Fabrication and Electrical Characterization of AlGaN/GaN HEMTs with BST Gate Dielectric: A Comparative Study of Two- and Three-Terminal Devices

<title>Abstract</title> <p> AlGaN/GaN high-electron-mobility transistors (HEMTs) are promising for high-frequency and high-power electronic applications because of their high electron transport capability and polarization-induced two-dimensional electron gas at the AlGaN/GaN heterointerface. In this work, a systematic fabrication and electrical characterization study of AlGaN/GaN HEMT structures incorporating barium strontium titanate (BST) as a gate dielectric is presented. Initially, two-terminal devices were fabricated using photolithographic patterning and Al/Cr/Au metallization with thicknesses of 150/40/250 nm, followed by rapid thermal annealing at 850°C for 40 s to promote low-resistance ohmic contact formation. The two-terminal current-voltage characteristics were investigated before and after annealing over different voltage ranges. Subsequently, a 200-nm-thick BST layer was deposited over the designated gate region by RF sputtering using a commercial 2-inch BST target at an RF power of 100 W, an Ar flow rate of 60 sccm, and a working pressure of 2.0 × 10 ^− 2 mbar. The BST-integrated two-terminal devices exhibited predominantly linear and approximately symmetric current-voltage characteristics over the investigated voltage ranges. Finally, a Ni/Au gate electrode with a thickness of 20/100 nm was fabricated over the BST layer to realize a three-terminal BST-gated AlGaN/GaN HEMT. The final device, having a gate length of 14.03 µm and gate width of 1.343 mm, exhibited clear gate-dependent drain-current modulation. The output characteristics showed a maximum drain current of approximately 5.0 × 10 ^− 4 A, while the transfer characteristics exhibited drain-current modulation with applied gate voltage. SEM and EDS analyses further verified the fabricated device morphology and elemental distribution of the BST, AlGaN/GaN, and metal-contact regions. The results demonstrate the feasibility of integrating a sputter-deposited BST layer into AlGaN/GaN HEMT structures and establish a systematic electrical comparison from two-terminal contact structures to a three-terminal BST-gated device. </p>