The first high-temperature characterization of GaN150 HEMT devices is presented from ambient temperature to 400°C. With a 2-gate length of 150nm, three configurations of GaN150 are investigated. They have gate widths of 40µm (T1), 100µm (T2) and 200µm (T3) respectively. The stability with temperature of the electrical characteristics of the AlGaN/GaN devices are measured with tungsten probes while the tested die is heated by a hot plate. The packaging is a standard tungsten based metallization, with Ni and Au plating, supporting a 92% alumina ceramic substrate. The drain saturation current decreases from 50mA, 150mA and 290mA at room temperature (RT) to 35mA, 110mA and 175mA at 400°C for T1, T2 and T3 respectively. The peak transconductance value of T1 is dropped from 22mS at ambient temperature to 8mS at 250°C. The pinch-off voltage is stable at VGS = −5V during the whole temperature change.