GaN and related materials have recently attracted significant interest for applications in high power electronics capable of operation at elevated temperatures. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor, is more mature, the AlGaInN material system offers numerous advantages. These include wider bandgaps, excellent transport properties, and the availability of bandgap engineering. All these advantages have led to rapid progress in the realization of various GaN-based electronic devices, especially the AlGaN/GaN heterojunction field effect transistor (HFET). This paper presents recent progress in the development of fabrication processes and device performance of AlGaN/GaN HFETs at the University of Illinois. A unity current gain cut-off frequency (fT) of over 100 GHz and a maximum oscillation frequency (fmax) of 155 GHz were demonstrated for AlGaN/GaN HFETs with a gate length of 0.12 μm on SiC substrates grown by MOCVD. These devices exhibited excellent microwave noise performance with a minimum noise figure (NFmin) of 0.53 dB and an associated gain (Ga) of 12.1 dB at 8 GHz achieved. Results of devices on sapphire substrates are also presented.