Recessed-gate AlGaN/GaN HFETs with lattice-matched InAlGaN quaternary alloy capping layers

In this paper, we present recessed AlGaN/GaN heterojunction field-effect transistors (HFETs) with lattice-matched InAlGaN capping layers, which reduce both ohmic contact resistance and series resistance between the AlGaN and the capping layer. The lattice-matched alloy epitaxial layer with both In and Al high compositions are successfully grown by metal-organic chemical vapor deposition. The grown lattice-matched In_0.09Al_0.32Ga_0.59N capping layer has close total polarization and bandgap to those of the underlying Al_0.26Ga_0.74N layer. The balanced polarization eliminates the depletion of electrons at the In_0.09Al_0.32Ga_0.59N/Al_0.26Ga_0.74N interface, which can reduce the series resistance across it. It is also noted that the fabricated HFET exhibits very low ohmic contact resistance of 1.0×10^-6 Ω·cm² or less. Detailed analysis of the source resistance reveals that the series resistance at the In_0.09Al_0.32Ga_0.59N/Al_0.26Ga_0.74N interface is one fifth as low as the resistance at the conventional GaN/Al_0.26Ga_0.74N interface.