High Electron Mobility Transistors With Al0.65Ga0.35N Channel Layers on Thick AlN/Sapphire Templates

We report Al-0.85 Ga-0.15 N/Al-0.65 Ga0.35N high-electron mobility transistors on low-defect AlN buffer layers over basal plane sapphire substrates. A new epilayer design consisting of a doped barrier layer yielded sheet resistivity values as low as 1800 Omega/square. Devices with the source-drain spacing of 5.5 mu m and a gate length of 1.8 mu m exhibited peak drain-currents as high as 250 mA/mm at a gate bias of +4 V. We also show that the 3-mu m thick low-defect AlN buffer layers over sapphire to provide sufficient thermal conduction which enables stable device operation up to at least 40 V at 250mA/mm with no current droop.