Use of high-k gate dielectrics in AlGaN/GaN heterostructure field-effect transistors (HFETs) may reduce gate leakage and improve device reliability without adversely impacting transconductance and pinchoff voltage. To achieve this, AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors have been formed by incorporating barium strontium titanate (BST) deposited by rf magnetron sputtering as the gate dielectric. The maximum current achieved was slightly lower than realized in standard devices without BST, while the gate leakage of the devices was reduced by similar to5 orders of magnitude compared to a conventional HFET for the as-deposited devices and 4 orders of magnitude for films annealed in N-2. The transconductance and pinchoff voltage were found to vary with different dielectric constants of the BST films, being reduced by similar to25% for a 40 nm film with a dielectric constant of 20, and by 14% upon annealing in N-2 (dielectric constant similar to66) when compared to a baseline device with no oxide. It was found that the BST deposition temperature greatly affected mobility in the AlGaN/GaN structure, with higher temperatures significantly reducing the HFET mobility. (C) 2004 American Vacuum Society.