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]

Display Technologies on Your Smartphone Screen: A Myth Buster

When you are looking for a new smartphone, you often come across some of these display technologies in technical specifications—LCD, LCD IPS, LED, WLED, OLED, SLCD, TFT, Retina Display, AMOLED, Super AMOLED, PLS, Super PLS, and so on. What the heck are these acronyms? How can you find out what is what and what matters the most? It’s a difficult job indeed. There are quite a number of things you need to understand and appreciate. But here, in this article, let me clear a few doubts and myths.

Display Technology

 

When we consider a smartphone screen, there are three major aspects to consider—the basic technology used, the resolution, and the technology used for wide-angle viewing.

The first part is the actual display technology, which you probably know about. In yesteryear computers, you have CRT (Cathode Ray Tube), and then came LCD for flat panel displays. Realistically speaking, if you go into the detail of display technologies, there are actually only two types of major display technologies—CRT and LCD. LCD is extremely popular and they find use in almost all of the smartphones you have there.

The other technologies, like TFT, IPS, Retina, etc., are enhancements to the LCD panel for various end results. Let’s see.

LCD (Liquid Crystal Display)

 

I am just trying to give you a general idea of the Liquid Crystal Display and will not go into the nuances of the technology.

LCD finds its roots back in your old calculators and digital watches that have monochrome LCD panels. The structure of an LCD panel is shown here.

structure of an LCD panel

 

The main element of this display panel is the liquid crystal layer that is aligned between the transparent positive and negative electrodes. As the electricity is applied through these electrodes, the LCD will either let the light pass or not. This creates the image what we see on the screen.

an old calculator uses LCD display

 

One important aspect of this panel is the final layer you see in the image, which could be either a reflective surface, such as a mirror or a backlighting source. In the case of regular monochrome LCD displays such as the calculator you saw above, there is no backlighting source. The display is visible only in daylight. It simply reflects back the light it receives for you to see the image. On other applications, such as your smartphone or the laptop display, there is a backlight, which makes it possible for us to see the images in darkness.

LED Display

 

Is there really a technology known as LED display? When you are watching a sport event in a large stadium, you may notice the huge display panels set up that show live footage of the game. Also, you see billboards all the time. Buses have this marquee-style display lit up with a number of LEDs to show you the route information.

true LED display

 

All of these displays are true LED displays, because they have a number of Light Emitting Diodes (a teeny-tiny light bulb) that collectively show images.

LEDs

 

In actuality, there is no display technology involved here. Then what is the LED, WLED display panels advertised by your TV maker? If you believe that they are actually made of millions of microscopic LEDs, you couldn’t be more wrong. They use exactly the same technology as the LCD above. Then what is the difference?

Remember we told you about the backlighting panel in the LCD structure above? That backlighting panel is usually made using a technology known as CCFL, which stands for Cold Cathode Fluorescent Lamps. This is a thin white tube that emits light. A horizontal panel made using a number of these CCFL tubes form the final layer of LCD displays. It makes the display thick and a little more power-consuming.

CCFL tubes

 

As an alternative, the industry came up with LED backlighting. Instead of using this series of CCFLs, we created an LED panel to provide light for the LCD display upfront. This is what we call LED display, and there is nothing it has to do with regular LED displays you see in a billboard.

First LED display panels involved white LEDs instead of colored ones. This is the reason why they are called WLEDs (White LED displays). Then came along colored LEDs, in three colors primarily—Red, Green, and Blue—later known as RGB LED displays. These panels are easy on the eyes since they give better colors than your regular WLED.

So, in essence, you have only one display technology, LCD, and LED is just an enhancement to it.

The Resolution

 

When it comes to smartphone world, we keep hearing quite a number of terms—VGA, QVGA, XGA, WXGA, UXGA, QXGA, HD, FHD, 2K, 4K, and so on. All of these terms represent only the display’s resolution, rather than the technology used.

For instance, VGA (Video Graphics Array) is a resolution of 640×480 pixels; QVGA is Quarter VGA with a resolution of 320×240; SXGA is Super Extended Graphics Array and represents a resolution of 1280×1024.

Aspect ratio is also closely related to this. For instance, iPhone 5 has a resolution of 1136×640, which corresponds to 16:9 aspect ratio. In the same way, iPad 4 has a resolution of 2048×1536, which corresponds to 4:3 aspect ratio.

Check out this image showing all kinds of resolutions on smartphones (click to enlarge).

resolutions and names

 

To know more about resolution, you can read our 4K TV article.

Another aspect of your display that closely relates to resolution is the ppi ratio (pixels per inch). iPhone 5 has a ppi of 326, which Apple likes to call Retina Display, while HTC Droid DNA has a ppi of 441. This has nothing to do with the display technology used, but everything to do with the size of the display and the resolution.

For instance, if you have a 4 inch display and 1136×640 of resolution, you cannot build it without having 326 pixels every inch. Droid DNA has a 5 inch display and with 441 pixels on every inch, it is capable of getting Full HD resolution of 1920×1080. Hence, the ppi ratio is directly proportional to the resolution and inversely proportional to the size of the display.

