Industrial diamond is considered the supreme semiconductor for cutting losses during power transmission over long distances. While silicon is today the star of semiconductors, diamond could replace it in the following decades. With superior electronic and thermal capacities, this replacement provides many possibilities in power transmission, bionics, or aeronautics, if its cost drops.
One of the major difficulties of energy transition technologies is their high cost, especially due to the use of rare materials and metals. In this context, using diamonds, an overly expensive material, could be considered prohibitive. However, this is the opinion of Étienne Gheeraert, coordinator of the European project GreenDiamond.
Purpose of the project: replacing silicon-based semiconductors of a converter, with a diamond, to cut energy losses in power transmission from offshore wind farms.
“The idea of the European project GreenDiamond is to reduce energy losses as much as possible,” says Étienne Gheeraert. For power electronic applications, one of the most important factors is the breakdown field of the material. In the case of silicon, it is equivalent to 300,000 V/cm, which is quite low. In the case of diamond however, the breakdown field is 10 MV/cm.
To withstand a voltage of 1,000 V, a converter would require a layer of 100 microns, which leads to a significant resistance, compared to 1 micron for diamond. With this change, it is estimated that energy losses can be reduced by four-fold.
At an equivalent power, but with a reduced size, diamond provides new paths for applications in bionics and transport. The weight of the material being the number 1 enemy in vehicle design, this replacement becomes very interesting.
If weight reduction is involved in smaller volumes of materials, high temperature resistance is also a major argument in these applications. Beyond 150°C, silicon’s properties change and are no longer optimal. Diamond can easily rise to 300°C. As a result, where a silicon converter requires a complex cooling system, diamond offers simpler and easier solutions. “In the case of aircraft electrification, we can imagine the transition from a 400 kg to 50 kg converter, which would be a huge gain,” explains Étienne Gheeraert.
In general, semiconductors are materials, inorganic or organic, which have the capacity to control their functionality in relation to the chemical structure, temperature, lighting, and the presence of dopants. The name of semiconductor comes from the fact that these materials have an electric conductivity between that of a metal, such as copper, gold etc. and an insulator, such as glass. They have an energy gap of less than 4eV (approximately 1eV). In solid state physics, there is an energy gap between the valence band and the conduction band in which electronic states are forbidden. Unlike conductors, electrons in a semiconductor must obtain energy (for example, from ionizing radiation) to cross the forbidden band and reach the conduction band. The properties of semiconductors are determined by the energy gap between the valence bands and the conduction bands.
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamonds are also very good electrical insulators which strangely is both useful and problematic for electrical devices. Diamond is a wide-band gap semiconductor (Egap = 5.47 eV) with high potential as a material in many devices, including electronics. Diamond detectors are very similar to silicon detectors, but offer significant advantages, especially high radiation hardness and very low drift currents.
Essential features of diamond as an advanced semiconductor material
- Diamond is made of carbon. Each carbon atom is in a rigid tetrahedral network, where it is equidistant from neighbouring carbon atoms, giving it much of its superlative properties.
- Diamond can be made in the laboratory. Two techniques are used to synthesize it: HPHT (High Pressure High Temperature) or MPCVD (Microwave Plasma Chemical Vapor Deposition).
- Diamond can be doped to become more or less conductive. Diamond can be of type p, doped with boron (B), or of type n, doped with nitrogen (N) or phosphorus (P).
- Implantation doping is not effective in diamonds. That is why in situ doping is done: doping species are incorporated during the process.
- Diamond is also a superconductor at low temperatures (-269°C).
- Diamond is a very wide band gap material. Its band gap is 5.5 eV, five times greater than the silicon one at 1.1 eV. The wider the band gap, the lower the influence of temperature on intrinsic carrier concentrations. This property led it to be classified as an insulator until doping was successful.
- Diamond is a better heat dissipator than metals. Its thermal conductivity is around 2200 W/mK or 5 times copper conductivity.
- Diamond’s breakdown field is 30 times greater than silicon’s, which allows diamond’s diodes and transistors to withstand voltages of thousands of volts in the off state.
- More current due to fast charging. The electrons in diamond have great mobility, which allows to reduce the joule effect losses while the diamond’s diodes and transistors are in on state.
- With a Young modulus of 1000 GPa the diamond is by far the hardest semiconductor.
Applications of diamond converters
If the efficiency of a good silicon converter is around 97%, it is possible to reach 99% with a diamond converter. This difference may seem small. However, the benefits are great for certain specific applications, such as long-distance HVDC (high voltage direct current) power transmission, as is the case with offshore wind farms. To bring 1 GW of power on the continent, 20 MW are saved!
The second application is transport in general, because any technology that saves weight on a vehicle is potentially interesting. If conventional electric cars are equipped with silicon converters, some high-end vehicles use silicon carbide (SiC), which can be considered as an intermediate between silicon and diamond. For example, Toyota has estimated that it is possible to gain 20% range by switching from Si to SiC. This means that even greater gains can be made by switching to diamonds. However, the car ranks second in diamond converter projects, the main target market being aerospace, as it is a sector that is constantly looking to reduce the weight of its aircraft.
Natural diamond is associated with luxury
Diamond is not a luxury product, but an industrial product. Moreover, the industrial process of making synthetic diamonds has long been mastered. PECVD machines used to make diamonds make it possible to deposit an area of 50 diamonds the size of a grain of sand on a 2-inch-diameter surface. When it comes to making a diamond, it is not difficult: the process needs hydrogen that can be obtained by hydrolysis of water and methane for the carbon source.
Based on known production in India and China, it is estimated that 12% of diamond jewellery is manufactured industrially without the knowledge of the jeweller, and this rate is increasing. Therefore, the manufacturers sell us diamonds at the same price as the jewellers. But this situation cannot last; things will change in the coming years as technology opens up important new markets that will completely disrupt traditional trade. Nothing will stop diamond technology and the price will drop to the value of an industrial market. This will happen as soon as research demonstrates a widespread industrial application of diamond in electronics.
Is this technology ready to see the light of day on an industrial scale?
From a research standpoint, GreenDiamond has long been involved in the manufacture of diamond converters. The purpose of the GreenDiamond project is also to involve industry. At the end of this project, a start-up was born: DiamFab, created in March 2019, and which won a Grand Prix i-Lab in 2019. In addition, they have already created diodes and transistors ready to be installed, so they are getting closer to industrialization. The question that arises is related to the cost of diamond. Why is diamond…
Read More: Supreme Semiconductor: Industrial Diamond