The high temperature stability of AlGaN/GaN and lattice-matched InAlN/GaN heterostructure FETs has been evaluated by a stepped temperature test routine under large-signal operation. While AlGaN/GaN high-electron mobility transistors (HEMTs) have failed in an operating temperature range of 500 degrees C, InAlN/GaN HEMTs have been operated up to 900 degrees C for 50 h (in vacuum). Failure is thought to be still contact metallization stability related, indicating an extremely robust InAlN/GaN heterostructure configuration.