Ammonia-MBE growth techniques were developed to allow the production of AlGaN and GaN layers suitable for electronic applications on sapphire [0001] substrates. DC and RF characterization of AlGaN/GaN HEMT devices have been carried out over a temperature range from -40 degreesC to 200 degreesC. The devices characterized were two finger x 50 mum wide designs with measured gate lengths ranging from 1 mum to 3.5 mum Hall and CV measurements indicate a 2DEG sheet charge density at the AlGaN/GaN heterojunction interface as high as 1.9x10(13)/cm(2) and 1.5x10(13)/cm(2) respectively. Low field mobilities in excess of 950 cm(2)/Vxs have been measured. Room temperature measurements for a device with a gate length of 1 mum and Vds set to 10 V exhibited a pinch off voltage of -5 volts. A maximum drain current of 946 mA/mm and a peak transconductance of 160 mS/mm were also measured. The off state drain to source breakdown voltage is 33 volts. Room temperature RF characterization with Vds=10 volts indicate intrinsic device f(T) and f(MAX) to be 15.6 GHz and 49.4 GHz respectively At maximum power dissipation and chuck temperature, the channel temperature is estimated to be > 320 degreesC.