Effect of thermal annealing on 120-nm-T-shaped-Ti∕Pt∕Au-gate AlGaN∕GaN high electron mobility transistors

Abstract

We fabricated 120-nm-long-T-shaped-Ti∕Pt∕Au-gate AlGaN∕GaN high electron mobility transistors (HEMTs) on sapphire substrates and annealed them thermally to improve their Schottky contacts (i.e., to reduce interface traps, increase the Schottky barrier height, and get better adhesion), thereby reducing gate leakage current, achieving better gate controllability, and obtaining better dc and rf characteristics compared to the HEMTs with as-deposited Schottky gate metals. Rapid thermal annealing (RTA) was carried out successively at 500, 600, and 650°C. The drain-source current Ids decreased with RTA temperature up to 600°C and turned to increase at 650°C. The maximum transconductance gm̱max, on the other hand, increased up to 650°C, indicating that RTA improved the dc characteristics of the HEMTs. We confirmed the improvement of Schottky contacts by measuring gate leakage current Igs and calculating the Schottky barrier height ϕB. After RTA at 600°C, the Igs at a gate-source voltage Vgs of −10V was as much as three orders of magnitude lower than it was in the as-deposited condition and the ϕB was 0.27eV greater than it was in the as-deposited condition (1.16eV versus 0.89eV). After the RTA at 650°C, the ϕB was slightly less than it was after the RTA at 600°C. RTA at 600 and 650°C also improved the rf characteristics, increasing the cutoff frequency fT from 80to84GHz (5% increase) and increasing the maximum oscillation frequency fmax from 102to122GHz (20% increase).