We have reported SiC integrated circuits (IC’s) with two levels of metal interconnect that have demonstrated prolonged operation for thousands of hours at their intended peak ambient operational temperature of 500 °C [1, 2]. However, it is recognized that testing of semiconductor microelectronics at temperatures above their designed operating envelope is vital to qualification. Towards this end, we previously reported operation of a 4H-SiC JFET IC ring oscillator on an initial fast thermal ramp test through 727 °C . However, this thermal ramp was not ended until a peak temperature of 880 °C (well beyond failure) was attained. Further experiments are necessary to better understand failure mechanisms and upper temperature limit of this extreme-temperature capable 4H-SiC IC technology. Here we report on additional experimental testing of custom-packaged 4H-SiC JFET IC devices at temperatures above 500 °C. In one test, the temperature was ramped and then held at 727 °C, and the devices were periodically measured until electrical failure was observed. A 4H-SiC JFET on this chip electrically functioned with little change for around 25 hours at 727 °C before rapid increases in device resistance caused failure. In a second test, devices from our next generation 4H-SiC JFET ICs were ramped up and then held at 700 °C (which is below the maximum deposition temperature of the dielectrics). Three ring oscillators functioned for 8 hours at this temperature before degradation. In a third experiment, an alternative die attach of gold paste and package lid were used, and logic circuit operation was demonstrated for 143.5 hours at 700 °C.