This paper describes approaches that we have used to improve the high-frequency device characteristics of GaN-based heterostructure field-effect transistors (HFETs). We developed three novel techniques to suppress short-channel effects: high-Al-composition and thin barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. These techniques were used to enhance the high-frequency device characteristics of Al0.4Ga0.6N/GaN HFETs, which had a record current-gain cutoff frequency of 181 GHz. In addition, high-performance depletion- and enhancement-mode AlN/GaN HFETs, which had DC and RF device characteristics fully comparable with those of state-of-the-art AlGaN/GaN HFETs, were also fabricated with these techniques. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)