One of the main expected benefits of AlGaN HEMT technology for microwave applications is related to the higher operating bias voltage achievable with these devices. However, various technological issues, concerning material properties and device technology must be properly tailored to fully exploit the potential of that kind of devices. In this work we report on the realization of HEMT device showing improved performance in terms of breakdown voltage, device isolation and reverse current leakage achieved by improved epilayer buffer properties and optimized field plate gate geometry. In particular the low defect density and the high resistivity obtained by using an HT-AlN crystallization layer for the growth of the GaN layer has lead to an effective 2DEG carrier concentration of 8 times 1012 cm-2 with related mobility of 1700cm2/Vs and corresponding devices with a very high voltage breakdown (VB > 200V), excellent active device isolation and limited reverse current leakage.