So, a smartphone display can show images well without high resolution, while a TV display has to have a high resolution to show images in good quality.

You can read about ppi ratio in our article about Retina Display.

One important thing to note is building too high resolution is also sort of an overkill. You don’t need anything higher than the Full HD on your smartphone, and HD, which is 1280×720, is also quite acceptable. Too high resolution on a smaller display will only make it look weird.

Over time, LCD has also had its improvements, and SLCD (Super LCD) used in most of the devices today (including iPhone, HTC One X, etc.) has come about, and it is manufactured by Sony. SLCD should not be confused with S-LCD, which is a manufacturer of LCD panels (and it is South Korean subsidiary of Samsung Electronics and Sony).

TFT, IPS, & Super PLS

 

You are hearing a lot about TFT and IPS lately, I suppose? These are not new display technologies, but are enhancements to the existing LCD panels. TFT stands for Thin Film Transistor, which is simply a layer on an LCD panel to make it address the pixels better. Just as there are tiny LEDs on a large LED panel, there are millions of tiny pixels on an LCD panel. And these pixels have to be individually managed to make an image appear on the screen.

This management of pixels is done by a Thin Film Transistor layer that has transistors across the rows and columns, directing charge to the pixel array.

The following is a typical LCD pixel array. As you can see, each individual pixel consists of actually three subpixels—red, green, and blue. Each of these subpixels has its own transistor that passes some amount of power to it to light it up.

Subpixel array on LCD panel

 

If you take an individual pixel, you can see that it has a TFT associated with it.

TFT on LCD panel

 

IPS (In-Plane Switching) is another enhancement to LCD TFT technology. If you are familiar with old LCD panels found in laptops and TVs, you know that they have terrible viewing angles. If you try to watch the screen from the side or from slightly above, you literally can’t see anything. But today’s LCD panels have much improved viewing angles, don’t they?

The technology behind this was first developed by a Japanese conglomerate known as Hitachi. In-Plane Switching improves your LCD panel’s viewing angle, and it is used in most of the smartphone display panels these days, may it be iPhone, Nexus 4, or HTC One. They all use LCD (SLCD) IPS panels.

Naturally, IPS would get its share of competition from others, and Samsung did announce another technology that improves viewing angles even further . The Korean beauty shows off its Super PLS (Plane to Line Switching) technology along with IPS panels.

Super PLS vs IPS
IPS and Super PLS side by side

 

Samsung says Super PLS improves the viewing angle and brightness of the display. So, which smartphones use this technology? Several of Samsung’s products in fact, including Galaxy Tab 2, Google Nexus 10, Galaxy Ace 2, Ativ Tab, etc.

AMOLED & Super AMOLED

 

So far, we have seen a lot of technologies, but only one corresponds to the actual technology used in a display, and that is LCD (and its advanced variant, Super LCD). All smartphones out there, iPhone, Nokia Lumia 920, HTC One, Nexus 4, and tablets, Nexus 10, iPad 4, etc., use LCD for their display.

What about a different kind of display technology? Samsung has the other side of the revolution going on.

Samsung has its own in-house solution to display technology, known as AMOLED—Active-Matrix Organic Light Emitting Diode. AMOLED is a type of OLED display in which a different kind of technology is used, rather than LCD.

AMOLED actually qualifies as a basic display technology. Other technologies like TFT, IPS, Retina Display, are all various enhancements or marketing gimmicks to existing LCD display with LED backlighting. AMOLED is a different kind of technology altogether.

AMOLED technology structure

 

As you can see, AMOLED panel also uses TFT for pixel addressing. To know various differences between AMOLED and regular TFT panels, visit this article. Super AMOLED naturally provides amazing viewing angles and very impressive black levels; also, AMOLED consumes much less power than LCD. These are major advantages of the display, but it has been identified that the panels can cause color oversaturation. Samsung has later on improved the AMOLED technology and came up with other variants like Super AMOLED, Super AMOLED Plus, etc. The basic technology remains the same.

Quite a number of phones have AMOLED display on them. The first generation Nexus phone, Google Nexus One manufactured by HTC had an AMOLED display; then came Lumia 900 from Nokia with AMOLED panel. Afterwards, quite a number of Samsung smartphones have sported this display panel— specifically Galaxy S series.

Samsung doesn’t have the manufacturing infrastructure to create AMOLED panels for all OEMs out there, and this is one of the reasons why HTC, Nokia, and others have moved on to Super LCD panels, created by Sony.

Sony also has the mobile BRAVIA engine shipping within its smartphones—more recently the Xperia Z. Mobile BRAVIA is nothing but a software program that enhances the display, so that the rendered images are much better. You will see some of the images in that article about Xperia Z smartphone.

In Conclusion

 

The article has already become too long for you to read. It was just meant to be an introductory article to all these display panels. You should check out each of those links as well. They are important links with further information. We have given care not to go into more technical aspects of these display technologies. If you wish to know any specific aspect, please tell us through the comments.

[Image credit: OutdoorLEDDisplayscreen, Wikipedia, Samsung, Reefbuilders]