Semiconductors are undoubtedly one of the most important inventions in human history. Without them, the entire digital revolution would not have been possible. And the world we know today would be a different one. With the physical properties of silicon, the most common semiconductor, pushed to its limits, the hunt for new material begins. And carbon, especially carbon nanotubes might be just the right candidate for usage in the electronics industry.
Silicon, the grand-father of semiconductors
The first semiconductor properties were discovered in the early 19th century by Thomas Johann Seebeck who observed decreasing resistance of silver-sulfide with increasing material temperature. Which is one of the key characteristics of a semiconductor.
In general, semiconductors are classified as material with electrical conductivity between a conductor (e.g. copper) and an insulator (e.g. glass). The properties of the material can be altered by a process called „doping“. By implementing impurities into the wafer material, electrical, optical and structural characteristics of the resulting product can be adapted to individual needs.
However, unwanted impurities pose a big threat to contaminating and destroying entire production badges. Silicon semiconductors are grown from high purity quartz sand. In a process called Czochralski method, this sand is melted into a monolithic crystal of silicon. The resulting silicon ingot then gets cut, ground and polished into round wafers of nearly perfect flatness. The unevenness of these wafers is narrowed down to sizes smaller than 1 nanometer. The standard diameter of wafers is 300mm, with a thickness of 775 micrometers. These serve as a substrate material for electronic components. The miniaturization of integrated circuits and electronic components has reached its limits with silicon. Without the capacity to reduce ICs further, silicon can’t continue delivering the performance it has up until this point. Meeting this endeavor may require reexamining how we produce gadgets, or even whether we need a substitute for silicon itself.
Carbon, Graphite, Graphene, Carbon nanotubes
Carbon is an essential element on this planet. It is present in all living tissue, but also in a variety of different physical forms. The properties that make carbon so interesting for the electronics industry, is its special electron configuration. Due to four electrons available to form chemical bonds, it is able to create complex molecules. However, under normal conditions, carbon is highly unreactive.
For use in the electronics industry, Carbon is just a generic term. More precisely, we are interested in its allotropes. The first one being Graphite. Which is simply a very pure form of Carbon. The atoms in Graphite are arranged in a hexagonal crystal grid which is key for its characteristics. Graphite is a good conductor of electricity and heat, that’s why it is very well used in the production of electrodes. About 42% of artificially produced Graphite is manufactured into electrodes. Next to its electrical properties, Graphite is an excellent lubricant, which makes it a key resource in the production of self-lubricating bearings and sealings.
A close structural relation to Graphite maintains the modified carbon element Graphene. Each carbon atom is connected at a 120° angle, which leads to the same honeycomb or hexagonal structure as in Graphite. But Graphite consists of multiple layers, which gives it a 3D structure. Whereas Graphene is present in 2D. This „flat“ pattern of Graphene is a key factor for the usage of carbon in the electronics industry. This way it can be folded into a tubular form, which leads to astonishing characteristics. Also, Graphene is capable of being doped with other substances. For example, the introduction of Ammonia leads to increased reliability in RAM devices.
A sheet of Graphene folded into a tube is called a carbon nanotube (CNT). A nanotube is defined as a tubular geometrical form with a diameter less than 100 nanometers. CNTs can be produced as a single tube, or numerous tubes stacked into each other, to further increase strength and efficiency. Overall, efficiency and strength are the core competencies of carbon nanotubes. There is no material stronger and stiffer in regard to tensile strength. Also, they incorporate either metallic or semiconductoral behaviour, depending on the notation, which makes them very versatile. Furthermore, CNTs have remarkable electrical conductivity.
Practical use of Carbon nanotubes
Despite being present for a couple of years already, CNTs have yet to be commercially exploited. Production processes are still very expensive, compared to silicon semiconductors. One possible application is the Carbon nanotube field-effect transistor (CNFET). Which is based on a classic metal-oxide-semiconductor field-effect transistor (MOSFET) structure, but uses CNTs in the design. CNFETs have a variety of advantages over classic MOSFETS. The higher electron mobility leads to a gate capacitance nearly twice the value of a MOSFET. Also, CNFETs have a higher current density and transconductance. In addition to that, the self-heating effect is less likely to destroy the component.
On the other hand, CNFETs are still expensive to produce, and their lifetime and reliability are still not on the level of silicon-based FETs. New production methods need to be found in order to reach the needed purity of carbon nanotubes of 99.999999%.
To show the capabilities this technology offers, Analog Devices and the Massachusetts Institute of Technology have developed a 16-Bit microcontroller, based on RISC-V-architecture. The RV16X-Nano named controller is composed of 14,000 complementary FETs and is able of running the complete RISC-V command set. What’s remarkable about this microcontroller, is the production method used. The CNFETs are positioned in a way that requires a lower CNT purity of 99,99x%, which current manufacturing processes already allow. CPUs and microcontrollers based on CNTs are expected to be ten times more energy-efficient and way faster than current generations of processors.
Recent accomplishments show, how much of a potential lays within the usage of carbon in the electronics industry. However, in this stage, it is still not possible to determine, when carbon-based electronic components are going to be available on a commercial scale.