As the electric vehicle (EV) market expands, manufacturers must reduce initial purchase and operating costs while simultaneously increasing driving range and decreasing charging time – which are all functions played out using SiC-based power electronics.
Coherent’s expertise in II-VI has inspired them to form a subsidiary dedicated to providing 200mm substrates and epi wafers for power electronics devices, with Denso being an automotive supplier and Mitsubishi Electric serving multiple industries as its key customers.
High-Resistivity Substrates
SiC substrates are integral parts of power devices that enable high breakdown voltages and low switching losses needed for efficient power conversion in electric vehicles, wind turbines, solar panels and other industrial applications. To remain cost effective under extreme conditions these devices need a substrate with high resistance which minimizes energy lost when passing current through them.
Resistance of mono-SiC substrates is determined by their doping density. While mono-SiCs typically consist of high-purity silicon carbide with no defects and minimal doping density, using heavy metals like copper (Cu) or gold in bonding layers increases their resistivity by approximately 2.5 micro Ohm-cm per Cu/Au added.
SiC-on-Si (single crystal silicon carbide on silicon) substrates offer lower doping densities while maintaining crystal quality, enabling less costly Cu or Au bonding layers to reduce resistance significantly.
By switching from 6-inch substrates to 8-inch substrates, manufacturers can significantly cut costs by decreasing waste and increasing utilization rates – according to Chinese substrate maker TankeBlue Semiconductor this can save them 50% per wafer! As a result, high-resistivity substrates have become popular within the industry.
Semi-Insulating Substrates
RF devices such as GaN heteroepitaxy require semiconductor substrates with high electrical resistivity, low electron drift mobility and semi-insulating characteristics – typically made of n-type SiC and produced using epitaxy or ion implantation techniques. Their quality has a direct bearing on device performance, yield and reliability.
One conventional method to form semi-insulating silicon carbide (SiC) single crystal involves adding an element such as aluminum that has deep level acceptor levels in SiC; this overcompensates for any nitrogen impurities which reduce donor levels and produces conductive SiC single crystals.
One alternative method for producing semi-insulating SiC single crystals involves employing intrinsic point defects as deep levels to compensate for free carriers introduced into SiC by shallow donor and acceptor dopants, including shallow donor dopants such as shallow acceptor dopants and donor dopants. For this to work efficiently, however, an equal concentration of deep level acceptors and donors must be doped into each corner (NN+ND=NA+NTE).
Semi-Insulating Silicon Carbide Substrates are manufactured using advanced manufacturing techniques designed to minimize crystal defects and ensure material uniformity, yielding high-performance, stable and reliable semiconductor substrates that can be used in RF and power devices like amplifiers and transistors as well as high temperature/extreme environments applications like radar systems, seeker networks and satellite communications.
Wide-Bandgap Substrates
Wide-bandgap substrates allow the production of high-performance RF power amplifiers, lasers, and other semiconductor devices with wide bandgap substrates. These semiconductor devices offer superior performance over traditional silicon-based devices due to their larger bandgap, which allows for operation at higher temperatures; the higher temperature allows more damage-proofing against wear-and-tear, thus increasing lifespan and reliability of these semiconductor devices.
Wide-bandgap materials also boast significantly faster switching speeds than silicon; Gallium nitride for instance boasts an electron mobility of 2,000 cm2/Vs – 10 times faster than silicon! This increased switching speed makes wide bandgap materials ideal for high frequency applications such as telecom, data communications and power electronics.
The US Department of Defense established eight Microelectronics Commons regional innovation hubs in 2022, such as North Carolina’s Commercial Leap Ahead for Wide-Bandgap Semiconductors (CLAWS) Hub. Their goal is to promote ecosystems that minimize risk while incentivizing large-scale private investments into large-diameter production and breakthrough technologies to help accelerate commercialization of wide-bandgap semiconductors. Wolfspeed operates within this ecosystem.
Coherent announced in 2023 that they had received a $1 billion investment from automotive firms Mitsubishi Electric and Denso to create a vertically integrated subsidiary dedicated to manufacturing SiC substrates, epiwafers, power devices and modules from its substrate production through module design. Khan leads this new subsidiary company; their focus lies on engaging customers at every step – from making substrates and epiwafers right through designing devices and modules, with maximum learning being their motto of operation.
High-Voltage Substrates
As demand for wide bandgap power semiconductors grows in applications like electric vehicles (EVs), renewable energy inverters and smart grid infrastructure, manufacturers must ensure their components can withstand high currents and voltages without failing. Substrates often serve as the basis of such devices and must be capable of withstanding high voltage levels in these challenging environments to prevent device failure.
Substrates are thin discs of monocrystalline semiconductor material used as the foundation for MOSFETs and IGBTs to be constructed via epitaxial growth, typically using monocrystalline SiC MOSFETs and IGBTs found in automotive modules like onboard chargers, traction inverters and DC-DC converters as an example. Silicon (Si) is often the preferred substrate material.
Under higher voltages, however, the metallized ceramic substrate can experience issues where it meets its encapsulating material – particularly at its edges where it touches. This interaction triggers a strong increase in electric field strength which results in Partial Discharges that eventually cause its failure and could obliterate your module entirely.
Recently, a novel substrate has been created to address this problem of high-voltage power devices. Leveraging silicon-on-insulator technology – consisting of polycrystalline layers adhered to monocrystalline SiC – this new substrate makes fabrication of AlN metal-semiconductor field effect transistors (MESFETs) possible without needing complex regrown or graded contact layers, providing excellent drain saturation current and on/off ratio results.
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