Threshold Voltage Control in $\hbox {Al}_{0.72} \hbox {Ga}_{0.28}\hbox {N/AlN/GaN}$ HEMTs by Work-Function Engineering

The first demonstration of high-Al-composition (> 70%) AlGaN high electron mobility transistors (HEMTs) is reported. High electron mobility (~1300 cm²/Vs at room temperature) was achieved in novel high-Al-composition AlGaN 2-D electron gas structures. The threshold voltages (V_th) of Al_0.72Ga_0.28N/AlN/GaN HEMTs were shifted from -1.0 to -0.13 V by employing different gate metal stacks, Al/Au and Ni/Au, respectively. With a 4-nm Al₂O₃ gate dielectric on top of the nitride heterostructures, the ~0.9-eV work-function difference between Al and Ni induced ~0.9-V V_th shift in the pairs of the Al/Au and Ni/Au gate HEMTs, which indicates that the Fermi level is unpinned at the ALD Al₂O₃/AlGaN interface. The results were reproducible for HEMTs of various gate lengths. The results suggest that it is possible to obtain enhancement- and depletion-mode AlGaN HEMTs using work-function engineering which can enable integrated monolithic digital circuits without postgrowth recess etching or ion implantation.