Positive threshold voltage shift in AlGaN/GaN HEMTs with p-type NiO gate synthesized by magnetron reactive sputtering

In the present study, the threshold voltage turning of AlGaN/GaN heterostructure field-effect transistor (HFETs) by NiO gate electrode was evaluated. Firstly, NiO films were sputtered on sapphire substrates for material characterization. The composition, crystalline structure, electrical properties and optical properties of the as-grown films were all influenced by the substrate temperatures during magnetron sputtering. All NiO samples were indexed as (1 1 1) oriented face-centered cubic, while the sample synthesized at a substrate temperature of 30 °C showed the smallest (highest) resistivity (hole concentration). Then, this procedure was adopted in further studies using NiO as the gate electrode of AlGaN/GaN HFETs. Compared with the Ni/Au-gated device, the band structure adjustment introduced by the p-NiO gate was found to shift the threshold voltage positively and also cause a smaller drain current. With a band gap of 3.6 eV for the NiO film, the valence and conduction band offsets between NiO and AlGaN are estimated to be 1.6 eV and 1.4 eV, which may cause the positive threshold voltage shift.