Technical Article

How Power Designs Benefit From Carbon Advances

5 days ago by Steve Roberts

Carbon will play an increasingly important role in the future of electronics. This article explores advances in material science, where carbon is expected to revolutionize electronics.

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

 

Carbon (C) is an essential element. We are carbon-based lifeforms. Gaseous CO2 concentration, in combination with oxygen, is the barometer we use to measure our contribution to global warming. In solid form, pure carbon can be as soft as graphite or as hard as diamond. Carbon fibers reinforce countless products, from airplanes to fishing rods. Radiocarbon 14C dating is an essential tool in archaeology. A more influential element is hard to imagine.

 

Ultra-Wide Bandgap

Wide Bandgap (WBG) transistors based on Silicon Carbide (SiC) and Gallium Nitride (GaN) have already led to rapid advances in power switching performance. Wider Bandgap materials have a significantly higher intrinsic thermal conductivity and a higher dielectric breakdown voltage than traditional Silicon (Si) based MOSFET power transistors, meaning that the transistor substrates can be made smaller and thinner for the same performance ratings. The smaller size also reduces gate and terminal capacitances and resistances, leading to faster and more efficient switching with lower power dissipation. SiC transistors can handle higher voltages and switch faster and more efficiently than Si-MOSFETs, while High Electron Mobility Transistors (HEMT) based on GaN substrates can switch even faster than SiC-MOSFETS, making them useful for high-frequency electronics. The fast switching reduces the required size of the other inductive and capacitive components, allowing very compact, efficient, and high-power-density products to be manufactured.

These WBG advantages mean that SiC and GaN transistors are already extensively used in green technologies such as electric vehicles, photovoltaic converters, IoT Networks, and eco-design power supplies.

Carbon offers the next generation of ultra-wave bandgap (UWBG) transistors in this process. Instead of SiC or GaN substrates, pure diamond is used, which has even higher thermal conductivity (4x better than SiC), greater breakdown voltage (6x better than GaN), and a much wider bandgap value than both SiC and GaN (Table 1):

 

Table 1. Comparison of basic properties of silicon, WBG, and UWBG transistors
Property Si SiC GaN Diamond
Bandgap (eV) 1.1 3.0 3.5 5.5
Thermal conductivity (W/cm K) 1.5 4.9 1.3 22
Breakdown voltage (kV/mm) 0.3 2.5 3.3 20
Electron Mobility (cm²/V s) 1500 400 2000 1060

 

The performance of different transistor technologies can be enumerated as the Baliga Figure of Merit (BFOM) – the higher the BFOM value, the better. The scale is non-linear because critical performance indicators such as breakdown voltage and conductivity depend on the critical electric field value, which scales up as a sixth power of the semiconductor bandgap electron voltage. Thus, based on BFOM, WBG transistors are about 730 times better than Si-MOSFETS, and a carbon-based UWBG transistor is about 15 625 times better–a massive leap in performance that will be essential for transforming our global energy consumption from polluting fossil fuels to efficient green electrical energy.

 

Graphene Semiconductors

Graphene is a 2-dimensional form (allotrope) of carbon formed from only one atom thick nanolayers, with the atoms arranged in a honeycomb-shaped planar lattice. It behaves like a semi-metal, allowing heat and electricity to flow easily along its plane but not transversely. As a bulk material, it absorbs light strongly across all visible wavelengths, yet it is nearly transparent in single sheets. Microscopically, it is the strongest material on Earth as each atom is double-bonded to each of its three neighbors. This rigidity creates an exceptionally high electron mobility, measured at 15 000 cm²/Vs (compare this value to those in Table 1), so it conducts electricity better than silver.

 

Figure 1. Crystalline structure of graphene. Image used courtesy of Wikipedia

 

Graphene additionally exhibits several unusual electrical properties: it is strongly affected by an external magnetic field, allowing sensitive hall-effect sensors to be built that can operate well at both room temperature and at cryogenic temperatures (down to less than 1°K above absolute zero) and it can be used to make graphene-based FETs (gFETs) that can be used as biosensors. A gFET uses a liquid gate where charged biomolecules affect the channel current, allowing measurements based on ions rather than charge injection. This permits real-time measurements of proteins, biomolecules, and nucleic acids, enabling cutting-edge technologies such as CRISPR gene editing, RNA drug research, detecting the presence of infectious diseases in humans, plants, and animals, and cancer research.

Research is continuing into the unique electrical properties of graphene that may open up the development of new electronic devices. One development area is spintronics, where information can be stored in the angular momentum of electrons (spin-up or spin-down). The regular and rigid array structure of graphene may be an ideal carrier material for room temperature, atomic level, and spintronic non-volatile memory (NVM), which would be faster than conventional RAM and yet retain all the data when switched off.

 

Carbon Nanotubes

If a graphene sheet were rolled into a cylinder, it would become a nanostructure with exceptional tensile strength and thermal conductivity properties. Thermal interface materials made of vertically aligned carbon nanotubes (CNTs) exhibit highly directional thermal conductivity, so the heat generated by power electronic devices can be efficiently transferred to a suitable heatsink without excessively warming adjacent components. In tests, thermal conductivities of nearly 15W/°K have been reached – about 3x higher than thermal grease.

In addition, carbon nanotubes can be formulated to act like semiconductors or semi-metals, depending on their physical dimensions and/or additional chemical doping. In theory, a carbon nanotube could carry 1000x more current than a similar-sized copper conductor, and because of its cylindrical structure, this current could be steered to flow only along the axis of the tube and not laterally, enabling many new kinds of electronic devices.

Other uses of carbon nanotubes are in photovoltaics, sensors, displays, smart textiles, and energy harvesters, but the most promising development is new types of Li-Ion batteries that use CNT cathodes. Existing Li-Ion batteries suffer from thermal expansion problems when fast charging or during high discharge rate conditions, which damages the internal structure. The higher mechanical strength of carbon nanotubes can withstand these thermal stresses without degradation. These new CNT cathode batteries can charge from 10% to 90% within 15 minutes and are lightweight, with double the WH/Kg energy density compared with conventional batteries. Furthermore, they will still have 90% of their original capacity after 800 charge/discharge cycles, promising a revolution in electric vehicle driving where a 1000 km range becomes commonplace.

 

This article originally appeared in Bodo’s Power Systems [PDF] magazine.