Experiments have revealed that stable breakdown-voltage characteristics can be achieved in silicon carbide rectifiers. Stable breakdown-voltage characteristics are essential to the ability to withstand overvoltage transients and are therefore necessary for reliability in high-power semiconductor switching and rectifying devices.
Silicon carbide semiconductor devices can function under high-temperature, high-power, and high-ionizing-radiation conditions beyond the endurance limits of silicon semiconductor devices. Consequently, SiC devices are undergoing development for eventual use in potential applications that include high-voltage switching in electric-power distribution and electric vehicles, increasingly powerful microwave electronic circuits for radar and cellular communications, and sensors and controls for advanced, cleaner-burning, more-efficient engines. However, prior to the experiments reported here, SiC semiconductor devices had exhibited unstable breakdown-voltage characteristics and were therefore questionable for incorporation into high-power circuits.
In this context, a stable or unstable breakdown-voltage characteristic of a semiconductor rectifier is synonymous with a positive or negative value, respectively, of the temperature coefficient of breakdown voltage. Silicon power rectifiers in use today are highly reliable, partly because they have positive temperature coefficients of breakdown voltages.
During large overvoltage transients, a device can become momentarily reverse-biased at a potential greater than its reverse breakdown voltage. If the device has a negative temperature coefficient of breakdown voltage, then local junction heating from breakdown current causes the local breakdown voltage to decrease, thereby giving rise to a further local increase in breakdown current. The breakdown current becomes concentrated into one or more high-current-density filaments at junction hot spots, which leads to physical junction damage and device failure. If the device has a positive temperature coefficient of breakdown voltage, then local junction heating from breakdown current increases the local breakdown voltage, preventing local concentration of breakdown current; thus, breakdown current distributes nearly evenly across the entire area of the diode junction.
The experiments were performed to determine whether the unstable breakdown observed previously is a fundamental property of SiC or whether it arises because of impurities and crystalline imperfections that could be reduced by improvements in techniques for growing SiC crystals. For the experiments, SiC rectifier diodes were fabricated by use of the crystal-growth process described in "Chemical Vapor Deposition of Silicon Carbide With Controlled Doping" (LEW-15803), NASA Tech Briefs, Vol. 20, No. 12 (December 1996), page 80. The figure shows the diode structure and current and voltage waveforms recorded when one of these diodes was subjected to an overvoltage pulse with a duration of 200 ns. These waveforms show that as the device becomes heated by the breakdown current during the pulse interval, the voltage across the device increases, while the current through the device decreases; this behavior is consistent with a positive temperature coefficient of breakdown voltage and thus with a stable breakdown-voltage characteristic needed for reliability. The diode sustained repeated overvoltage pulses without measurable degradation of its junction.
This work was done by Philip G. Neudeck of Lewis Research Center and Chris Fazi of U.S. Army Research Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.comunder the Electronic Components and Circuits category.
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