Why is Diamond an Electrical Engineer’s Best Friend? Diamond Materials, Part 2Gary Kardys | August 13, 2018
Diamond is much more than a pretty stone. Diamond has unique properties such as the highest thermal conductivity of any material, high electron mobility, an extremely wide band gap, broad optical transparency from UV to infrared and outstanding strength and hardness.
For many years, abrasive and mechanical components have leveraged the super-hardness and extreme wear resistance of synthetic diamond, which is manufactured in high pressure and high temperature (HPHT) equipment (as discussed in "Is Diamond Really a Super Materials? Diamond Materials Part 1"). New cost-effective single crystal and polycrystalline diamond growth methods based on chemical vapor deposition (CVD) have been commercialized over the last 25 years. Advancements in HPHT technology have been made as well. These developments in diamond synthesis will enable expanded use of diamond’s optical, thermal, electrochemical, chemical and electronic properties. The electrical, thermal, quantum, optical, thermal and mechanical properties make diamond an outstanding material platform many electrical and electronic devices. Adam Khan provided a good overview of the benefits and promise of diamond materials for electronics applications in his 2016 talk, "Akhan Semiconductor's Next Big Idea," at the National Competitiveness Forum, shown in the video above.
Diamond Semiconductors – A Super-Semiconducting Material
Diamond can be transformed from one of the best electrical insulators into a highly conductive pseudo-metal and ultimately a superconductor by adding impurities or dopants. Optical characteristics such as color are altered with doping or impurity additions as well. Diamond is doped with boron (B) to make a p-type region and phosphorous for n-type regions. Nitrogen (N) doping also produces n-type semiconductors, but these are likely to be more useful for biosensors, magnetometers, quantum photonics and spintronics devices.
In theory, diamond semiconductor devices can drive circuits at frequencies as high as 400 GHz. Diamond-based semiconductors are poised to provide advancements in complex devices such as high speed and power transistors (e.g., high-frequency field-effect transistors [FETs]), RF and microwave electronics, high-power switches, MEMS and high-efficiency passive devices. These faster, thinner and cooler diamond devices will result in more powerful supercomputers, advanced radar and telecommunications, hyper-efficient hybrid vehicles, robust electronics for extreme environments and next-generation avionics instruments. Diamond MEMS devices can be specifically designed for the capacitive switching arrays to provide better dynamic tuning of high-end smartphone antennas. Diamond technology may even extend Moore’s law beyond its anticipated demise.
Diamond materials for semiconductor applications are typically produced by converting methane gas into single or nanocrystalline diamond material using a microwave plasma chemical vapor deposition (MPCVD) or plasma ball reactor. The process is more environmentally friendly compared to silicon technology because it consumes 20% less water and could convert waste methane gas into a semiconductor. Monolithic substrates derived from HPHT diamond might be useful for heatsinks, wear parts and optics, but the defect density is too high for semiconductor applications.
Diamond and compound semiconductors such as silicon carbide (SiC), gallium arsenide (GaAs), and gallium nitride (GaN) are wide bandgap (WBG) semiconductors. WBG semiconductors exhibit superior power handling at high operating frequencies compared to silicon. Diamond WBG semiconductors have the potential to outperform traditional compound semiconductors like SIC, GaAs and GaN. Conventional silicon and GaAs devices typically cannot operate above 300° C due to packaging, interconnect, thermal increase in carrier concentration and other factors. Diamond semiconductor devices should be immune to the effects of thermally generated carriers, which makes them ideal for high-temperature and high-voltage applications such as power electronics (e.g., PIN diodes, junction field effect transistors [JFET] and thyristors) or electronic controls in hot engines. High-voltage PIN diodes with high-temperature and high-frequency capabilities could make high-performance, high-voltage electrical power switches, power rectifiers, attenuators, photodetectors and RF switches.
Synthetic diamond transistors have been produced in the laboratory. They are functional at much higher temperatures than silicon devices and are resistant to chemical and radiation damage. The optical phonon energy (Eoptical = 160 meV) of diamond is higher than any other semiconductor material. High optical phonon energies result in high-saturation carrier velocity. The semiconductor conductivity becomes a function of saturation velocity instead of mobility in high electric fields. Diamond is unique in retaining its high saturation velocity even at high field strengths, which is an important factor in enhancing the performance of FETs operating at high frequencies. Diamond is an ideal platform for low-noise amplifiers because diamond can sustain stable bipolar operation, unlike GaN and SiC, even in harsh environments.
