High Electron Mobility Transistors With Al_0.65Ga_0.35N Channel Layers on Thick AlN/Sapphire Templates

We report Al_0.85 Ga_0.15 N/Al_0.65 Ga_0.35N 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 /\Box $ . Devices with the source–drain spacing of $5.5~\mu \text{m}$ and a gate length of 1.8 $\mu \text{m}$ exhibited peak drain–currents as high as 250 mA/mm at a gate bias of +4 V. We also show that the 3- $\mu \text{m}$ 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 250 mA/mm with no current droop.