In this paper, we report on the development of an RF high power amplifier, based on normally-on AlGaN/GaN Heterostructure Field-Effect Transistors (HFETs) on semi-insulating SiC substrates. The amplifier is derived from a transistor with a total gate periphery of 120 mm that exhibits a breakdown voltage better than 420 V. The transistor yields pulsed drain current levels of up to 53 A and therefore is found suitable for the ISM frequency band (industrial, scientific, medical) power applications at 13.56 MHz. The realized amplifier shows good performance in cw mode with an output power of 139 W and an efficiency of 71%, respectively. In pulsed mode, the amplifier exhibits an output power of 431 W for a duty cycle of 10% at a frequency of 13.56 MHz, which emphasizes the high potential of the III-V-compound semiconductor AlGaN/GaN for ISM applications. The comparison of the obtained values with standard silicon based semiconductor devices used for this frequency range furthermore shows the impressive advantages of AlGaN/GaN based devices for parameters like current density and power density that are at least by a factor of 10 higher. In a next step, the ruggedness of the realized amplifier was investigated. Operating the amplifier up to a VSWR of more than 15:1, no damage was observed. The junction temperature during VSWR mismatch was calculated to be more than 249 degrees C. (c) 2012 Elsevier Ltd. All rights reserved.