Diamond has come to be known as the ultimate wide bandgap (WBG) semiconductor material due to its inherent properties. Diamond conducts heat five times better than copper and 22 times better than silicon. While diamond is an excellent thermal conductor, diamond is also an excellent electrical insulator. The diamond’s high dielectric strength allows thin diamond layers to isolate massive voltages compared to current technologies. In isolating 10,000 V, the volume of diamond needed is 50 times less than that of silicon. Devices with smaller dimensions enable faster switching. CVD diamond’s lower dielectric constant compared to other WBG semiconductors should reduce crosstalk in integrated circuits, which is an issue with decreasing feature size. Diamond-like carbon (DLC) films modified with N, fluorine (F) or silicon (Si) with dielectric constants as low as 2.5 were synthesized by Alfred Grill in IBM’s research labs, and were reported in “Electrical and optical properties of diamond-like carbon”. Dielectric diamond films with closed internal porosity should have even lower dielectric constant values.
Diamond materials will likely have the greatest impact in the power electronics realm. Photonics and high-power RF devices will also be revolutionized by diamond materials platforms. According to the International Energy Agency (IEA), worldwide electrical energy production is expected to increase by 75% over the 20 years as industrialization increases and technology spreads. Power electronics are a key component in transforming electrical energy from coal, fuels, gas or renewable primary sources into the forms required by end-users in households, offices and plants. Fifty percent of the world’s electricity is converted or controlled by silicon-based power electronics devices. These power converters are a major factor in the approximately 80% of the energy lost along the electrical power distribution system from primary generation sources to end-users. Wide-bandgap semiconductors such as GaN, SiC and diamond have the electrical properties to overcome silicon’s limitations to produce energy conversion devices with low losses and high efficiency. Figures 2 to 6 show how diamond properties and performance potential far outshine all other semiconductor materials for enhanced power electronic devices.
In summary, some of the advantages of diamond materials for electronics and electrical applications include:
- High operating temperature of over 300° C degrees (five times hotter than silicon), eliminating the need for cooling in some applications.
- Highest thermal conductivity of any known material; it can dissipate heat, if required.
- 90% more efficient or 90% lower energy loss compared to silicon.
- High dielectric strength; several orders of magnitude over silicon (1 x 107 diamond vs. 3 x 105)
- Low dielectric constant and reduced crosstalk in microelectronic circuits; diamond: 5.5 to 5.7; silicon 11.8 to 11.9; silicon dioxide (3.5).
- Excellent resistance to radiation due to extremely short carrier lifetimes (about two microseconds).
- High hardness and strength; beneficial in MEMS devices.
- Broad optical transparency from UV to infrared.
- Low coefficient of thermal expansion (CTE) equals reduced thermal interface stress.
- High strength, low CTE and high thermal conductivity impart high thermal shock resistance.
- Wide bandgap: Egap = 5.47 eV
- High carrier mobility: 4,500 cm2/(V·s) for electrons in single-crystal CVD diamond, desirable properties for high-frequency operation above 50 GHz and FETs.
- High saturation velocity (vs) at high field strengths due to high optical phonon energy (Eoptical = 160 meV).
- Absence of a native oxide allows UV radiation access to semiconductor regions for sensing or photovoltaics. The SiO2 layer on silicon absorbs UV.
Founded in 2013, Akhan Semiconductor is one of the companies at the cusp of the emerging diamond microelectronics industry. They are deploying a 200 mm Miraj electronics platform for fabricating diamond semiconductor devices. Diamond microelectronics will be faster, more efficient, one-thousandth thinner and have current densities a million times higher than the state of the art silicon technologies. Some of Akhan Semiconductor’s technology was licensed from Argonne National Laboratory (ANL), which has been on the forefront of innovative diamond research for many years. Advanced Diamond Technologies (ADT), another pioneer in nanostructured CVD diamond for semiconductor and industrial applications, also has roots with ANL.
Well-established companies in the diamond industry such as Element 6 (a unit of DeBeers), Applied Diamond Inc. and A.L.M.T. Corp. (Sumitomo Electric Industries) are expanding their portfolio of HPHT and CVD diamond products. The CVD diamond industry has seen many new entries such as Advent Diamond (2017), Diamond Materials (2004), EDP Corporation (2009), Hebei Plasma Diamond Technology (2009), Lake Diamond (2007), IIa Technologies Pte. Ltd (2005), NeoCoat (2013), Scio Diamond Technology Corporation (2009) and WD Lab Grown Diamonds (2008).
Modern diamond materials manufactured with either HPHT or CVD technologies can have properties such as hardness, thermal conductivity and electron mobility that are superior to those of most naturally formed diamonds. Natural diamond can be enhanced or modified through HPHT processing. Diamond materials will revolutionize many electronic and electrical applications such as high-power switches at power stations, high-frequency FETs, lasers, semiconductor devices, thermal management products and LEDs.
In Diamond Materials Part 3, the benefits diamond materials bring to additional electrical and electronics need, thermal management, will be examined.
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