the barrier. Also, after that, the passing beam and the reflected beam have lower altitude (height) and higher wavelength.

Quantum tunneling visualization

This tunneling phenomenon is what causes the extreme speed on MIM and MIIM diodes.

Ironically enough, as mentioned at the beginning of the article, quantum tunneling is a major reason behind power drain and extreme temperature in current electronic systems, but that is unwanted quantum tunneling.

John Conley Jr.

Extra insulator layer helps enhance an MIM diode. Dr. John Conley, Jr.,  of OSU that was part of the research into MIIM says:

This approach enables us to enhance device operation. It ... moves us closer to the real applications that should be possible with this technology.

 


This extra insulator layer causes something known as ‘step tunneling’ that allows highly precise control of the asymmetric diodes.

Advantages

Quantum tunneling and MIIM diodes have only advantages.

  • They are as easy to manufacture as Silicon based electronic components.
  • They are inexpensive.
  • They can form extremely fast electronic components as compared to any available technology today—imagine terahertz of processing power in place of gigahertz we have now.
  • They will find application in all kinds of electronic technologies—smartphones, tablets, LCD displays, TVs, consumer electronics, automobiles, etc.

Conclusion

I am not an electronics expert, but when it comes to technologies that will enhance what we have today in smartphones and tablets, I do take high level of interest. We do not have infrastructure today to make MIIM diodes on large scale. However, when it happens in the near future, we will all have extremely powerful smartphones and tablets. We will have in our hands devices better than the biggest and the most massive of today’s supercomputers.

[Image source: BSN, Wikipedia]