High-Temperature Integrated Circuits (ICs) Providing performance in extreme environments
Electronics today are dominated by semiconductor devices and ICs that are fabricated on silicon. As a result, most electronics are limited by the reliability and efficiency of their silicon devices to operate in environments where the temperatures of the silicon transistor junctions are kept below 125°C, with silicon insulator forms offering solutions up to 225° C. However, there are system applications where electronics must operate at higher temperatures for improved system performance. For these applications, alternative semiconductor materials must be used that do not have silicon's temperature limitation. One of these materials is silicon carbide (SiC).
SiC power components are emerging as viable alternatives to silicon power components; silicon carbide can also be used to make integrated circuits, where logic gates and analog components are combined to perform sensing and control functions while operating at high temperatures.
High-temperature SiC integrated circuit technology (Figure 1) is under development by Raytheon in Glenrothes, Scotland. The U.K. government, recognizing the potential of this technology as an enabler for higher efficiency systems, provides partial support for development under its Technology Strategy Board "Materials For Energy" research program. This technology will support the manufacture of complementary metal-oxide semiconductor (CMOS) integrated circuits that use SiC semiconductor material.
Table 1 compares selected properties of SiC with silicon. The wide bandgap property of SiC corresponds with low leakage current for SiC transistors at high temperatures, whereas the leakage current in silicon transistors (having a narrower bandgap) increases significantly with temperature, ultimately causing device failure. It is predominantly this characteristic that enables silicon carbide devices to operate reliably and efficiently at temperatures well above that of silicon devices.
Gas Turbine Engine Application
Raytheon is applying SiC technology to a European Union-funded aerospace research project under the Clean Sky program. One of the program goals is to develop technologies that reduce aircraft noise and increase fuel efficiency. If the fuel efficiency of a jet engine is improved, and its weight is reduced even by a small amount, total fuel consumption can be significantly reduced over the service life of the aircraft. One way to improve engine efficiency is to better control the combustion process. This requires accurate temperature sensing of the hot gases as they flow through the engine.
Raytheon is working with Aero Engine Controls (Rolls-Royce Goodrich Engine Control Systems, Ltd) to evaluate materials used in the engine control system. Raytheon's high-temperature SiC CMOS circuits are being evaluated as a way to improve the accuracy of temperature measurements taken at critical locations on the gas turbine engines.
The temperature of a gas turbine engine increases from the intake to the exhaust throughout the length of the engine (Figure 2). Electronic control of the engine for efficient operation requires an accurate measurement of the gas temperatures in the combustion chamber and exhaust sections. Due to the limitations of current silicon semiconductor technology, sensitive temperature measurement electronics must be remotely located near the cooler intake. Specialized cabling runs the length of the engine to the sensing elements near the exhaust. This long cable is expensive and must be routed through engine compartment bulkheads, adding extra weight and cost. Additionally, the long cabling introduces electronic noise and adds signal loss to the sensing system, which reduces the accuracy of the temperature sensing function.
Raytheon and Aero Engine Controls are working together to develop a thermocouple signal multiplexing unit that uses high-temperature SiC CMOS technology. When fully developed, the SiC CMOS circuit should operate reliably and efficiently at temperatures up to 300ºC, allowing the unit to sit closer to the sensors at the hot end of the engine. This facilitates more accurate and failsafe sensing. After multiplexing and amplification, the sensor data can be sent via simple and inexpensive twisted pair cabling back to the main control electronics module mounted within the cool section of the engine.
High Temperature Power Converter Application
SiC is an emerging alternative to silicon semiconductors for use in the power switches and rectifiers employed in power supplies. The SiC material properties of higher breakdown electric field, higher bandgap energy, higher thermal conductivity and a higher saturated drift velocity are desirable because they allow switches to operate at a higher efficiency with higher switching speeds, higher voltages and higher power densities. SiC, which combines these properties with the ability to operate and maintain its efficiency at a higher temperature, represents an enabler for many new power applications.
In power supplies, the higher (5 to 10 times) switching speeds of SiC power switches allow the associated energy storage components (inductors and transformers) to be proportionally smaller — reducing their weight and volume by the same factor.1 Power densities of 30 kW/l have been reported.2 Additionally, higher allowable device junction temperatures and high thermal conductivity reduce the need for bulky thermal management solutions, leading to smaller, lighter and more densely packaged power converter modules. Dense packaging, in turn, enables the close integration of control circuitry. This minimizes series inductance, improving stability and efficiency.
Raytheon's high temperature SiC CMOS technology will provide solutions to the challenges of achieving higher performance with greater energy efficiency in extreme environments. This new development provides opportunities with new classes of products, all enabled by high operating temperature electronics.
1.Infineon "thinQ™ Silicon Carbide Schottky Diodes: An SMPS Circuit Designer's Dream Comes True!"
2. SemiSouth press release,"Highest output power density inverter uses SiC JFETs from SemiSouth," at http://semisouth.com/archives/1056
© Raytheon Systems Limited 2012
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