Why Silicon Carbide for Electric Power?
Silicon carbide (SiC) is a compound semiconductor made from silicon and carbon atoms bonded in a single crystalline structure. Its biggest issue today is extremely high cost; nevertheless, it’s benefits are profound if produced in commercially available quantities at reasonable prices.
Of the many crystalline atomic arrangements (polytypes) that exist for SiC, the 4H polytype possesses the best combination of electronic, thermal, and chemical properties for robust high-voltage, high-power electronic device applications. The key attributes of SiC are compared to those of silicon in Table 1.
The key attributes that establish SiC as superior to silicon are:
- high breakdown field- that permits large blocking voltages to be attained with minimal semiconductor thicknesses
- wider band gap- that permits efficient operation of the power device at elevated temperatures, thereby reducing cooling requirements.
- 400 times lower resistance- that results in increased system efficiency
- 10 times higher switching frequency- that permits smaller passive components, resulting in smaller, lighter power systems.
Altogether, SiC presents a more than hundred-fold improvement compared to silicon in potential power performance.
These and other properties of SiC have positioned the material to be of considerable commercial importance in a variety of other markets; including substrates for visible and white light emitting diodes, short wave-length lasers, and microwave power transistors, as well as an active material for medium-voltage (1200v or less) power electronics and ultra-high-frequency power transistors.
For example, high-torque, high power (watts/kg) axial-flux motor-generators like the one pictured above could produce transformational capabilites using 10,000v MOSFETs made from SMI's technological capabilities. Instead of a 100kW motor that is 305mm diameter x 105mm long x 27kg using 700V at 400 A, it could be producing over 1,400kW (2,380 HP) with less heat for the same form factor. That would be revolutionary since multiple units can be placed on one drive shaft!
However, significant hurdles remain in the areas of extended defect reduction and low, controlled electron concentrations with very long “free carrier” lifetimes. Nevertheless, SMI can deliver those requirements for both commercial applications as well as the high-performance, high-reliability, high-voltage power devices needed by military applications. Here is why:
As with any epitaxial materials growth effort, the quality of the substrate is of critical importance to the quality of epilayers grown and intended for device applications. To ensure a thorough understanding of the SiC substrate technology, which demonstrates varying degrees of perfection at this time, a number of full-wafer characterization techniques are employed. It has been proven over time that a thorough understanding of substrate quality is important not only to refining epitaxial growth processes and material quality, but also to advancing the quality and maturity of SiC substrates.
How Do You Make Better Substrates?
The SiC epitaxial growth environment is an extreme environment and a very challenging one to investigate. Typically it is comprised of hydrogen gas under a high flow rate (80 liters per minute) carrying a small mass fraction (<1%) of silane and propane through a reaction zone held at 1600 °C. For this reason, there have been very few efforts to characterize the growth process in situ.
However, gaining a better understanding of this environment is critical to the development of epilayers suitable for high-voltage power electronic devices since the required layers of such device structures imply long growths (8–16 hours) and are very sensitive to impurities (at the parts per billion level) and to variations in the gas phase carbon-to-silicon ratio. Such sensitivity is extremely important to the controlled growth of very thick (100 μm), very lightly doped (5 × 1014 cm–3) drift regions of SiC power devices.
Thanks for contributions in NRL Review- by C.R. Eddy, Jr., D.K. Gaskill, K.-K. Lew, B.L. VanMil, R.L. Myers-Ward, and F.J. Kub- US Naval Research Lab- Electronics Science and Technology Division
STELA Materials has the capabilities to deliver such high-voltage, high-efficiency SiC substrates today for your R&D testing purposes. Contact us now.