The End of the Age of Silicon
Chips are becoming faster as individual transistors continue to shrink. This reduction in size enables more transistors to fit on a piece of silicon as Moore’s Law has described for years (i.e., the number of transistors that can fit on a chip doubles every 18 months). It is this shrinking of feature size that makes modern chips better than their predecessors.
However, Moore’s Law is shortly coming to an end because transistors can only be made so small before quantum phenomena become too much of a problem. For example, MOSFET devices rely on the insulation that lies between the gate and the bulk semiconductor material to function correctly. The insulated gate is one of the factors that help to reduce the amount of current draw which, in turn, reduces the heat generated by that device. However, if the gate is too thin, quantum tunneling can kick in, which involves electrons “teleporting” from the bulk material into the gate which results in current flow. This current flow increases the temperature of an individual transistor by a small amount—but when 1 billion transistors are all generating a small amount of heat, the combined effect can be devastating.
All is not lost, however, as there are alternatives to the reliance on transistor reduction for technological advancement. For example, software could be written more carefully while employing new, faster algorithms. Hardware architecture could also be altered to take advantage of multiple chip packages which all could handle different tasks (such as co-processors like Floating Point Units). There are some engineers and scientists who are looking at more exotic methods for increasing computational power including gold-coated DNA, carbon nanotubes, and even quantum computers.
One group of researchers at the Institute of Science Education and Research in Pune, India, have spent the past four years on an alternative material to silicon for future devices.
The Pune Group's MOF Semiconductors
The research group has spent the past four years trying to turn insulating Metal-Organic Frameworks (MOFs) into a semiconductor. Using nanochemistry, they have combined the cadmium-based MOF with a conductive polymer to increase the conductivity by a billion times. Not only is the material semiconducting, but it's semiconducting at room temperature, which is a vital property for any practical application.
The MOF semiconductor. Image courtesy of IISER Pune
To create the silicon-free semiconductor, the researchers fill the pores in a cadmium-based MOF with pyrrole monomers. Then, iodine is introduced to the MOF, which oxidizes the monomers into polymers which are electrically conductive. It is these conductive polymers that make the MOF a semiconductor.
Interestingly, the electrical conductivity of the MOF was expected to remove its fluorescent capability. It was believed that this quality would vanish due to the fact that fluorescence is dependent on electrons being able to jump between energy levels. In conductors, electrons are loosely held, which makes the material unlikely to be fluorescent. But, as it turns out, the MOF is still fluorescent even as a semiconductor. It's this same non-covalent, weak interaction between the MOF and the polymer that makes the increased conductivity possible.
While the new material is a semiconductor with a huge boost in conductivity (as compared to the original MOF), it is still much less conductive than silicon. Despite this, the researchers have shown that organic materials still show promise in future technologies.
PhD student Barun Dhara (right) and Dr. Nirmalya Ballav, members of the research team. Image courtesy of IISER Pune
MOFs are typically found in gas storage and solvent separation which places MOF-based semiconductors into two probable applications: gas sensing and supercapacitors. While gas sensors may benefit from such devices, supercapacitors could be changed entirely thanks to MOF semiconductors. Quick charge and discharge batteries are very sought after as they allow for convenient charging of handheld devices such as tablet and smartphones.
Supercapacitors also benefit applications that require large amounts of energy in a short time period, including electric cars and even magnetic launchers (for example, coil guns require strong magnetic fields that last for a short period to pull projectiles).
Details of Mazda's i-ELOOP EDLC system in the Mazda6. Image courtesy of Mazda.
The importance of supercapacitors and new sensing technologies cannot be overestimated, which is why the research by the Pune Group is so important. Even though their MOF semiconductor has a long way to go, it is never the less a fantastic achievement that may provide the future with instant charging batteries and advanced semiconductor sensors.