A high-temperature silicon carbide (SiC) semiconductor transistor was fabricated, packaged, and electrically operated continuously at 500 °C for over 2000 hr in an air ambient. For the first 500 hr of electrical operation, less than 10-percent change in operational transistor parameters was observed. This is the first semiconductor transistor to demonstrate continuous electrical operation in such a harsh high-temperature oxidizing air environment with excellent stability over such an extended period of time. This demonstration of 500 °C transistor durability represents an important step toward significantly expanding the operational envelope of sensor signal processing electronics for harsh environments such as the high-temperature regions of combustion engines and the surface of Venus.
The emergence of wide bandgap semiconductors, including SiC, diamond, and gallium nitride (GaN), has enabled short-term electrical device demonstrations at ambient temperatures from 500 to 650 °C, well beyond the reach of the silicon semiconductor material that is used to manufacture today’s computer and communication electronics. However, wide bandgap transistors had not previously demonstrated sufficient long-term durability when electronically operated at these high temperatures to be considered viable for most envisioned applications. This was especially true when the transistors were tested in air ambient, which chemically attacks, oxidizes, and degrades the electrical ability of metal-semiconductor contacts to carry electrical signals in and out of the semiconductor. Therefore, this new SiC transistor fabrication process, which enabled durability in air, featured multiple levels of high-temperature metal and dielectric passivation to prevent contamination(particularly oxygen) from reaching electrically sensitive interfaces.
Left: Scanning electron micrograph of SiC metal-semiconductor field effect transistor structure. The large lighter regions are the metal contacts. Right: Operational transistor drain current versus drain voltage characteristics of SiC field effect at 500 °C. Applied gate voltage is indicated in –2-volt steps. Substrate bias voltage, Vsub, –20 V.
Long description of figure 1.
Long description of figure 2.
The transistor was a metal-semiconductor field effect transistor (MESFET) fabricated on a 6H-SiC wafer, identical to the device shown in the photomicrograph. A thick-film metallization-based ceramic package with conductive die-attach material and gold wire bonds facilitated long-term testing under electrical bias at 500 °C. The graph shows the transistor current-voltage characteristics at the start of the test and after 558 hr of continuous electrical operation in air at 500 °C. The small change observed in the transistor characteristics is due to increasing leakage current from the metal-semiconductor gate terminal as anneal time increased. This gradual degradation mechanism could be eliminated by a modification to the design of the gate to the SiC transistor. The remaining electrical contacts and packaging continued to operate successfully without observable electrical degradation for the full duration of the 500 °C test, which exceeded 2000 hr.
Okojie, Robert S.: Thermally Stable Ohmic Contacts on Silicon Carbide Developed for High-Temperature Sensors and Electronics. Research & Technology 2000. NASA/TM-2001-210605, 2001, pp. 59-60. http://www.grc.nasa.gov/WWW/RT/RT2000/5000/5510okojie.html
Chen, Liang-Yu: Packaging Technology Designed, Fabricated, and Assembled for High-Temperature SiC Microsystems. Research & Technology 2002. NASA/TM-2003-211990, 2003, pp. 69-70. http://www.grc.nasa.gov/WWW/RT/RT2002/5000/5510chen.html
Find out more about SiC electronics at Glenn: http://www.grc.nasa.gov/WWW/SiC/
Glenn contacts:
Dr. Philip G. Neudeck, 216-433-8902, Philip.G.Neudeck@nasa.gov; Dr. Robert S. Okojie, 216-433-6522, Robert.S.Okojie@nasa.gov; and Dr. Glenn M. Beheim, 216-433-3847, Glenn.M.Beheim@nasa.gov
Ohio Aerospace Institute (OAI)contacts:
David J. Spry, 216-433-3361, David.J.Spry@nasa.gov; and Dr. Liang-Yu Chen, 216-433-6458, Liangyu.Chen-1@nasa.gov
Author:
Dr. Philip G. Neudeck
Headquarters program office:
Aeronautics Research
Programs/Projects:
GMI, UEET
Last updated: October 16, 2006
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